This site is designed as a convenient location for our visitors to view the various Java tutorials that we have constructed to aid in teaching concepts in light and color.

Interactive Lens Action Tutorials - This set of tutorials examines lens action as a function of lens shape. Examine the difference between negative and positive lenses on image formation and visit our variable lens Java and Flash tutorials.

Prism Refraction - The prism tutorial explores how changes in the thickness and angle of incidence of a visible light beam affect how light is refracted by a prism.

Human Eye Accommodation - Exploring how images are created on the retina of a human eye, this tutorial allows the student to adjust the distance of an object from the eye to vary the size of the image.

Lasers - This tutorial explores how a ruby laser crystal works when excited by a xenon flash tube. As photon energy is "pumped" into the crystal by the flash tube, the internal crystal photons reflect on the mirrored ends of the crystal until they have achieved enough energy to escape in the form of a laser beam.

Concave Spherical Mirrors - Concave mirrors have a curved surface with a center of curvature equidistant from every point on the mirror's surface. An object beyond the center of curvature forms a real and inverted image between the focal point and the center of curvature. This interactive tutorial explores how moving the object farther away from the center of curvature affects the size of the real image formed by the mirror. Also examined in the tutorial are the effects of moving the object closer to the mirror, first between the center of curvature and the focal point, and then between the focal point and the mirror surface (to form a virtual image).

Concave Spherical Mirrors (3-Dimensional Version) - Concave mirrors have a curved surface with a center of curvature equidistant from every point on the mirror's surface. An object beyond the center of curvature forms a real and inverted image between the focal point and the center of curvature. This interactive tutorial explores how moving the object farther away from the center of curvature affects the size of the real image formed by the mirror.

Convex Spherical Mirrors - Regardless of the position of the object reflected by a convex mirror, the image formed is always virtual, upright, and reduced in size. This interactive tutorial explores how moving the object farther away from the mirror's surface affects the size of the virtual image formed behind the mirror.

Convex Spherical Mirrors (3-Dimensional Version) - Regardless of the position of the object reflected by a convex mirror, the image formed is always virtual, upright, and reduced in size. This interactive tutorial explores how moving the object farther away from the mirror's surface affects the size of the virtual image formed behind the mirror. This tutorial utilizes three-dimensional graphics.

Acoustical Model of Anisotropy - The anisotropic character of materials relates to those properties that have different values when measurements are made in different directions within the same material. This interactive tutorial explores how sound waves exhibit anisotropic character as a function of grain structure when traveling through a wooden block, which serves as an excellent model for the behavior of light passing through anisotropic crystals.

Double Refraction (Birefringence) - Calcite is a form of calcium carbonate, commonly referred to as Iceland spar, which has a rhombohedral crystalline shape. Light passing through a crystal of calcite is refracted into two rays, which are separated by a wide margin due to the strong birefringence of the crystal. This interactive tutorial simulates viewing of a ball-point pen and a line of text through a crystal of Iceland spar, producing a double image from the refracted light rays.

Birefringence in Calcite Crystals - As light travels through an anisotropic material, the electromagnetic waves become split into two principal vibrations, which are oriented mutually perpendicular to each other and perpendicular to the direction that the waves propagate. The wave whose electric vector vibrates along the major axis of the index ellipse is termed the slow wave, because the refractive index for this wave is greater than the refractive index for the other wave. The wave vibrating perpendicular to the slow wave is termed the fast wave. This tutorial explores double refraction or birefringence in calcite (calcium carbonate), a colorless, transparent, rhombohedral crystalline salt that is the most common such material found naturally.

The Fresnel or Refractive Index Ellipsoid - The Fresnel, or refractive index, ellipsoid describes the dielectric properties measured in all directions through a material. Measurements through the radius yields the refractive index (n) or the square root of the dielectric constant for waves whose electric displacement vectors lie in the direction of the ellipsoid radius. This tutorial explores variations in the shape and dimensions of the ellipsoid as a function of refractive index.

Birefringence Variations with Crystal Orientation - When a beam of light is incident on a birefringent crystal, the waves are split upon entry into orthogonal polarized components (termed ordinary and extraordinary) that travel through the molecular lattice along different pathways, depending on their orientation with respect to the crystalline optical axis. If the incident beam is oblique to the optical axis, the waves diverge during their journey through the crystal. In contrast, the orthogonal wave components follow a co-linear pathway when the incident light beam enters the crystal either parallel or perpendicular to the optical axis. This interactive tutorial explores variations in birefringence that result from orientational variations between the crystal optical axis and the incident light beam.

Birefringent Crystals in Polarized Light - In order to examine how birefringent anisotropic crystals interact with polarized light in an optical microscope, the properties of an individual, isolated crystal can be considered. The specimen material in this tutorial is a hypothetical tetragonal birefringent crystal having an optical axis oriented in a direction that is parallel to the long axis of the crystal. Light entering the crystal from the polarizer will be traveling perpendicular to the optical axis of the crystal, regardless of the crystal orientation with respect to the polarizer and analyzer transmission axes. The virtual microscope viewport presents the crystal as it would appear in the eyepieces of a microscope under crossed-polarized illumination as it is rotated around the microscope optical axis.

Interactive Michel-Levy Birefringence Chart - Quantitative analysis of the interference colors observed in birefringent samples is usually accomplished by consulting a Michel-Levy chart similar to the one illustrated in the tutorial window below. As is evident from this graph, the polarization colors visualized in the microscope and recorded onto film or captured digitally can be correlated with the actual retardation value, thickness, and birefringence of the specimen. The chart is relatively easy to use with birefringent samples if two of the three required variables are known. This interactive tutorial enables visitors to determine the interference color associated with all three values by clicking on selected regions of the interactive chart. A large version of the tutorial is also available.

Color Temperature in a Virtual Radiator - Investigate the apparent "color" of a virtual radiator (in this case, a black metal pot) as it is slowly heated through a wide temperature range by external energy. The concept of color temperature is based on the relationship between the temperature and radiation emitted by a theoretical standardized material, termed a black body radiator, cooled down to a state in which all molecular motion has ceased. Hypothetically, at cessation of all molecular motion, the temperature is described as being at absolute zero or 0 Kelvin, which is equal to -273 degrees Celsius.

Color Temperature Nomograph - The color temperature nomograph is a useful tool with which to determine the necessary color balancing and/or correction filter(s) that are necessary to convert a light source from one color temperature to another. To use this type of graph, a straight edge ruler is placed at the color temperature of the original source and is pivoted to connect to the desired color temperature. The region where the ruler intersects the central axis identifies the necessary filter to achieve the color conversion. This interactive Java nomograph tutorial can be employed to quickly determine the appropriate filter under a variety of illumination scenarios.

White and Black Balance - The overall color of a digital image captured with an optical microscope is dependent not only upon the spectrum of visible light wavelengths transmitted through or reflected by the specimen, but also on the spectral content of the illuminator. In color digital camera systems that employ either charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) image sensors, white and/or black balance (baseline) adjustment is often necessary in order to produce acceptable color quality in digital images.

Diffraction of Light - Several of the classical and most fundamental experiments that help explain diffraction of light were first conducted between the late seventeenth and early nineteenth centuries by Italian scientist Francesco Grimaldi, French scientist Augustin Fresnel, English physicist Thomas Young, and several other investigators. These experiments involve propagation of light waves though a very small slit (aperture), and demonstrate that when light passes through the slit, the physical size of the slit determines how the slit interacts with the light. This interactive tutorial explores the diffraction of a monochromatic light beam through a slit of variable aperture.

Particle Size and Diffraction Angles - The phenomenon of diffraction is observed when a specimen consisting of fine particles is illuminated with a beam of semi-coherent, collimated light. Good examples of this effect are a microscope slide containing particles of various sizes, and the spreading of automobile headlights on a foggy night. In both cases, diffraction is manifested through the scattering of light by small particles having linear physical dimensions similar to the wavelength of the illumination. This interactive tutorial demonstrates the effects of diffraction at an aperture and explores the spreading of light by a specimen composed of individual particles.

Line Spacing Calculations from Diffraction Gratings - By definition, a diffraction grating is composed of a planar substrate containing a parallel series of linear grooves or rulings, which can be transparent, semi-transparent, or opaque. When the spacing between lines on a diffraction grating is similar in size to the wavelength of light, an incident collimated and coherent light beam will be strongly diffracted upon encountering the grating. This interactive tutorial examines the effects of wavelength on the diffraction patterns produced by a virtual periodic line grating of fixed line spacing.

Light Diffraction Through a Periodic Grating - A model for the diffraction of visible light through a periodic grating is an excellent tool with which to address both the theoretical and practical aspects of image formation in optical microscopy. Light passing through the grating is diffracted according to the wavelength of the incident light beam and the periodicity of the line grating. This interactive tutorial explores the mechanics of periodic diffraction gratings when used to interpret the Abbe theory of image formation in the optical microscope.

Airy Pattern Formation - When an image is formed in the focused image plane of an optical microscope, every point in the specimen is represented by an Airy diffraction pattern having a finite spread. This occurs because light waves emitted from a point source are not focused into an infinitely small point by the objective, but converge together and interfere near the intermediate image plane to produce a three-dimensional Fraunhofer diffraction pattern. This interactive tutorial explores the origin of Airy diffraction patterns formed by the rear aperture of the microscope objective and observed at the intermediate image plane.

Airy Pattern Basics - The three-dimensional diffraction pattern formed by a circular aperture near the focal point in a well-corrected microscope is symmetrically periodic along the axis of the microscope as well as radially around the axis. When this diffraction pattern is sectioned in the focal plane, it is observed as the classical two-dimensional diffraction spectrum known as the Airy pattern. This tutorial explores how Airy pattern size changes with objective numerical aperture and the wavelength of illumination; it also simulates the close approach of two Airy patterns.

Numerical Aperture and Image Resolution - The Airy pattern formed at the microscope intermediate image plane is a three-dimensional diffraction image, which is symmetrically periodic both along the optical axis of the microscope, and radially across the image plane. This diffraction pattern can be sectioned in the focal plane to produce a two-dimensional diffraction pattern having a bright circular disk surrounded by an alternating series of bright and dark higher-order diffraction rings whose intensity decreases as they become further removed from the central disk. Usually only two or three of the circular luminous rings are visible in the microscope (this number is dependent upon the objective numerical aperture), because the higher orders are absorbed by stray light and are not visible.

Conoscopic Images of Periodic Gratings - The purpose of this tutorial is to explore the reciprocal relationship between line spacings in a periodic grid (simulating a specimen) and the separation of the conoscopic image at the objective aperture plane. When the line grating has broad periodic spacings, several images of the condenser iris aperture appear in the objective rear focal plane. If white light is used to illuminate the line grating, higher order diffracted images of the aperture appear with a blue fringe closer to the zeroth order (central) image and with a green-yellow-red spectrum appearing further out towards the objective aperture periphery.

Spatial Frequency and Image Resolution - When a line grating is imaged in the microscope, a series of conoscopic images representing the condenser iris opening can be seen at the objective rear focal plane. This tutorial explores the relationship between the distance separating these iris opening images and the periodic spacing (spatial frequency) of lines in the grating.

Airy Patterns and the Rayleigh Criterion - Airy diffraction pattern sizes and their corresponding radial intensity distribution functions are sensitive to both objective numerical aperture and the wavelength of illuminating light. For a well-corrected objective with a uniform circular aperture, two adjacent points are just resolved when the centers of their Airy patterns are separated by a distance r. This tutorial examines how Airy disk sizes, at the limit of optical resolution, vary with changes in objective numerical aperture and illumination wavelength and how these changes affect the resolution of the objective.

Periodic Diffraction Images - When a microscope objective forms a diffraction-limited image of an object, it produces a three-dimensional diffraction pattern that is periodic both along the optical axis and laterally within the intermediate image plane. This tutorial explores diffraction images produced by a periodic object at several focal depths.

Electromagnetic Radiation - This interactive tutorial explores the classical representation of an electromagnetic wave as a sine function, and enables the visitor to vary amplitude and wavelength to demonstrate how this function appears in three dimensions. Whether taking the form of a signal transmitted to a radio from the broadcast station, heat radiating from a fireplace, the dentist's X-rays producing images of teeth, or the visible and ultraviolet light emanating from the sun, the various categories of electromagnetic radiation all share identical and fundamental wave-like properties.

Basic Electromagnetic Wave Properties - Electromagnetic radiation is characterized by a broad range of wavelengths and frequencies, each associated with a specific intensity (or amplitude) and quantity of energy. This interactive tutorial explores the relationship between frequency, wavelength, and energy, and enables the visitor to adjust the intensity of the radiation and to set the wave into motion.

Electromagnetic Wave Propagation - Electromagnetic waves can be generated by a variety of methods, such as a discharging spark or by an oscillating molecular dipole. Visible light is a commonly studied form of electromagnetic radiation, and exhibits oscillating electric and magnetic fields whose amplitudes and directions are represented by vectors that undulate in phase as sinusoidal waves in two mutually perpendicular (orthogonal) planes. This tutorial explores propagation of a virtual electromagnetic wave and considers the orientation of the magnetic and electric field vectors.

Electron Excitation and Emission - Electrons can absorb energy from external sources, such as lasers, arc-discharge lamps, and tungsten-halogen bulbs, and be promoted to higher energy levels. This tutorial explores how photon energy is absorbed by an electron to elevate it into a higher energy level and how the energy can subsequently be released, in the form of a lower energy photon, when the electron falls back to the original ground state.

Jablonski Diagram - Fluorescence activity can be schematically illustrated with the classical Jablonski diagram, first proposed by Professor Alexander Jablonski in 1935 to describe absorption and emission of light. This tutorial explores how electrons in fluorophores are excited from the ground state into higher electronic energy states and the events that occur as these excited molecules emit photons and fall back into lower energy states.

Tuning a Radio Wave Receiver - Variable capacitors are used in conjunction with inductor coils in tuning circuits of radios, television sets, and a number of other devices that must isolate electromagnetic radiation of selected frequencies in the radio wave region. This interactive tutorial explores how a variable capacitor is coupled to a simple antenna transformer circuit to tune a radiofrequency spectrum.

Wave Interactions in Optical Interference - The classical method of describing interference includes presentations that depict the graphical recombination of two or more sinusoidal light waves in a plot of amplitude, wavelength, and relative phase displacement. In effect, when two waves are added together, the resulting wave has an amplitude value that is either increased through constructive interference, or diminished through destructive interference. This interactive tutorial illustrates the effect by considering a pair of light waves from the same source that are traveling together in parallel, but can be adjusted with respect to coherency (phase relationship), amplitude, and wavelength.

Interference Phenomena in Soap Bubbles - Most of us observe some type of optical interference almost every day, but usually do not realize the events in play behind the often-kaleidoscopic display of color produced when light waves interfere with each other. One of the best examples of interference is demonstrated by the light reflected from a film of oil floating on water. Another example is the thin film of a soap bubble, which reflects a spectrum of beautiful colors when illuminated by natural or artificial light sources. This interactive tutorial explores how the interference phenomenon of light reflected by a soap bubble changes as a function of film thickness.

Thomas Young's Double Slit Experiment - In 1801, an English physicist named Thomas Young performed an experiment that strongly inferred the wave-like nature of light. Because he believed that light was composed of waves, Young reasoned that some type of interaction would occur when two light waves met. This interactive tutorial explores how coherent light waves interact when passed through two closely spaced slits.

Interference Between Parallel Light Waves - If the vibrations produced by the electric field vectors (which are perpendicular to the propagation direction) from waves that are parallel to each other (in effect, the vectors vibrate in the same plane), then the light waves may combine and undergo interference. If the vectors do not lie in the same plane, and are vibrating at some angle between 90 and 180 degrees with respect to each other, then the waves cannot interfere with one another. Analogous to the wave tutorial linked above, this interactive tutorial illustrates the effect by considering a pair of light waves from the same source that are coherent (having an identical phase relationship) and traveling together in parallel.

Complex Waveforms and Beat Frequencies in Superposed Waves - In general, the process of describing interference through the superposition of sine waves generates simple waveforms that can be adequately represented by a resultant sine curve in a plot of amplitude, wavelength, and relative phase displacement. If the recombined waves have appreciably different frequencies, the resulting waveform is often complex, yielding a contour that is no longer a sine function with a simple, single harmonic. This interactive tutorial explores the complex waveforms and beat frequencies generated by the superposition of two light waves propagating in the same direction with different relative frequencies, amplitudes, and phases.

Laser Cavity Resonance Modes and Gain Bandwidth - In a typical laser, the number of cavity resonances that can fit within the gain bandwidth is often plotted as a function of laser output power versus wavelength. This interactive tutorial explores how varying the appropriate frequencies can alter curves describing the number of cavity modes and gain bandwidth of a laser.

Laser Energy Levels - A population inversion can be produced through two basic mechanisms, either by creating an excess of atoms or molecules in a higher energy state, or by reducing the population of a lower energy state. This tutorial explores metastable states for both three-level and four-level laser systems.

Spontaneous and Stimulated Processes - One of the most important concepts necessary in understanding laser operation is the fact that quantization of energy in the atom results in discrete energy levels. In addition, transitions from one energy level to another must be possible in order for light emission to occur, and these transitions include both spontaneous and stimulated emission. This tutorial explores the concepts of spontaneous absorption and emission, as well as stimulated emission.

Stimulated Emission in a Laser Cavity - The amplification of light by stimulated emission is a fundamental concept in the basic understanding of laser action. This interactive tutorial explores how laser amplification occurs starting from spontaneous emission of the first photon to saturation of the laser cavity and the establishment of a formal equilibrium state.

Argon-Ion Lasers - As a distinguished member of the common and well-explored family of ion lasers, the argon-ion laser operates in the visible and ultraviolet spectral regions by utilizing an ionized species of the noble gas argon. Argon-ion lasers function in continuous wave mode when plasma electrons within the gaseous discharge collide with the excited laser species to produce light.

Diode Lasers - Semiconductor diode lasers having sufficient power output to be useful in optical microscopy are now available from a host of manufacturers. In general these devices operate in the infrared region, but newer diode lasers operating at specific visible wavelengths are now available. Diode lasers coupled to internal optical systems that improve beam shape have sufficient power and stability to rival helium-neon lasers in many applications. This interactive tutorial explores the properties of typical diode lasers and how specialized anamorphic prisms can be utilized for beam expansion.

Ti:Sapphire Mode-Locked Lasers - The self mode-locked Ti:sapphire pulsed laser is currently one of the preferred laser excitation sources in a majority of multiphoton fluorescence microscopy investigations. This tutorial explores the operation of Ti:sapphire lasers over a broad range of near-infrared wavelengths with variable pulse widths and an adjustable applet speed control.

Nd:YLF Mode-Locked Pulsed Lasers - An increasing number of applications, including new illumination techniques in fluorescence optical microscopy, require a reliable high average-power laser source that enables efficient frequency conversion to ultra violet and visible wavelengths. Several variants of the diode-pumped solid state laser have been developed, and of these, the Nd:YLF (neodymium: yttrium lithium fluoride) laser produces the highest pulse energy and average power in the repetition rate ranging from a single pulse up to approximately 6 kHz. This tutorial explores the operation of a Nd:YLF multi-pass slab laser side-pumped by two collimated diode-laser bars.

Compact Disk Lasers - A pre-recorded compact disk is read by tracking a finely focused laser across the spiral pattern of lands and pits stamped into the disk by a master diskette. This tutorial explores how the laser beam is focused onto the surface of a spinning compact disk, and how variations between the height of pits and lands determine whether the light is scattered by the disk surface or reflected back into a detector.

Simple Bi-Convex Thin Lenses - A simple thin lens has two focal planes that are defined by the geometry of the lens and the relationship between the lens and the focused image. Light rays passing through the lens will intersect and are physically combined at the focal plane, while extensions of the rays passing through the lens will intersect with the rays emerging from the lens at the principal plane. The focal length of a lens is defined as the distance between the principal plane and the focal plane, and every lens has a set of these planes on each side (front and rear). This interactive tutorial explores how changes to focal length and object size affect the size and position of the image formed by a simple thin lens.

Simple Magnification - A typical magnifying glass consists of a single thin bi-convex lens that produces a modest magnification in the range of 1.5x to 30x, with the most common being about 2-4x for reading or studying rocks, stamps, coins, insects, and leaves. Magnifying glasses produce a virtual image that is magnified and upright. This interactive tutorial demonstrates how a simple, thin bi-convex magnifying lens works to produce a magnified virtual image on the retina.

Magnification with a Bi-Convex Lens - Single lenses capable of forming images (like the bi-convex lens) are useful in tools designed for simple magnification applications, such as magnifying glasses, eyeglasses, single-lens cameras, loupes, viewfinders, and contact lenses. This interactive tutorial explores how a simple bi-convex lens can be used to magnify an image.

Image Formation with Converging Lenses - Positive, or converging, thin lenses unite incident light rays that are parallel to the optical axis and focus them at the focal plane to form a real image. This interactive tutorial utilizes ray traces to explore how images are formed by the three primary types of converging lenses, and the relationship between the object and the image formed by the lens as a function of distance between the object and the focal points.

Image Formation with Diverging Lenses - Negative lenses diverge parallel incident light rays and form a virtual image by extending traces of the light rays passing through the lens to a focal point behind the lens. In general, these lenses have at least one concave surface and are thinner in the center than at the edges. This interactive tutorial utilizes ray traces to explore how images are formed by the three primary types of diverging lenses, and the relationship between the object and the image formed by the lens as a function of distance between the object and the focal points.

Geometrical Construction of Ray Diagrams - A popular method of representing a train of propagating light waves involves the application of geometrical optics to determine the size and location of images formed by a lens or multi-lens system. This tutorial explores how two representative light rays can establish the parameters of an imaging scenario.

Perfect Lens Characteristics - The simplest imaging element in an optical microscope is a perfect lens, which is an ideally corrected glass element that is free of aberration and focuses light onto a single point. This tutorial explores how light waves propagate through and are focused by a perfect lens.

Perfect Two-Lens System Characteristics - During investigations of a point source of light that does not lie in the focal plane of a lens, it is often convenient to represent a perfect lens as a system composed of two individual lens elements. This tutorial explores off-axis oblique light rays passing through such a system.

Radius and Refractive Index Effects on Lens Action - The action of a simple bi-convex thin lens is governed by the principles of refraction (which is a function of lens curvature radius and refractive index), and can be understood with the aid of a few simple rules about the geometry involved in tracing light rays through the lens. This interactive tutorial explores how variations in the refractive index and radius of a bi-convex lens affect the relationship between the object and the image produced by the lens.

Photosynthesis - Green plants absorb water and carbon dioxide from the environment, and utilizing energy from the sun, turn these simple substances into glucose and oxygen. With glucose as a basic building block, plants synthesize a number of complex carbon-based biochemicals used to grow and sustain life. This process is termed photosynthesis, and is the cornerstone of life on Earth. The tutorial demonstrates the basic molecular steps in the photosynthetic process.

Solar Cell Operation - Solar cells convert light energy into electrical energy either indirectly by first converting it into heat, or through a direct process known as the photovoltaic effect. The most common types of solar cells are based on the photovoltaic effect, which occurs when light falling on a two-layer semiconductor material produces a potential difference, or voltage, between the two layers. The voltage produced in the cell is capable of driving a current through an external electrical circuit that can be utilized to power electrical devices. This tutorial explores the basic concepts behind solar cell operation.

Hydrogen Fuel Cell Basics - Fuel cells are designed to utilize a catalyst, such as platinum, to convert a mixture of hydrogen and oxygen into water. An important byproduct of this chemical reaction is the electricity generated when hydrogen molecules interact (through oxidation) with the anode to produce protons and electrons. This interactive tutorial explores the major steps in fuel cell operation.

Interaction of Photons with Silicon - In a charge-coupled device (CCD) incident light must first pass through a silicon nitride passivation coating as well as several thin films of silicon dioxide and polysilicon gate structures before being absorbed into the silicon substrate. This interactive tutorial explores the interaction of photons with silicon as a function of wavelength.

Building A Charge-Coupled Device - Explore the steps utilized in the construction of a charge-coupled device (CCD) as a portion of an individual pixel gate is fabricated on a silicon wafer simultaneously with thousands or even millions of neighboring elements. The interactive tutorial examines and illustrates each individual stage in the fabrication of the CCD photodiode sensor element.

Full-Frame CCD Operation - Full-frame charge-coupled devices (CCDs) feature high-density pixel arrays capable of producing digital images with the highest resolution currently available. This popular CCD architecture has been widely adopted due to the simple design, reliability, and ease of fabrication.

Photomultiplier Tube Operation - In the end-on photomultiplier tube design, photons impact an internal photocathode and transfer their energy to electrons, which then proceed through a chain of electron multipliers termed dynodes ending in the anode. This tutorial explores how electrons are generated by the photocathode and amplified by passing through a dynode chain.

Absorption Filters - Absorption filters, commonly manufactured from dyed glass or pigmented gelatin resins, are one of the most widely used types of filter for brightfield and fluorescence microscopy. These filters operate by attenuation of light through absorption of specific wavelengths, so that spectral performance is a function of the physical thickness of the filter and the amount of dye present in the glass or gelatin matrix. This interactive tutorial explores how absorption filters pass certain wavelengths of light while blocking others.

Interference Filters - Recent technological achievements in bandpass filter design have led to the relatively inexpensive construction of thin-film interference filters featuring major improvements in wavelength selection and transmission performance. These filters operate by transmitting a selected wavelength region with high efficiency while rejecting, through reflection and destructive interference, all other wavelengths. Explore how interference filters operate by selectively transmitting constructively reinforced wavelengths while simultaneously eliminating unwanted light with this interactive tutorial.

Exposure and Color Balance in Photomicrography - Photography through the microscope is complicated by a wide spectrum of unexpected color shifts and changes that affect how the image is rendered on the film emulsion or digital electronic image capturing device. These unexpected imaging results are caused by a number of factors ranging from incorrect color balance between the light source and the film emulsion to optical artifacts such as aberration and lamp voltage fluctuations. This tutorial explores how exposure and color balance affect the qualities of color photographs using both everyday subjects and photomicrographs captured in the microscope.

Filters for Black & White Photomicrography - In black & white photography through the microscope, filters are used primarily to control contrast in the final image captured either on film or with a CCD digital camera system. Specimens that are highly differentiated with respect to colored elements from biological stains are translated into shades of gray on black & white film and will often appear to have equal brightness. When this occurs, important specimen details may be lost through a lack of contrast. Use this interactive Java tutorial to determine the appropriate starting point for contrast control in Black & White Photomicrography using Kodak Wratten filters.

Filter Control of Image Contrast in Black & White Photomicrography - As a general rule when employing color filters in black & white photomicrography, utilize filters that are complementary to specimen stain color (they absorb most of the predominant wavelengths transmitted by the stain) to maximize the amount of contrast in final images. To achieve a medium level of contrast, use filters that only partially absorb colors displayed by features of interest. Finally, to reduce contrast to a minimum, use filters that have colors identical to those of the specimen. A combination of filters can be used to enhance detail contrast in specimens stained with more than one color. This tutorial explores the use of Kodak Wratten color filters for contrast control in black & white photomicrography when using stained specimens.

Didymium Filters for Color Photomicrography - The intensities and hues generated by most biological tissue stains will reproduce very well on color film, but some stains tend to appear washed out or have their colors shifted, especially in multiple stain mixtures. In many instances, color compensating filters can help restore most or all of the lost color, but too much filtration can affect the color of neighboring stained features and the background. This problem occurs with the popular stains Eosin, Fuchsin, and Methylene Blue, which are not reproduced very well on most color films. Often, tissues stained with these dyes, either singly or in mixtures, appear muddy and lacking in color saturation. Explore, in this interactive tutorial, how didymium filters enhance intensities and hues of specimens stained with a variety of dyes for color photomicrography.

Liquid Crystal Tunable Filters - Liquid crystal tunable filters (LCTFs) use electrically controlled liquid crystal elements to select a specific visible wavelength of light for transmission through the filter at the exclusion of all others. This type of filter is ideal for use with electronic imaging devices, such as charge-coupled devices (CCDs), because it offers excellent imaging quality with a simple linear optical pathway.

Particle and Wave Reflection - One point of view envisions light as wave-like in nature, producing energy that traverses through space in a manner similar to the ripples spreading across the surface of a still pond after being disturbed by a dropped rock. The opposing view holds that light is composed of a steady stream of particles, much like tiny droplets of water sprayed from a garden hose nozzle. This interactive tutorial explores how particles and waves behave when reflected from a smooth surface.

Particle and Wave Refraction - When a beam of light travels between two media having different refractive indices, the beam undergoes refraction, and changes direction when it passes from the first medium into the second. According to the wave theory, a small portion of each angled wavefront should impact the second medium before the rest of the front reaches the interface. This portion will start to move through the second medium while the rest of the wave is still traveling in the first medium, but will move more slowly due to the higher refractive index of the second medium. Because the wavefront is now traveling at two different speeds, it will bend into the second medium, thus changing the angle of propagation. In contrast, particle theory has a rather difficult time explaining why particles of light should change direction when they pass from one medium into another.

Particle and Wave Diffraction - Particles and waves should behave differently when they encounter the edge of an object and form a shadow. Newton was quick to point out in his 1704 book Opticks, that "Light is never known to follow crooked passages nor to bend into the shadow". This concept is consistent with the particle theory, which proposes that light particles must always travel in straight lines. If the particles encounter the edge of a barrier, then they will cast a shadow because the particles not blocked by the barrier continue on in a straight line and cannot spread out behind the edge. On a macroscopic scale, this observation is almost correct, but it does not agree with the results obtained from light diffraction experiments on a much smaller scale.

Astigmatism - Astigmatism aberrations are similar to comatic aberrations, however these artifacts are not as sensitive to aperture size and depend more strongly on the oblique angle of the light beam. The aberration is manifested by the off-axis image of a specimen point appearing as a line or ellipse instead of a point. Depending on the angle of the off-axis rays entering the lens, the line image may be oriented in either of two different directions, tangentially (meridionally) or sagittally (equatorially). The intensity ratio of the unit image will diminish, with definition, detail, and contrast being lost as the distance from the center is increased.

Chromatic Aberration - Chromatic aberrations are wavelength-dependent artifacts that occur because the refractive index of every optical glass formulation varies with wavelength. When white light passes through a simple or complex lens system, the component wavelengths are refracted according to their frequency. In most glasses, the refractive index is greater for shorter (blue) wavelengths and changes at a more rapid rate as the wavelength is decreased.

Comatic Aberration - Comatic aberrations are similar to spherical aberrations, but they are mainly encountered with off-axis light fluxes and are most severe when the microscope is out of alignment. When these aberrations occur, the image of a point is focused at sequentially differing heights producing a series of asymmetrical spot shapes of increasing size that result in a comet-like (hence, the term coma) shape to the Airy pattern.

Curvature of Field - Modern microscopes deal with field curvature by correcting this aberration using specially designed objectives. These specially-corrected objectives have been named plan or plano (for flat-field) and are the most common type of objective in use today, providing ocular fields ranging between 18 and 26 millimeters, which exhibit sharp detail from center to edge.

Geometrical Distortion - Distortion is an aberration commonly seen in stereoscopic microscopy, which is manifested by changes in the shape of an image rather than the sharpness or color spectrum. The two most prevalent types of distortion, positive and negative (often termed pincushion and barrel, respectively), can often be present in very sharp images that are otherwise corrected for spherical, chromatic, comatic, and astigmatic aberrations. In this case, the true geometry of an object is no longer maintained in the image.

Spherical Aberration - The most serious of the monochromatic defects that occurs with microscope objectives, spherical aberration, causes the specimen image to appear hazy or blurred and slightly out of focus. The effect of spherical aberration manifests itself in two ways: the center remains more in focus than the edges of the image and the intensity of the edges falls relative to that of the center. This defect appears in both on-axis and off-axis image points.

Brewster's Angle - Light that is reflected from the flat surface of a dielectric (or insulating) material is often partially polarized, with the electric vectors of the reflected light vibrating in a plane that is parallel to the surface of the material. Common examples of surfaces that reflect polarized light are undisturbed water, glass, sheet plastics, and highways. In these instances, light waves that have the electric field vectors parallel to the surface are reflected to a greater degree than those with different orientations. This tutorial demonstrates the polarization effect on light reflected at a specific angle (the Brewster angle) from a transparent medium.

Polarized Light Waveforms - The ordinary and extraordinary light waves generated when a beam of light traverses a birefringent crystal have plane-polarized electric vectors that are mutually perpendicular to each other. In addition, due to differences in electronic interaction that each component experiences during its journey through the crystal, there is usually a phase shift that occurs between the two waves. This interactive tutorial explores the generation of linear, elliptical, and circularly polarized light by a pair of orthogonal light waves (as a function of the relative phase shift between the waves) when the electric field vectors are added together.

Polarization of Light - When light travels through a linear polarizing material, a selected vibration plane is passed by the polarizer, while electric field vectors vibrating in all other orientations are blocked. Linearly polarized light transmitted through a polarizer can be either passed or absorbed by a second polarizer, depending upon the electric vector transmission azimuth orientation of the second polarizing material. This tutorial explores the effect of rotating two polarizers on an incident beam of white light.

Polarization of Light (3-D Version) - When non-polarized white light encounters a linear polarizer that is oriented with the transmission azimuth positioned vertically to the incident beam, only those waves having vertical electric field vectors will pass through. Polarized light exiting the first polarizer can be subsequently blocked by a second polarizer if the transmission axis is oriented horizontally with respect to the electric field vector of the polarized light waves. The concept of using two polarizers oriented at right angles with respect to each other is commonly termed crossed polarization and is fundamental to the concept of polarized light microscopy. This tutorial explores the effects of two polarizers having adjustable transmission axes on an incident beam of white light, and enables the visitor to translate the optical train in three dimensions.

Nicol Prisms - Several versions of prism-based polarizing devices were once widely available, and these were usually named after their designers. The most common polarizing prism (illustrated in the tutorial window) was named after William Nicol, who first cleaved and cemented together two crystals of Iceland spar with Canada balsam in 1829. Nicol prisms were first used to measure the polarization angle of birefringent compounds, leading to new developments in the understanding of interaction between polarized light and crystalline substances. This interactive tutorial explores the generation of orthogonal or mutually perpendicular (ordinary and extraordinary) waves as the result of light transmission through a Nicol prism.

Polarized Light Virtual Microscopes - When a birefringent material is placed between crossed polarizers in an optical microscope, light incident upon the material is split into two component beams whose amplitude and intensity vary depending upon the orientation angle between the polarizer and permitted vibration directions of the material. Use this link to explore our tutorials on polarized light microscopy.

Primary Additive Colors - Light is perceived as white by humans when all three cone cell types are simultaneously stimulated by equal amounts of red, green, and blue light. Because the addition of these three colors yields white light, the colors red, green, and blue are termed the primary additive colors. This tutorial explores how the three primary additive colors interact with each other, both in pairs or all together.

Primary Subtractive Colors - The complementary colors (cyan, yellow, and magenta) are also commonly referred to as the primary subtractive colors because each can be formed by subtracting one of the primary additives (red, green, and blue) from white light. This tutorial explores how the three primary subtractive colors interact with each other, both in pairs or all together.

Color Filters - Examine how color filters operate to change the color of objects visualized under filtered illumination. The tutorial enables visitors to drag and drop red, green, and blue virtual color filters over objects illuminated both with white light and also previously filtered with one of the primary additive colors.

Color Separation - Pigments and dyes are responsible for most of the color that humans see in the real world. Books, magazines, signs, and billboards are printed with colored inks that create colors through the process of color subtraction. This interactive tutorial explores how individual subtractive primary colors can be separated from a full-color photograph, and then how they can be reassembled to create the original scene.

Common Reflecting Prisms - The angular parameters displayed by various prism designs cover a wide gamut of geometries that dramatically extend the usefulness of prisms as strategic optical components. Reflecting prisms are often designed to be located in specific orientations where the entrance and exit faces are both parallel and perpendicular to the optical axis. This interactive tutorial explores image deviation, rotation, and displacement exhibited by common reflecting prisms.

Right-Angle Prisms - The right-angle prism possesses the simple geometry of a 45-degree right triangle, and is one of the most commonly used prisms for redirecting light and rotating images. This interactive tutorial explores light reflection and image rotation, inversion, and reversion by a right-angle prism as a function of the prism orientation with respect to incident light.

Refraction by an Equilateral Prism - Visible white light passing through an equilateral prism undergoes a phenomenon known as dispersion, which is manifested by wavelength-dependent refraction of the light waves. This interactive tutorial explores how the incident angle of white light entering the prism affects the degree of dispersion and the angles of light exiting the prism.

Transmission and Reflection by Beamsplitters - A beamsplitter is a common optical component that partially transmits and partially reflects an incident light beam, usually in unequal proportions. In addition to the task of dividing light, beamsplitters can be employed to recombine two separate light beams or images into a single path. This interactive tutorial explores transmission and reflection of a light beam by three common beamsplitter designs.

Dielectric Plate Beamsplitters - The simplest configuration for a beamsplitter is an uncoated flat glass plate (such as a microscope slide), which has an average surface reflectance of about 4 percent. When placed at a 45-degree angle, the plate will transmit most of the light, but reflect a small amount at a 90-degree angle to the incident beam. Plate beamsplitters are, as the name implies, optical crown glass plates having a partially silvered coating designed to produce a desired transmission-to-reflection ratio. These ratios usually vary between 50:50 and 20:80, depending upon the application.

Beam Steering by Wedge Prisms - Circular prisms having plane surfaces positioned at slight angles with respect to each other are termed optical wedges, and deflect light by refraction rather than reflection. Although wedges are prismatic in nature, they can be manipulated to act as beamsplitters or beam steerers. This interactive tutorial explores how two wedge prisms operate together to deflect an incident light beam.

Birefringent Polarizing Prisms - Polarizing prisms are utilized in a wide spectrum of applications ranging from optical microscopy and spectroscopy to complex laser systems. This interactive tutorial explores how various common birefringent polarizing prisms operate to split light waves into ordinary and extraordinary components.

Reflection of Light - Reflection of light (and other forms of electromagnetic radiation) occurs when the waves encounter a surface or other boundary that does not absorb the energy of the radiation and bounces the waves away from the surface. This tutorial explores the incident and reflected angles of a single light wave impacting on a smooth surface.

Specular and Diffuse Reflection - The amount of light reflected by an object, and how it is reflected, is very dependent upon the smoothness or texture of the surface. When surface imperfections are smaller than the wavelength of the incident light (as in the case of a mirror), virtually all of the light is reflected equally. However, in the real world most objects have convoluted surfaces that exhibit a diffuse reflection, with the incident light being reflected in all directions. This interactive tutorial explores how light waves are reflected by smooth and rough surfaces.

Reflection and Refraction with Huygens Wavelets - Near the beginning of the eighteenth century, Dutch physicist Christiaan Huygens proposed that each point in a wave of light can be thought of as an individual source of illumination that produces its own spherical wavelets, which all add together to form an advancing wavefront. This interactive Java tutorial is designed to illustrate the reflection and refraction of light according to the multiple wavelet concept, now known as the Huygens' principle.

Antireflection Surface Coatings - The concept behind antireflection technology is to control the light used in an optical device in such a way that the light rays reflect from surfaces where it is intended and beneficial, and do not reflect from surfaces where this would have a deleterious effect on the image being observed. One of the most significant advances made in modern lens design, whether for microscopes, cameras, or other optical devices, is the significant improvement in antireflection coating technology. This tutorial explores various coatings and their reflectivities as a function of incident angle.

Refraction of Light - Refraction occurs as light passes from one medium to another only when there is a difference in the index of refraction between the two materials. The effects of refraction are responsible for a variety of familiar phenomena, such as the apparent bending of an object that is partially submerged in water and the mirages observed on a dry, sandy desert. The refraction of visible light is also an important characteristic of lenses that enables them to focus a beam of light onto a single point. This interactive tutorial explores how changes to the incident angle and refractive index differential between two dissimilar media affect the refraction angle of light at the interface.

Observing Objects in Water - An object seen in the water will usually appear to be at a different depth than it actually is, due to the refraction of light rays as they travel from the water into the air. This tutorial explores how fish, observed from the bank of a pond or lake, appear to be closer to the surface than they really are.

The Critical Angle of Reflection - An important concept in optical microscopy is the critical angle of reflection, which is a necessary factor to consider when choosing whether to use dry or oil immersion objectives to view a specimen at high magnification. Upon passing through a medium of higher refractive index into a medium of lower refractive index, the path taken by light waves is determined by the incident angle with respect to the boundary between the two media. This interactive tutorial explores the transition from refraction to total internal reflection as the angle of the incident wave is increased at constant refractive index.

Refraction of Monochromatic Light - Refraction occurs as light passes from one medium to another only when there is a difference in the index of refraction between the two materials. The effects of refraction are responsible for a variety of familiar phenomena, such as the apparent bending of an object that is partially submerged in water and the mirages observed on a dry, sandy desert. The refraction of visible light is also an important characteristic of lenses that enables them to focus a beam of light onto a single point. This interactive tutorial explores how changes to the incident angle and refractive index differential between two dissimilar media affect the refraction angle of monochromatic light at the interface.

Lightning: A Natural Capacitor - Lightning is one of the naturally occurring mechanisms that provided early mankind with the ability to understand and harness fire. This meteorological phenomenon occurs when water-filled clouds and the ground act in unison to mimic a huge natural capacitor. View the build-up of static electrical charges between storm clouds and the wet ground during a thunderstorm with this tutorial, which simulates capacitor-like lightning discharges.

Incandescent Lamp Filaments - Nearly every source of light depends, at the fundamental level, on the release of energy from atoms that have been excited in some manner. Standard incandescent lamps, derived directly from the early models of the 1800s, now commonly utilize a tungsten filament in an inert gas atmosphere, and produce light through the resistive effect that occurs when the filament temperature increases as electrical current is passed through. This interactive tutorial demonstrates the sub-atomic activity within a conducting incandescent lamp filament that results in resistance to current flow, and ultimately leads to the emission of infrared and visible light photons (with a corresponding rise in color temperature).

Light Emitting Diodes - Light emitting diodes (LEDs) are a general source of continuous light with a high luminescence efficiency, and are based on the general properties of a simple twin-element semiconductor diode encased in a clear epoxy dome that acts as a lens. This interactive tutorial explores how two dissimilar doped semiconductors can produce light when a voltage is applied to the junction region between the materials.

Speed of Light in Transparent Materials - When light traveling in a vacuum enters a new transparent medium, such as air, water, or glass, the speed is reduced in proportion to the refractive index of the new material. This interactive tutorial explores the reduction in the speed of light as a function of refractive index in common substances.

Moiré Patterns - Moiré fringes arise from interference patterns that are generated when two similar grids overlap each other. The result is a series of fringe patterns that change shape as the grids are translated with respect to one another. One of the more common occurrences of moiré patterns is found in computer monitors and television sets where we see an ordered wavy pattern superimposed over the screen in a series of ripples. Moiré patterns also are a common problem in scanned images due to optical interference between the printed dot pattern and the reflection of the image.

Java Development Laboratory - This tutorial allows students to visit our Java development laboratory at the National High Magnetic Field Laboratory where these tutorial-applets are created.

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