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Light and Color

Light is a complex phenomenon that is classically explained with a simple model based on rays and wavefronts. The light and color section of Molecular Expressions Science, Optics & You explores many aspects of visible light, beginning with an introduction to electromagnetic radiation and continuing through to the perception of color and the characteristics of lenses. Each section outlined below is an independent treatise on a limited aspect of light and color.

Electromagnetic Radiation - The light that can be perceived by the naked eye is only a small part of a larger family of wave-like energy known as electromagnetic radiation. The term is derived from the characteristic electric and magnetic properties common to all forms of electromagnetic radiation, which includes everything from high frequency gamma rays through X-rays, ultraviolet light, infrared radiation and microwaves to very low frequency radio waves.

Frequency and Wavelength of Light - Since the energy of photons depends upon the energy of source electrons, very high-frequency electromagnetic radiation, such as gamma rays, X-rays, and ultraviolet light, possess very short wavelengths and, therefore, a great deal of energy. On the other hand, lower frequency radiation, such as visible, infrared, microwave, and radio waves, have greater wavelengths, but correspondingly lower frequencies and energy.

Sources of Visible Light - During day-to-day living, most people encounter only a small number of visible light sources. When venturing outside, for instance, the vast majority of the light that can be seen is emitted from the sun, which also emits many other frequencies of radiation that do not fall in the visible range. Inside, however, visible light primarily comes from artificial sources, most often fluorescent or tungsten devices.

Reflection of Light - Reflection of light and other forms of electromagnetic radiation occurs when waves encounter a boundary that does not absorb the radiation's energy, but instead bounces the waves off its surface. In such cases, the incoming light wave is referred to as an incident wave and the wave that is bounced from the surface is called the reflected wave.

Refraction of Light - Refraction, or bending of light, occurs as light passes from one medium into another medium with a different refractive index. Refraction is an important characteristic of lenses, allowing them to focus a beam of light onto a single point, and is also responsible for a variety of familiar phenomena, such as the apparent distortion of objects partially submerged in water.

Diffraction of Light - Classically, light is thought of as always traveling in straight lines, but in reality, light waves tend to bend around nearby barriers, spreading out in the process. This phenomenon is known as diffraction and occurs when a light wave passes by a corner or through an opening or slit that is the approximate size of, or even smaller than, that light's wavelength.

Polarization of Light - Natural sunlight and most forms of artificial illumination transmit light waves whose electric field vectors vibrate equally in all planes perpendicular to the direction of propagation. When their electric field vectors are restricted to a single plane by filtration, however, then the light is polarized with respect to the direction of propagation.

Interference - When a film of oil floating atop a body of water reflects light, a swirling mass of colors seems to magically appear. Yet, even though many people have seen such a sight, few realize that the cause of this strange phenomenon is interference between light waves. A simple soap bubble is another common example of interference, reflecting a variety of beautiful colors when illuminated by natural or artificial light sources.

Optical Birefringence - When light enters the optical axis of anisotropic crystals, it acts in a manner similar to its interaction with isotropic crystals, passing through at a single velocity. However, when light enters a non-equivalent axis, it is refracted into two rays, each polarized so that they travel at different velocities and that their vibration directions are oriented at right angles to one another. This phenomenon is termed "double" or "bi" refraction and is seen to a greater or lesser degree in all anisotropic crystals.

Color Temperature - The concept of color temperature is based on the relationship between the temperature of a theoretical standardized material, called a black body radiator, and the energy distribution of its emitted light as the radiator is brought to increasingly higher temperatures. It is especially important in the field of photography, where film emulsions must be carefully balanced to accurately render color using different light sources.

Primary Colors - The colors red, green, and blue are classically considered the primary colors because they are fundamental to human vision. All other colors of the visible light spectrum can be produced by properly adding different combinations of these three colors. Moreover, adding equal amounts of red, green, and blue light produces white light and, therefore, these colors are also often described as the primary additive colors.

Light Filtration - Most natural and artificial light sources emit a broad range of wavelengths that cover the entire visible light spectrum. However, it is often desirable to produce light that has a restricted wavelength spectrum. This can be easily accomplished through the use of specialized filters that transmit some wavelengths and selectively absorb or reflect unwanted wavelengths.

Human Vision and Color Perception - Human vision is a complex process that is not yet completely understood, despite hundreds of years of study and research. The complex physical process of visualizing something involves the nearly simultaneous interaction of the eyes and the brain through a network of neurons, receptors, and other specialized cells.

Introduction to Lenses - The term lens is applied to a piece of glass or transparent plastic, usually circular in shape, that has two surfaces that are ground and polished in a specific manner designed to produce either a convergence or divergence of light. The two most common types of lenses are concave and convex lenses.

Light and Color Java Tutorials - Difficult concepts in the physics of light and the science of optics are much easier to understand with the aid of interactive tutorials that demonstrate various aspects of the principles involved. These Java tutorial-applets explore a wide range of concepts in light, color, and optics.

Contributing Authors

Mortimer Abramowitz - Olympus America, Inc., Two Corporate Center Drive., Melville, New York, 11747.

Shannon H. Neaves, Brian O. Flynn, Kirill I. Tchourioukanov, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.


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