The formation of an image in the microscope relies on a complex interplay between two critical optical phenomena: diffraction and interference. Light passing through the specimen is scattered and diffracted into divergent waves by tiny details and features present in the specimen. Some of the divergent light scattered by the specimen is captured by the objective and focused onto the intermediate image plane, where the superimposed light waves are recombined or summed through the process of interference to produce a magnified image of the specimen.

Fundamentals of Interference - The seemingly close relationship between diffraction and interference occurs because they are actually manifestations of the same physical process and produce ostensibly reciprocal effects. 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.

Augustin-Jean Fresnel (1788-1827) - Augustin-Jean Fresnel, was a nineteenth century French physicist, who is best known for the invention of unique compound lenses designed to produce parallel beams of light, which are still used widely in lighthouses. In the field of optics, Fresnel derived formulas to explain reflection, diffraction, interference, refraction, double refraction, and the polarization of light reflected from a transparent substance.

Christiaan Huygens (1629-1695) - Christiaan Huygens was a brilliant Dutch mathematician, physicist, and astronomer who lived during the seventeenth century, a period sometimes referred to as the Scientific Revolution. Huygens, a particularly gifted scientist, is best known for his work on the theories of centrifugal force, the wave theory of light, and the pendulum clock. His theories neatly explained the laws of refraction, diffraction, interference, and reflection, and Huygens went on to make major advances in the theories concerning the phenomena of double refraction (birefringence) and polarization of light.

Samuel Tolansky (1907-1973) - Born in Newcastle upon Tyne, England as Samuel Turlausky, Tolansky performed a significant amount of his research and developed the interference contrast microscopy technique that bears his name. Other research interests of Tolansky included the analysis of spectra to investigate nuclear spin and the study of optical illusions. Although he was primarily concerned with the spectrum of mercury, during World War II Tolansky was asked to ascertain the spin of uranium-235, the isotope capable of fission in a nuclear chain reaction.

Thomas Young (1773-1829) - Thomas Young was an English physician and a physicist who was responsible for many important theories and discoveries in optics and in human anatomy. His best known work is the wave theory of interference. Young was also responsible for postulating how the receptors in the eye perceive colors. He is credited, along with Hermann Ludwig Ferdinand von Helmholtz, for developing the Young-Helmholtz trichromatic theory.

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 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.

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.

Reference Listing - Leading investigators in the fields of optics and photonics have published a number of high-quality review articles on a variety of interference phenomena. This section contains periodical and book location information about these articles, as well as providing a listing of the chapter titles for appropriate sections dealing with the interference between wavefronts and related effects.

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