Light and Color

(Important Definitions)

Light is a complex phenomenon that is classically explained with a simple model based on rays and wavefronts. The Molecular Expressions Microscopy Primer explores many of the aspects of visible light starting with an introduction to electromagnetic radiation and continuing through to human vision and the perception of color.

Electromagnetic Radiation - Visible light represents only a small portion of the entire electromagnetic spectrum of radiation that extends from high-frequency gamma rays through X-rays, ultraviolet light, infrared radiation and microwaves to very low frequency long-wavelength radio waves.

Frequency and Wavelength of Light - This discussion centers on the relationship between the frequency (the number of oscillations per second) and the wavelength (the size of each oscillation) of light. Very high-frequency electromagnetic radiation such as gamma rays, X-rays, and ultraviolet light possess very short wavelengths and a great deal of energy. On the other hand, lower frequency radiation such as visible, infrared, microwave, and radio waves have correspondingly greater wavelengths with lower frequencies and energy.

Sources of Visible Light - The vast majority of the light we see is emitted from the sun, which also emits many other frequencies of radiation that do not fall in the visible range. When indoors, we are exposed to visible light that comes from "artificial" sources primarily originating from fluorescent and/or tungsten devices.

Refraction of Light - The refraction of visible light is an important characteristic of lenses that allows them to focus a beam of light onto a single point. Refraction (or bending of the light) occurs as light passes from a one medium to another when there is a difference in the index of refraction between the two materials.

Diffraction of Light - Diffraction of light occurs when a light wave passes by a corner or through an opening or slit that is physically the approximate size of, or even smaller than, that light's wavelength. Diffraction describes a specialized case of light scattering in which an object with regularly repeating features (such as a diffraction grating) produces an orderly diffraction of light in a diffraction pattern. In the real world most objects are very complex in shape and should be considered to be composed of many individual diffraction features that can collectively produce a random scattering of light.

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

Interference - An important characteristic of light waves is its ability, under certain circumstances, to interfere with one another. One of the best examples of interference is demonstrated by the light reflected from a film of oil floating on water or a soap bubble, which reflects a variety of beautiful colors when illuminated by natural or artificial light sources.

Optical Birefringence - Anisotropic crystals have crystallographically distinct axes and interact with light in a manner that is dependent upon the orientation of the crystalline lattice with respect to the incident light. When light enters a non-equivalent axis in an anisotropic crystal, it is refracted into two rays each polarized with the vibration directions oriented at right angles to one another, and traveling at different velocities. This phenomenon is termed double- or bi-refraction (or birefringence) 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 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. This model is useful in relating the emission spectrum of natural and artificial light sources to the emulsion characteristics of individual photographic films and electronic digital cameras.

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.