Although color is an inherent part of everything that we see, most individuals don't fully realize that visible light is the only source of color in our world. The most beautiful sunrise, picturesque mountain scene, or gaudiest clown costume are all simply reflections, transmissions, or absorptions of one or more of the colors that compose white light.

The interrelationship between the source of light, color, response of the human eye, and complex phenomena, such as refraction and diffraction, all play a role in how we visualize and understand our world. The discussions in this section are designed to introduce many of the basic concepts surrounding the physics of light and color, and how they combine to affect our everyday lives.

Electromagnetic Radiation - Electromagnetic radiation is the vehicle that carries energy through space, and covers the entire spectrum of wavelengths, ranging from high-energy gamma rays through ultraviolet and visible light, to the radio waves that provide music, news, and entertainment. Every form of electromagnetic radiation, including visible light, oscillates in a periodic fashion with peaks and valleys, and displays a characteristic amplitude, wavelength, and frequency that define the direction, energy, and intensity of the radiation.

Sources of Visible Light - Visible light comprises only a tiny portion of the entire electromagnetic spectrum of radiation, yet it contains the only region of light to which the rods and cones of the human eye will respond. The wavelengths that humans are typically able to visualize lie in a very narrow range between 400 and 700 nanometers in length. The wide variety of sources responsible for emission of electromagnetic radiation are categoried according to the spectrum of wavelengths generated by the source.

Primary Colors - The human eye is sensitive to a narrow band of electromagnetic radiation that lies in the wavelength range between 400 and 700 nanometers, commonly known as the visible light spectrum, which is the only source of color. When combined, all of the 300 or so wavelengths present in visible light, about a third of the total spectral distribution that successfully passes through the Earth's atmosphere, form colorless white light that can be refracted and dispersed into its component colors by means of a prism. The colors red, green, and blue are classically considered the primary colors because they are fundamental to human vision.

Human Vision and Color Perception - Human stereo color vision is a very complex process that is not completely understood, despite hundreds of years of intense study and modeling. Vision involves the nearly simultaneous interaction of the two eyes and the brain through a network of neurons, receptors, and other specialized cells. When an object is viewed, it is first focused through the cornea and lens, forming an image on the surface of the retina, a multilayered membrane that contains millions of light-sensitive cells. These photosensory cells detect the image and translate it into a series of electrical signals for transmission to the brain.

Light: Particle or a Wave? - Many distinguished scientists have attempted to explain how electromagnetic radiation can display what has now been termed duality, or both particle-like and wave-like behavior. At times light behaves as if composed of particles, and at other times as a continuous wave. This complementary, or dual, role for the properties of light can be employed to describe all of the known characteristics that have been observed experimentally, ranging from refraction, reflection, interference, and diffraction, to the results with polarized light and the photoelectric effect.

Reflection of Light - Reflection of visible light 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. The incoming light wave is referred to as an incident wave, and the wave that is bounced away from the surface is termed the reflected wave. The best surfaces for reflecting light are very smooth, such as a glass mirror or polished metal, although almost all surfaces will reflect light to some degree.

Refraction of Light - As visible light travels from one substance or medium into another, the light waves may undergo a phenomenon known as refraction, which is manifested by a bending or change in direction of the 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 phenomenon produces a wide variety of effects including mirages, rainbows, and bizarre optical effects such as making fish appear to be swimming in water more shallow than they really are, and is also an important characteristic of lenses that enables them to focus a beam of light onto a single point.

Diffraction of Light - When light waves pass near a barrier, they tend to bend around that barrier and spread at oblique angles. This phenomenon is known as diffraction of the light, and occurs when a light wave passes very close to the edge of an object or through a tiny opening such as a slit or aperture. The light that passes through the opening is partially redirected due to an interaction with the edges.

Interference - 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. The interaction between light waves that are very close together occurs so often that the phenomenon is often neglected and taken for granted. However, from its fundamental contributions to the physics of image formation, to the beautiful color patterns of halos and coronae in the atmosphere and a myriad of sensational insect disguises, interference of light waves helps to bring color to the world around us.

Polarized Light - Applications that benefit from polarized light include the ever-popular Polariod polarizing sunglasses, as well as the use of special polarizing filters for camera lenses. A variety of scientific instruments utilize polarized light, either emitted by lasers, or through polarization of incandescent and fluorescent sources by a host of techniques. Polarizers are sometimes used in room and stage lighting to reduce glare and produce a more even degree of illumination, and are worn as glasses to bestow an apparent sense of depth to three-dimensional movies. Crossed polarizers are even utilized in space suits to dramatically reduce the chances of light from the sun entering the astronauts' eyes during naps.

Optical Birefringence - Anisotropic crystals, such as quartz, calcite, and tourmaline, have crystallographically distinct axes and interact with light by a mechanism that is dependent upon the orientation of the crystalline lattice with respect to the incident light angle. When light enters the optical axis of anisotropic crystals, it behaves in a manner similar to the interaction with isotropic crystals, and passes through at a single velocity. However, when light enters a non-equivalent axis, 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 refraction or birefringence and is exhibited to a greater or lesser degree in all anisotropic crystals.

Light Filters - Successful use of filters requires attention to the technical details of absorption and transmission spectra, as well as other published filter characteristics. The key is to build a solid base of facts surrounding the physical properties of the light source, digital imaging or traditional photography criteria, and the effects of the filters obtained while acquiring solid experience with real-world applications. Some filters are used purely for technical applications, while others are implemented for their artistic qualities. Regardless of the target function, proper use of filters will dramatically improve the quality of photography and digital imaging.

Color Temperature - The color temperature model is based on the relationship between the temperature of a theoretical standardized material, known as a black body radiator, and the energy distribution of its emitted light as the radiator is brought from absolute zero to increasingly higher temperatures. The concept of color temperature is very important in the traditional photography arena, where film emulsions must be balanced to accurately render color using different light sources. In addition, digital imaging techniques must also address color temperature in the form of white balance circuitry that controls the red, green, and blue intensity levels to match the output spectrum of the illumination source.

Speed of Light - Starting with Ole Roemer's 1676 breakthrough endeavors, the speed of light has been measured at least 163 times by more than 100 investigators utilizing a wide variety of different techniques. Finally in 1983, more than 300 years after the first serious measurement attempt, the speed of light was defined as being 299,792.458 kilometers per second by the Seventeenth General Congress on Weights and Measures. Thus, the meter is defined as the distance light travels during a time interval of 1/299,792,458 seconds. In general, however, (even in many scientific calculations) the speed of light is rounded to 300,000 kilometers (or 186,000 miles) per second.

Light and Energy - Mankind has always been dependent upon energy from the sun's light both directly - for warmth, to dry clothing, to cook, and indirectly to provide food, water, and air. Our awareness of the value of the sun's rays revolves around the manner in which we benefit from the energy, but there are far more fundamental implications from the relationship between light and energy. Whether or not mankind devises ingenius mechanisms to harness the sun's energy, our planet and the changing environment contained within is naturally driven by the energy of sunlight.

Introduction to Light Emitting Diodes - The past few decades have brought a continuing and rapidly evolving sequence of technological revolutions, particularly in the digital arena, which has dramatically changed many aspects of our daily lives. The developing race among manufacturers of light emitting diodes (LEDs) promises to produce, literally, the most visible and far-reaching transition to date. Recent advances in the design and manufacture of these miniature semiconductor devices may result in the obsolescence of the common light bulb, perhaps the most ubiquitous device utilized by modern society.

Laser Fundamentals - In a few decades since the 1960s, the laser has gone from being a science fiction fantasy, to a laboratory research curiosity, to an expensive but valuable tool in esoteric scientific applications, to its current role as an integral part of everyday tasks as mundane as reading grocery prices or measuring a room for wallpaper. Any substantial list of the major technological achievements of the Twentieth Century would include the laser near the top. The pervasiveness of the laser in all areas of current life can be best appreciated by the range of applications that utilize laser technology.

Introduction to Lenses and Geometrical Optics - The action of a simple lens, similar to those used in the microscope, is governed by the principles of refraction and reflection and can be understood with the aid of a few simple rules about the geometry involved in tracing light rays through the lens. The basic concepts explored in this section, which are derived from the science of Geometrical Optics, will lead to an understanding of the magnification process and the properties of real and virtual images.

Introduction to Mirrors - Reflection of light is an inherent and important fundamental property of mirrors, and is quantitatively gauged by the ratio between the amount of light reflected from the surface to that incident upon the surface, a term known as reflectivity. Mirrors of different design and construction vary widely in their reflectivity, from nearly 100 percent for highly-polished mirrors coated with metals that reflect visible and infrared wavlengths, to nearly zero for strongly absorbing materials.

Prisms and Beamsplitters - Prisms and beamsplitters are essential components that bend, split, reflect, and fold light through the pathways of both simple and sophisticated optical systems. Cut and ground to specific tolerances and exact angles, prisms are polished blocks of glass or other transparent materials that can be employed to deflect or deviate a light beam, rotate or invert an image, separate polarization states, or disperse light into its component wavelengths. As the name implies, beamsplitters are utilized to redirect a portion of a light beam while allowing the remainder to continue on a straight path.

Optical Defects in the Microscope Lens System (Aberrations) - Microscopes are commonly plagued by lens errors that occur because of the spherical surfaces that are associated with defects (commonly referred to as aberrations) that distort the image by a variety of mechanisms. In general, the ultimate effect of optical aberrations in the microscope is to induce faults in the tiny features and specimen detail of an image that is being observed or digitally recorded.

Light and Color Interactive Java Tutorials - Exploring the concepts of light and color can be significantly enhanced through utilization of interactive tutorials that enable the student to immediately visualize alterations in physical phenomena in response to changes in both experimental and environmental conditions. This section contains 40 interactive Java tutorials that examine many of the basic principles associated with refraction, reflection, diffraction, interference, color, image formation, lasers, prisms, and general features of electromagnetic radiation.