Visible light is a complex phenomenon that is classically explained with a simple model based on propagating rays and wavefronts, a concept first proposed in the late 1600s by Dutch physicist Christiaan Huygens. Electromagnetic radiation, the larger family of wave-like phenomena to which visible light belongs (also known as radiant energy), is the primary vehicle transporting energy through the vast reaches of the universe. The mechanisms by which visible light is emitted or absorbed by substances, and how it predictably reacts under varying conditions as it travels through space and the atmosphere, form the basis of the existence of color in our universe.

The Nature of Electromagnetic Radiation - Coined by Sir James Clerk Maxwell, the term electromagnetic radiation is derived from the characteristic electric and magnetic properties common to all forms of this wave-like energy, as manifested by the generation of both electrical and magnetic oscillating fields as the waves propagate through space. Visible light represents only a small portion of the entire spectrum of electromagnetic radiation, which extends from high-frequency cosmic and gamma rays through X-rays, ultraviolet light, infrared radiation, and microwaves, down to very low frequency long-wavelength radio waves.

James Clerk Maxwell (1831-1879) - James Clerk Maxwell was one of the greatest scientists of the nineteenth century. He is best known for the formulation of the theory of electromagnetism and in making the connection between light and electromagnetic waves. He also made significant contributions in the areas of physics, mathematics, astronomy and engineering. He considered by many as the father of modern physics.

William Herschel (1738-1822) - Friedrich William Herschel was an eighteenth century German astronomer who is credited with the discovery of the planet Uranus. In addition, Herschel measured the heights of about one hundred mountains on the moon, carefully recorded the data, and prepared papers that were presented to the Royal Society of London. In the late 1700s, he began to build and sell telescopes. The high quality of Herschel's optics was soon widely known outside of England, and he utilized them to publish three catalogues containing data on 2500 heavenly objects, including the sixth and seventh moons of Saturn, Enceladus and Mimas. Herschel continued making observations and cataloging his discoveries until his death in 1822 at age 84.

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.

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.

Reference Listing - The reference materials listed in this section are an excellent source of additional information on the diverse topic of electromagnetic radiation. Included are references to books, book chapters, and review articles, which discuss the theory and applications of electromagnetic radiation and how they relate to the physics of light and color.

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