Light

## CONCEPT

Light exists along a relatively narrow bandwidth of the electromagnetic spectrum, and the region of visible light is more narrow still. Yet, within that realm are an almost infinite array of hues that quite literally give color to the entire world of human experience. Light, of course, is more than color: it is energy, which travels at incredible speeds throughout the universe. From prehistoric times, humans harnessed light's power through fire, and later, through the invention of illumination devices such as candles and gas lamps. In the late nineteenth century, the first electric-powered forms of light were invented, which created a revolution in human existence. Today, the power of lasers, highly focused beams of high-intensity light, make possible a number of technologies used in everything from surgery to entertainment.

## HOW IT WORKS

### Early Progress in Understanding of Light

The first useful observations concerning light came from ancient Greece. The Greeks recognized that light travels through air in rays, a term from geometry describing that part of a straight line that extends in one direction only. Upon entering some denser medium, such as glass or water, as Greek scientists noticed, the ray experiences refraction, or bending. Another type of incidence, or contact, between a light ray and any surface, is reflection, whereby a light ray returns, rather than being absorbed at the interface.

The Greeks worked out the basic laws governing reflection and refraction, observing, for instance, that in reflection, the angle of incidence is approximately equal to the angle of reflection. Unfortunately, they also subscribed to the erroneous concept of intromission—the belief that light rays originate in the eye and travel toward objects, making them visible. Some 1,500 years after the high point of Greek civilization, Arab physicist Alhasen (Ibn al-Haytham; c. 965-1039), sometimes called the greatest scientist of the Middle Ages, showed that light comes from a source such as the Sun, and reflects from an object to the eyes.

The next great era of progress in studies of light began with the Renaissance (c. 1300-c. 1600.) However, the most profound scientific achievements in this area belonged not to scientists, but to painters, who were fascinated by color, shading, shadows, and other properties of light. During the early seventeenth century, Galileo Galilei (1564-1642) and German astronomer Johannes Kepler (1571-1630) built the first refracting telescopes, while Dutch physicist and mathematician Willebrord Snell (1580-1626) further refined the laws of refraction.
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