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LIGHT Written by Brian Roberts

Introduction If you've ever walked into a closet and closed the door, you've been in a world without light. Without light, we can't see. But allowing us to see things is only one important thing about light. This book will reveal many of the fascinating facts that scientists have found out about light. These facts include what it is, where it comes from, how it moves, and what happens when light strikes different surfaces. There is much more to light than meets the eye.

What Is Light? Light is a kind of energy. It is called radiant energy. There are many kinds of radiant energy, but light is the kind you can see. Scientists usually describe the range of energy as a spectrum. A spectrum is a series of energy bands. The diagram above shows bands of radiant energy. Visible light is only a small part of the spectrum, in the middle On each side of visible light is invisible radiant energy. The bands on the left have less energy than the bands on the right. For example, radio waves are much less powerful than gamma waves.

How Light Moves There are two ways to describe light and how it moves. The first way is as tiny, invisible packets of energy called photons. They don't weigh anything—even billions of them together! Photons have different amounts of energy. High-powered gamma ray photons have more energy than photons of visible light. Even in visible light different colors of light, have different amounts of energy. For example, violet light photons have more energy than red light photons.

The second way of describing light is as a wave. Waves are measured by their frequency, or how closely together they move. High-frequency waves travel more closely together. Low-frequency waves are spread out. More high-frequency waves than low-frequency waves will pass the same place in the same time. Scientists measure frequency by a wave's wavelength. Wavelength is the distance between the peak of one wave and the peak of the wave next to it. Look at the diagram of two waves. The low-frequency wave is spread out. It has a longer wavelength. The longer the wavelength, the lower the wave's energy.

There are a few more things to know about light's movement. One is that it travels in straight lines. Another is that beams of light can change direction. You will read more about this in the section on how light behaves. A third thing to know about how light moves is that—unlike sound—it does not need anything to travel through, such as air, water, or wood. This makes radiant energy special—it can even travel through the emptiness of space. If radiant energy could not do this, sunlight would never reach Earth.