82 Light-Mid

Light Written by Brian Roberts

Introduction If you've ever walked into a closet and closed the door, you've experienced a world without light. Without light, we can't see a thing. But allowing us to see things is only one important aspect of light, especially sunlight. This book will reveal many of the fascinating facts that scientists have discovered 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. called radiant energy But the light you can see is only one kind and a very small part of radiant energy. There are many kinds of radiant energy, and scientists usually describe the whole range as a spectrum—a series of energy bands. The diagram above shows the major 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 of the diagram have less energy than the bands on the right. For example, radio waves are much less powerful than gamma waves. You can see bands of color in a rainbow. Each of these bands is made by radiant energy with a different amount of energy than the band next to it.

How Light Moves Scientists describe light and how it moves in two ways. The first way is as tiny, invisible packets of energy called photons. Photons are particles of energy. They don't weigh anything, even when billions get together! Not all photons are equal. High-powered gamma rays have photons with more energy than photons of visible light. Even in visible light, there is a difference in energy from one color to another. For example, violet light photons carry more energy than red light photons.

Light is also described as moving in waves. These waves are measured by their frequency, or how closely together they travel. High-frequency waves, travel more closely than low-frequency waves which are more spread out. More high- frequency waves pass a given point in a period of time than low-frequency waves. 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 a high-frequency wave. and a low-frequency wave The low frequency wave is more spread out. It has a longer wavelength. The longer the wavelength, the lower the wave's energy.

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