83 Light-High

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 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 form of energy called radiant energy. But the light you can see is only one kind of radiant energy. Scientists usually describe the range of radiant energy as a spectrum, or a series of energy bands. The diagram above shows all the major bands of radiant energy. Visible light bands are in the middle of this spectrum but are only a small part of it. On each side of visible light are bands representing invisible radiant energy. Those on the left side have less energy than the bands on the right side. For example, gamma rays are much more powerful than radio waves. Sunlight is actually a mixture of many colors. Light passing through a prism is separated into many bands of color— the same colors seen in a rainbow.

How Light Moves Scientists have two ways to describe light and how it moves. The first way describes light as tiny, invisible particles, or packets, of energy called photons. Because photons are energy, not matter, they have no weight— even if you gather together billions of them! Not all photons are equal. High-powered radiant energy, such as gamma rays, has photons that carry more energy than photons of visible light. Even in visible light, one color of light might have more energy than 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 in terms of their frequency, or how closely together they travel. High-frequency waves travel more closely together, meaning that more of them pass a given point in a period of time than low-frequency waves, which are more spread out. Scientists measure frequency by a property called 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 waves are more spread out— they have a longer wavelength. The longer the wavelength, the lower the wave's energy.

There are several things you should know about the movement of light. One is that it travels in a straight line. A second is that beams, or rays, of light can change direction. You will read more about this in the section on how light behaves. A third important thing you should know about light's movement is that, unlike sound, it does not need a medium such as air, water, or wood to travel through. This is what makes radiant energy so special— it can travel through the nothingness of space. If it were not for this special property of radiant energy, sunlight would never reach Earth.