Copyright © 2007 Dorling Kindersley
Light is a type of energy known as electromagnetic radiation. It is given out by hot objects such as the Sun, light bulbs, and LASERS. When light hits a surface, its energy can be absorbed (soaked up), REFLECTED, or deflected by REFRACTION.
Light is made up of little packets of energy called photons. Most of these photons are produced when the atoms in an object heat up. Heat “excites” the electrons inside the atoms and they gain extra energy. This extra energy is then released as a photon. The hotter an object gets, the more photons it gives out.
Light travels as a wave. But unlike sound waves or water waves, it does not need any matter or material to carry its energy along. This means that light can travel through a vacuum—a completely airless space. (Sound, on the other hand, must travel through a solid, a liquid, or a gas.) Nothing travels faster than light energy. It speeds through the vacuum of space at 186,400 miles (300,000 km) per second.
Light waves travel out from their source in straight lines called rays. Rays do not curve around corners, so when they hit an opaque object (one that does not allow light to pass through it), they are blocked from reaching the other side of that object. We see a dark shadow in the area from which light is blocked.
When light falls on a material, the energy in its photons can affect the atoms in the material. In some materials, such as metal, the atoms absorb some of the photons so light does not pass through them. These materials are opaque. In other materials, such as glass, the atoms cannot absorb the photons and light passes through them. These materials are transparent.
Light rays reflect (bounce) off objects. The Moon shines because it reflects light from the Sun. Smooth surfaces, such as mirrors, reflect light in one direction.
At first sight, your image is identical to you. But a closer look shows that as you lift your right hand, your image raises its left. Reflection always flips an image from left to right. If you hold up a sheet of paper with writing on it, the image in the mirror shows the writing in reverse.
Light travels more slowly through some materials than others. The change in speed can cause light rays to change direction. This directional change is called refraction.
Refraction can make things look closer than they really are. The difference in speed between light traveling through water and through air means that, from the surface, a 13-ft (4-m) pool appears to be just 10 ft (3 m) deep. Glass is another material that refracts light. It is used to make eyeglasses and other lenses.
A laser produces an incredibly powerful, concentrated form of light. Inside a laser, light waves are bounced back and forth between two mirrors to build up energy before being released as a narrow beam.
Laser light does not spread out in the way that light from other sources does. All the light waves in laser light are precisely in step with each other. As a result, laser light can be concentrated and controlled far more accurately. It can carry television and other signals over great distances without losing quality.