What do X-ray machines, microwave ovens, and heat lamps have in common with police radar, television, and radiation therapy? They all use waves. You are surrounded by such waves all the time. But you may not realize it, because most waves are invisible.

With X-rays, you can take pictures of your bones. Your dentist uses X-rays to examine the inner structure of your teeth. Microwaves cook or reheat your meals and carry cell phone conversations between you and your friends. Radio waves bring your favorite music to your radio from the radio station. Ultraviolet rays can give you a sunburn. Without waves, the girl in Figure 1 wouldn't be able to talk to her friends on a cell phone. Without waves, you wouldn't be able to watch your favorite TV show. You wouldn't be able to see colors. In fact, without waves you wouldn't be able to see anything at all.

Figure 1  The waves that carry this girl's cell phone conversation are not visible. The girl may not even know they exist. But their existence is what makes cell phone technology possible.