Without them, my friends would never be able to reach me and my parents couldn't find me in an emergency. More than 68 million Americans now use cell phones, and at least as many carry pagers. Cell phones are less common because they're too expensive. Both tools receive these waves, and cell phones also shoot them out so you can place a call any moment of the day. Hold your hand in the air for three seconds.
Light is one type of electro-magnetic wave.
Electromagnetic waves vary in frequency, or the amount of energy they contain. Every time you turn your radio dial to hit a different station, your radio picks up a new frequency of waves. Every cell phone in a cell, or area of wireless service, uses a slightly different frequency between 800 and 900 megahertz. As soon as Mara switches on her phone, powerful computers inside a building called a switching office assign her phone an unused frequency. Say Mara is calling her friend Jeremy's cell phone in Los Angeles.