Electromagnetic radiation encompasses a wide range of wavelengths and frequencies, including visible light, radio, microwaves, and X-rays. Generally, radiation with wavelengths much shorter than visible light has enough energy to strip electrons from atoms. Scientists call this ionising radiation. The shorter the wavelength, the greater the danger. Although longer wavelengths also have their hazards, very short wavelengths such as X-rays and gamma rays can easily damage living tissue.
X-rays have wavelengths ranging from .001 to 10 nanometres, or billionths of a meter. These waves are smaller than an atom and can pass through most materials as sunlight passes through glass. Although X-rays have many beneficial applications, using them requires caution since exposure can cause blindness, cancer, and other injuries. X-rays once had novelty uses, such as the shoe-store gadgets that let you see your foot inside a shoe to judge how well it fits. These devices have long since been outlawed. Today, states requires a license for operating X-ray devices.
Ultraviolet light, or UV, gets its name from the fact that its wavelengths are shorter than violet visible light. Its wavelengths lie in the range of 10 to 350 nanometres and come in several bands, such as UVA and UVB. The sunlight that reaches the Earth's surface has naturally-occurring amounts of UV. Too much can cause sunburn, skin cancer, and retinal damage. Hospitals use shorter-wavelength UV to kill germs in the air. Light from a germicidal UV lamp can cause blindness if you look directly at it. Because it has longer wavelengths than X-rays, UV causes less damage to tissue; even so, it is still not completely safe.
Beyond X-rays are shorter wavelengths called gamma rays. Nuclear processes in atoms produce this kind of radiation, which has more energy and greater penetrating power than X-rays. Food producers use gamma ray devices to kill mould, germs, and parasites in fruits and vegetables. People can work with gamma radiation only behind thick lead shielding.
Although microwaves have wavelengths too long to be ionising, the power in microwaves can make them dangerous. Microwaves have wavelengths between .01 and 5cm, much longer than those of visible light. They produce heat by causing certain molecules, like water, to vibrate strongly. Cell phones and other gadgets emit microwaves although these are much too weak to affect living tissue. Your microwave oven, on the other hand, can produce more than 1,000 watts of microwaves and cause serious harm. Fortunately, microwaves are easily shielded.