Light is a traveling wave of electromagnetic energy. Like water or sound waves, it can have different frequencies that depend on how much energy is put into it. (Unlike water and sound waves, light waves always travel at the speed of light, 186000 miles a second.) The more energy in the light wave, the higher its frequency, and the shorter its wavelength. There are certain ranges of values for these wavelengths in which light interacts with different kinds of material. Sometimes, historically, this interaction was discovered before it was realized that the wave in question was just another kind of light. And so we have divided up all the possible wavelengths that light can have into regions. The most familiar region will be that of the optical, or visible, light: the range of wavelengths that interact with your eyes' retinas, and allow you to see. gamma-rays are the most energetic form of light, with wavelengths on the scale of atomic nuclei.

Here is a table of the standard regions of light:
Name Size Scale (cm) Examples of
things that size
Radio 102 and bigger Buildings, People
Microwave 100 A bee
Infrared 10-2 Pinhead
Optical 10-5 Bacteria
Ultraviolet10-6 Molecules
X-ray 10-8 Atoms
Gamma-ray 10-10 and smaller Nuclei

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