On earth, we are constantly bombarded by radiant energy, or electromagnetic radiation, from galaxies and stars all around us.

This radiation arrives in the form of waves, with wavelengths of varying frequency. Some wavelengths, such as radio waves, are up to a kilometer across, while others such as cosmic rays are miniscule. The standard measurement for these energy forms is the nanometer (nm), which is about 1 billionth of a meter wide. This range of wavelengths is known as the electromagnetic spectrum. Most wavelengths in the electromagnetic spectrum are not visible to humans, but the human eye is sensitive to a small portion or bandwidth called the visible spectrum, or light, consisting of photons moving at wavelengths that make them detectible to photoreceptor cells in the eye. This visible spectrum falls in the range from roughly 400-700 nm, and encompasses all the colors that humans can see.

At the short-wavelength end of this visible spectrum, near 400 nm, lies light in the blue and violet colors that we see, and below it lie ultraviolet, x-ray, and gamma rays. On the longer end, near 700 nm, lies the reddish colored light that we see, and beyond this range lie infrared and radio waves.

While the types of radiation beyond the violet range have shorter wavelengths, they occur much more frequently and with more energy than their longer counterparts. This is why such wavelengths as ultraviolet (UV), x-ray, and gamma rays can be destructive to human tissues, causing sunburns, cancers, and eye damage.