The electromagnetic spectrum is the range of possible electromagnetic radiation frequencies. They are usually described in terms of either their wavelengths (the distance between waves) or their frequency (the number of waves per second). From high to low wavelengths (low to high frequencies), the spectrum covers:
- radio waves - wavelengths ranging from about 1,000 meters down to about 1 meter, roughly on the scale of buildings and people, with radio and television frequencies ranging from AM radio at about 1,000 to 100 meters, and FM radio at about 10 to 1 meters;
- microwaves - wavelengths of between 1 meter and 1 millimeter (or 0.001 of a meter), roughly on the scale of people and insects;
- infrared - wavelengths from about 1 millimeter down the edge of visible light at about 750 nanometers (or 0.00000075 of a meter), roughly the size of the point of a needle;
- visible light - wavelengths of about 750 nanometers (0.00000075 of a meter) down to about 400 nanometers (0.0000004 of a meter), roughly the size of cells, following the familiar color spectrum of red, orange, yellow, green, blue, indigo, violet;
- ultraviolet - wavelengths of 400 nanometers (0.0000004 of a meter) down to 10 nanometers (0.00000001 of a meter), roughly the size of molecules;
- x-rays - wavelengths of 10 nanometers (0.00000001 of a meter) down to 0.01 nanometers or 10 picometers (0.00000000001 of a meter), roughly the size of atoms;
- gamma rays - wavelengths below 10 picometers (0.00000000001 of a meter), roughly the size of atomic nuclei.