Microwaves are electromagnetic waves that have lower frequencies (and longer wavelengths) than infrared light. They are higher on the frequency scale of the electromagnetic spectrum than radio waves. Their wavelengths range between 1 mm and 30 cm. The existence of microwaves was predicted by James Clerk Maxwell from his electromagnetic wave equations but only experimentally verified by Heinrich Hertz. While we do not usually think of microwaves as a form of light, they follow the same laws and possess the same properties as other forms of electromagnetic radiation, including visible light. Microwaves can show refraction, reflection, and diffraction patterns just as visible light does.

Microwaves are seen all over the universe-literally. The cosmic background radiation, which seems to be roughly isotropic, or the same in every direction, is composed of microwaves left over from the energy of the big bang. These microwaves have interacted with matter very little, if at...