The colours of visible light by which we see the world around us are only a small part of the total spectrum of electromagnetic radiation. Other parts of this invisible spectrum have an effect on us though. We're all familiar with the effects of ultraviolet light, which has a shorter wavelength than visible light. UV is what gives us a suntan and makes our teeth, underwear and dandruff glow in a nightclub. It can also give us skin cancer and make inkjet prints fade.
At the other end of the spectrum, longer wavelengths are known as infrared, and we feel these wavelengths as heat. Infrared light is emitted by the sun and other hot objects. It is used in thermograph cameras for tracking criminals, and by alien predators tracking Arnold Schwarzenegger.
The electromagnetic spectrum doesn't end there. Beyond ultraviolet lie Gamma rays and X-rays, while longer wavelengths than infrared include microwaves and radio waves, but for today we're interested in the infrared portion of the spectrum.