What we perceive as color is really various frequencies of light. Visible light is a form of electromagnetic radiation, along with Radio, X-rays, and the like. Often we describe the wavelength of the light, which is related to the frequency by:
Speed of Light= Frequency times Wavelength.
In increasing order of frequency, we have: IR-R-O-Y-G-B-UV.
IR (infrared) and UV (ultraviolet) aren't visible. Red light (R) has the lowest frequency of the visible spectrum. Blue Light (B) has the highest frequency. White light is the combination of all the frequencies of light.
The color something reflects is the color you see. So, if you wear a black shirt on a hot day, you'd absorb all the frequencies of light (every color). According to the first law of thermodynamics, energy can neither be created nor destroyed. So when light hits an object, the energy has to go somewhere. Energy can change forms. So the light energy changes into heat energy, heating the air molecules in the Earth's atmosphere and heating the surface of whatever the color is on, like your shirt.
How hot the colored surface gets depends on what color it is because that describes what frequency(s) light it absorbs. The best way to get something hot is to make it absorb all of the frequencies (in other words, paint it black).
the effects of light on objects (including our skin) depend not only on how much total energy is coming in but also on what size packets of energy are absorbed in single steps. It turns out that the energy per packet (called a quantum of energy) is proportional to the frequency. Since UV has high frequency, its quanta are big, and the energy per quantum is able to do a lot of damage to molecules. Thus UV causes sunburn by damaging various molecules in your skin. The same amount of heat coming in as visible light doesn't do that damage.
Jason (w. some changes by Mike W.)
(republished on 07/27/06)