How Light Works

Visible light has a wavelength in the range of about 380 nanometres (nm), or 380×10−9 m, to about 740 nanometres. Every color that we see corresponds to a different wavelength, from the very long (red to orange - 600-700nm) to short (violet to ultraviolet - 440-360 nm). Beyond that, you have the rest of the electromagnetic spectrum, stretching in each direction as far as the eye can see -- or can't see, as the case may be. Visible light makes up a very small portion of the electromagnetic spectrum.

(courtesy of wikipedia)

All of the light we perceive falls within that tiny range. One thing that's interesting here: you'll notice that white light doesn't show up on the image above. That's because white light isn't actually a color - it's all the colors mixed together (or rather, they appear to be mixed, because full spectrum light activates all three types of cone cells in your eye - red green and blue). It's called additive mixing. Consequently, when you see white light, you are actually seeing the full color spectrum.

Here is a picture of a pretty standard form of visible light -- the sun!

If you take a prism and scatter the light from the sun, you can see the individual colors that make up white light. You can actually use a second prism to reassemble the spectrum i.e. turn it back into white light. Alternatively, you can look at the spectrum to get a much more precise idea of what wavelengths are present in your light source.

(courtesy of fdecomite)

Different kinds of white light

Incandescent

Fluorescent

White LED

Sunlight