Visible light - that which most people can actually see - has
wavelengths between 400 and 700 nm.
Violet light has the shortest
wavelength, followed by indigo, blue, green, yellow, orange, and finally
red, which has the longest. But there's more. As the wavelength of light
becomes even shorter than 400 nm, it seems, to your eye, to disappear. But
all film still reacts to the energy of these extremely short wavelengths
even more than to some wavelengths of visible light. This light, since it
is even more energetic than violet, is called ultraviolet.
situation exists at the other end of the spectrum. As the wavelength of red
light gets longer and longer, after it passes 700 nm, it, too, becomes
invisible to your eye.
But there are special films that can be made to
react to this invisible energy called infrared (below red). Both of these
light forms have importance in photography.
The visible spectrum
Now seven colors in the visible spectrum is really more complicated
than necessary. In the early 1800's, even before photography was invented,
a physicist discovered that by mixing only three colors, he could produce
all the other colors of visible light. The three colors he found are red,
green, and blue. In fact, these are the same three colors which are used in
a color television picture. If you look very closely at a color TV picture,
you'll see that it is composed of tiny dots of these three colors.
adding these colors together in different proportions, all the colors in the
picture are reproduced.
These three colors - red, green, and blue, are called additive primary
colors, because they add up to white. You can do this by using three slide
projectors. Put a filter of a different primary color over the lens of each
projector and aim the light beams at a white wall so that they overlap.
When all three projectors are on at once, the result will be white light.
Actually, the "white" might be a little off unless you are very careful to
balance the intensities of the three projectors to exactly the right
brightnesses, and the three filters are precisely the right color. But it
will be close(fig 2-2).