Occupying a very small part of the entire electromagnetic spectrum, the visible light region is the only range of energies that human eyes can detect. From the low-energy red (at about 7000 Å) to the high-energy violet (at about 4000 Å), the visible spectrum conveys the world to our brains in brilliant hues and shades.

Why can we even see things? Our eyes have developed special cells called "rods" and "cones" on the inside layers of the back of the eye, the retina. The rod-shaped cells are sensitive to different intensities of light (how many photons per second) but not to different energies (the colors). When we try to see in the dark, we are using the rods in our eyes. Notice that it's very hard to tell different colors in the dark -- everything seems grey at best. This is because the color-sensitive cones don't work well at low-light levels, and so the rods take over, registering only the difference between light and dark. In most people, the rods are sensitive enough that less than 10 photons can be seen impinging on the eye.

The cone-shaped retinal cells come in three different types: those sensitive to energies corresponding to the red portion of the spectrum, the green, and the blue. The fact that each type of cone is sensitive to different colors allows us to see many different shades and mixtures of colors.

The cones sensitive to red are actually more sensitive to the same intensity light as the green and blue cones, so we tend to see reds better than blues. Actually, the maximum sensitivity of our eyes is in the yellow (which is a mixture of green and red energies), probably because we have evolved on a planet which is bathed in yellow sunlight.

Some people's eyes are sensitive to wavelengths (energies) beyond the normal visible region; they can see a little into the ultraviolet and down into the infrared. Most people, though, are content with the rainbow of colors visible in everyday phenomena.

To read more about color, light, and vision go to the Reading Supplement.