I don't know exactly how they work, but I think I know what the basic
idea is. One part of the scanner shines a light over every part of a
picture. Then another part of the scanner measures the light that is
reflected back from the picture. White light is a combination of all
colors of light. Depending on the color of a part of a picture, some
light is absorbed by the picture, and some is reflected by the picture.
In this way, the part of the scanner that measures what is reflected
back, can determine what color that part of the picture was. Then it
tells the computer to remember what the color of each part of the
picture was.

Scanners work in much the same manner as copy machines.As you may have already noticed, when you put something on the scanner bed, a light bar moves across the object being scanned. The light is reflected off the scanned object to varying degrees. Darker colors absorb more light than lighter colors do (just like how black shirts absorb more sunlight and get hotter than white shirts). The reflected light, which is now composed of light of varying intensities, then hits a grid composed of thousands of cells. Each cell records the intensity of light that hits it by assigning it a number according to that intensity. The values recorded by all the cells can then be organized by the computer to reproduce the original object.