The human eye is actually a form of camera. When you look around, your eyes actually 'take pictures' of the things that you see. The lens in your eye acts like the lens in a camera. The retina of your eye acts like a film in your camera.
In a camera light acts on a specially prepared sensitive surface of the film. If light didn't have any effect on certain chemically prepared substances, photography would be impossible.
When we open and close the shutter of a camera quickly, light comes in and strikes the film. When this happens, a chemical reaction takes place on the film. Certain tiny grains of silver bromide undergo a change.
The film is taken out of the camera and treated with various chemicals to make the print visible and prevent further changes when exposed to light.
Digital cameras are a recent development. They convert the image they receive into electrical signals that are stored. These signals can then be read by computer and used to produce a picture on the screen, which can then be printed out.
Credit : Tell Me how?