How the Camera Works
In a nutshell, a camera’s lens captures light rays to make a permanent image. Film has an extremely light sensitive coating, which reacts differently to different colors and intensities of light, in order to produce and capture an image. Light energy leaves a permanent trace by causing chemical reactions and physical changes of the film. When the shutter is opened, light rays pass through the lens and come in contact with the film. The lens allows for the image to be scaled down and helps to concentrate light energy so the image forms on the film more quickly, and with minimal distortions.
After the light passes through the camera, the film retains an inverted image. Photographic film is made from plastic coated with an emulsion made from tiny crystals (silver halides) suspended in gelatin. These crystals consist of various ratios of silver and other elements, such as chlorine, iodine, and bromine. These compounds are known as silver halides. When these light-sensitive particles come into contact with light, a chemical reaction occurs which changes the halides into pure, metallic silver. As the image receives more exposure, or bright light, more grains of silver halides are changed by the reaction. The light rays leave an invisible, chemical trace on the film, known as a latent image.
On a Microscopic Level: When a photon of light is absorbed by a silver-halide grain, the energy of a single electron is raised into the conduction band from the valence band. A conduction-band electron can then go on to combine with a positive hole in the silver-halide lattice and form a single atom of silver. A silver-halide grain contains billions of silver-halide molecules, and it only takes two to four atoms of uncombined silver to form the latent-image site.
In color photography, the film is made so that each color (red, blue, green) has a separate layer that is sensitive to that particular light emission. For example, red light forms a latent image in the red-sensitive layer of the film, and so on.