In a positive, light areas in the scene are light in the picture. Colour photos use three layers of silver salts, sensitive to red, blue or green light. These layers are converted to coloured dyes during processing. When the picture is taken, light subtly alters some of the film grains. Processing To reveal the negatives, the film is developed in chemicals in a lightproof tank to change the altered film grains into silver.
More chemicals dissolve away the unaltered grains to fix the image.
Enlarging To make a positive print, each negative is placed in an enlarger. This has a lens to project a magnified version of the picture on to white print paper coated with light sensitive grains like film. The print A light in the enlarger is switched on briefly to expose the print paper. The print is then developed and fixed like the film to give the final print.