All film and indeed photography is based on the fact that 
light - darkens silver!
The more light that falls on a piece of film or paper the darker it becomes.

For the @ the Speed of Light installation, I used this fact to make images only with light during the period of the exhibition.

If we focus an image through the lens of the camera onto the film the areas that receive the most light become darkest while the areas that receive the least light become the lightest. 

In fact, because there is this proportional response to light we have what is called a NEGATIVE image of the subject. This is what happens when we expose film in a camera, the areas of the film which receive the most light are most effected.