Threshold concept #2: All photography is the capturing of light (radiant energy) and includes images that are made without a camera or film.
PINHOLE PHOTOGRAPHY: THE EARLIEST FORM OF CAMERA?
Pinhole photography operates in a simliar way to that of the human eye. The eye allows light to travel through the pupil, a hole only made smaller or larger by the iris, meaning that the outside imagery is reflected (upside down) into the eye, not unlike the eye the camera needs light to operate allowing the scenery on the other side to be projected onto the opposite surface.