Hole darkroom

A dark hole chamber consists of a device formed by a box of totally opaque walls, and in the middle of one face there is a small hole.

When placing an object, of size Ofacing the hole at a distance P, it is noted that a reflected image of i, appears on the opposite side of the box at a distance P'but inverted. As illustrated in the figure:

Thus, from a geometric similarity one can express the following equation:

This is known as the darkroom equation.