As we've seen in this article, even the most basic, completely manual SLR is a complex, intricate machine. A pinhole camera is simply a box with a tiny hole in one side and some film or photographic paper on the opposite size. The scientific principle behind this is very simple. If you were to shine a flashlight in a dark room, through a tiny hole in a wide piece of cardboard, the light would form a dot on the opposite wall. In a larger visual scene, every particular visible point acts like this flashlight.
Light reflects off each point of an object and travels out in all directions.
A small pinhole lets in a narrow beam from each point in a scene. The beams travel in a straight line, so light beams from the bottom of the scene hit the top of the piece of film, and vice-versa. In this way, an upside down image of the scene forms on the opposite side of the box. Since the hole is so small, you need a fairly long exposure time to let enough light in. One of the most popular designs uses an ordinary cylinder oatmeal box, coffee can, or similar container.