Figure 1. White light emanates from the sun and strikes a leaf on a tree. The leaf absorbs the energy of some of the colors of the spectrum and reflects the color green in all directions. One stream of green light travels through the hole of the camera obscura angling down toward the bottom of the rear wall. Another ray of sunlight strikes the dark brown root at the base of the tree and bounces through the hole angling upward to the top of the rear wall. Light bounces off all other portions of the tree in the same manner creating an inverted image.

The Latin term 'camera obscura' means 'chamber of darkness'. A small hole in the wall allows light to enter the chamber, projecting an inverted image of the outside scene onto the interior surfaces. The optical effect is based on the way light travels in straight lines. The camera obscura principle is a fundamental law of nature as old as light itself. The first pinhole/camera obscura eyes evolved in sea creatures some 540 million years ago during the Cambrian period. Every eyeball in the animal kingdom, every still camera, and every video camera owes it existence to this simple principle.

The evolution of the lens greatly increased the brightness of the image in the eye. The invention of the glass lens greatly increased the brightness of the image in the box. A lens is advantageous, but not necessary. A small hole will do. It is important to note, when the objects outside of the camera obscura are moving their images move correspondingly on the inside. In this respect, the images seen inside a camera obscura are more like a movie than a still photograph.

The existence of the camera obscura has been documented in various places and cultures throughout the historic period (since written records). To date, the oldest discovered document to reference a camera obscura was written by Mo Ti in the fifth century BC, describing the image of a building projected inside the enclosed space of an adjacent building. Shielding ourselves from the outside world has led to the accidental formation of camera obscuras for a very long time.

Enclosed spaces continue to randomly create camera obscuras to this day (though not as often due to the prevalence of the glass window pane, which has too great a surface area to form an image). Contemporary artist Madison Cawein rented studio space in an old factory building where many of the windows were boarded up or painted over. A small hole in one of those windows turned one room into a camera obscura. When photographer Bruce Cook was a private in the military he had to sit in the back of a dark truck, where to his amazement, a hole in the side of the truck projected the image of the passing street scene.

There are thousands of others who've had similar experiences.