Why does a tiny hole in a dark room show the world upside down?

The picture flips because light travels in straight lines and the rays cross at the hole. Light from the top of a tree outside heads down through the small opening and lands low on the far wall; light from the tree's base heads up and lands high. Left and right swap in the same way. What appears inside the dark room is the whole scene, in colour and correct perspective, but upside down and reversed. This is the camera obscura, Latin for 'dark chamber'.

People noticed it very early. A Chinese text called the Mozi, from the 4th century BC, already explains that the image is inverted by a crossing point that collects the light. Around 1027 Ibn al-Haytham explained that light travels in straight lines, and in 1502 Leonardo da Vinci wrote that the images appear smaller and upside down 'on account of crossing of the rays at that aperture'. In the 11th century the Chinese scholar Shen Kuo dismissed the idea that an upside-down image of a pagoda came from its reflection in the sea: the image is simply inverted by the small hole.

The trick had many uses: watching eclipses without hurting your eyes, scaring audiences with projected 'devils', and helping painters trace accurate scenes with perfect perspective. Fitted with lenses and, later, light-sensitive materials, the dark chamber became the photographic camera. Your own eye works the same way, projecting an inverted image onto your retina.