Pinhola

Meet Pinhola, a box camera with no lens, just a tiny hole.

Light travels in straight lines. Rays from the top of a scene pass through a small hole and continue down to the bottom of the back wall, while rays from the bottom land near the top. The result is an image upside down and flipped left to right. Similar triangles on each side of the hole give its size: h_i/h_o = d_i/d_o. This camera obscura effect was described in ancient China and Greece, and later used by painters.

A tree 10 m tall standing 20 m away, viewed in a box 0.2 m deep, makes an image h_i = 10 × 0.2 / 20 = 0.1 m tall, 10 cm on the back wall.

A pinhole projector is a safe way to watch a solar eclipse: you look at the Sun's image on card instead of at the Sun. The same geometry underlies how lenses and eyes form images.

A smaller hole gives a sharper but dimmer picture. Make it too small and diffraction blurs the image again, so each box depth has an ideal hole size.