How we see

This lesson asks you to identify the sensory organs and the devices that make use of the properties of light and sound. The museum serves only the light part: an eye and a camera as one medieval scholar understood them. Ears, sound and the devices that use it are for your class to bring. ¹

The expectations ask you to name organs and devices that use light. Think of what must reach an organ or a device before it can form a picture, and what happens when that is missing.

Ibn al-Haytham set several candles before one hole into a dark place. On the wall inside appear as many spots of light as there are candles, each opposite its own candle. When one candle is covered, only its spot goes out ². The record says this is why a camera's picture is sharp: the light from each point outside reaches its own point inside, along its own straight line ². In his theory of seeing, the record says, the eye is affected by what arrives from the object ³.

In the candle test, every candle gets its own spot because its light travels in a straight line through the hole. If the hole were much bigger, would each candle still get its own clear spot, and why or why not?

• Ibn al-Haytham's dark room: the candle test and the link the record draws to the camera.
• Ibn al-Haytham's account of vision: what must reach the eye for us to see.
• Ibn al-Haytham: the scholar behind both.

Read the account of vision and the dark room. Draw one diagram of light, an object and an eye, and another of light, an object and a hole in a wall.