The pinhole camera

Can we only see images in mirrors? No. A pinhole camera is a device where light from an object passes through a tiny hole (a pinhole) and forms an image on a screen.

Activity 11.9 makes the simplest one (with a lighted candle, under adult supervision). Make a small hole in a piece of cardboard. In a dim room, hold the cardboard a little way from a screen and put a lit candle in front of it.

Light from the flame goes through the hole and forms an image of the flame on the screen. And here is the surprise: the flame in the image is upside down (inverted). Why? Light travels in straight lines. Light from the top of the flame goes down through the hole and lands low on the screen; light from the bottom goes up and lands high. So top and bottom swap.

Activity 11.10 builds a sliding pinhole camera for outdoors. Take two cardboard boxes, one that slides snugly inside the other, and cut open one side of each.

• On the opposite face of the larger box, make a small hole in the middle.
• On the opposite face of the smaller box, cut a square about 5–6 cm across and cover it with thin tracing paper: this is the screen where the image forms.
• Slide the smaller box into the larger one, tracing paper inside.

Point the pinhole at a distant object, such as a tree or building in bright sunlight. Look in through the open side of the smaller box, with a dark cloth over your head and the camera to keep stray light out. Slide the small box in or out until a clear image appears on the tracing paper.

Notice two things: does the image show the object's colours, and is it upright or upside down?

Compare the two kinds of image. A pinhole camera turns the image upside down. A mirror swaps left and right (lateral inversion), but its image is not upside down.