Shadow Theatre
Why does your shadow puppet get huge when you hold it near the torch? Drag the light and the rabbit and watch the shadow on the screen change.
- Light to rabbit (d)
- 50 cm
- Light to screen (D)
- 170 cm
- Shadow height
- 68.0 cm
- Shadow is
- 3.4× the rabbit
The maths, live
Similar triangles shadow = h × D ÷ dshadow = 20 × 170 ÷ 50 = 68.0 cmLight travels in straight linesA tiny light gives sharp edgesMove the rabbit close to the light, then close to the screen. When is the shadow biggest? Switch to the big lamp to get fuzzy edges.
How it works
Light travels in straight lines. When something blocks it, the light can't bend round, so a dark patch with the same shape appears behind it. That patch is a shadow.
The rays from a tiny torch to the edges of the puppet carry on to the screen, making two similar triangles. So the shadow is bigger by the same amount the screen is further away: shadow height = puppet height × D ÷ d. A big lamp gives a dark middle (the umbra) with a fuzzy, partly lit edge (the penumbra).
Try this
- Put the rabbit halfway between the torch and the screen. How much bigger is the shadow?
- Where do you put the rabbit to make the smallest shadow?
- Switch to the big lamp and move the rabbit towards the screen. What happens to the fuzzy edges?