The Shadow on the Wall
During a science lesson, Ravi holds a small torch 30 cm from a wall. He places a card with a triangular hole between the torch and the wall. The bright patch on the wall is triangular. He then moves the torch to 60 cm from the wall, keeping the card in the same place. The bright patch becomes larger and dimmer. Finally, he replaces the card with one that has a round hole of the same size.
1. Explain why the bright patch is triangular in the first setup, and why it grows larger when the torch is moved farther away.
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Light travels in straight lines from the torch. Rays pass through the triangular hole and continue straight until they hit the wall, so the patch takes the shape of the hole. The hole acts like a small opening that shapes the beam.
When the torch is moved farther from the wall, the rays spread out more before reaching the wall. The same angular spread covers a larger area, so the patch grows. The same light is spread over a bigger area, so the patch looks dimmer. This is the same reason a shadow gets bigger when an object is closer to the light source.
2. Predict the shape of the bright patch when the round-holed card is used, and explain your reasoning.
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The patch will be round. The hole shapes the beam because light travels in straight lines through the opening, so the patch on the wall matches the shape of the hole. Since the round hole is the same size as the triangular one and is placed in the same position, the patch will be about the same size and brightness as before, but circular.
This assumes the torch is small compared with the hole. If the torch were very large, the edges of the patch would blur and the shape would be less clear.