With two torches shining towards a screen, an object can block light from one torch at a point on the screen while the other torch still lights that point. Where the object blocks light from both torches, the overlapping part looks darker.
Light travels in straight lines. To explain the pattern, check the straight path from each torch to the point on the screen. Imagine the torches as small light sources in a dim room.
- Pick a point on the screen.
- Check whether the object blocks the straight path from each torch.
- Compare how many torches can light that point.
Worked example | Three points on one screen
A card stands between two torches and a screen. At point P, the card blocks torch A but light from torch B reaches the screen. At Q, the card blocks both torches. At R, light from both torches reaches the screen.
Q receives the least light from the torches and is the darkest of the three points. P is partly lit by torch B. R receives light from both torches. In a real room, reflected light can still reach Q.
What happens if you switch one torch off?
Switch off torch B while leaving the card and torch A in place. P loses its only direct light because torch A was already blocked there. Q was already blocked from both torches. R still receives light from torch A.
Check the prediction by switching on one torch at a time and marking its shadow outline on paper at the screen. When both torches are on, the area inside both outlines is where the card blocks both sources.
Explain a lighter part of the pattern
At point S, torch A lights the screen but the card blocks torch B. Compare S with a point where the card blocks both beams.
Answer. S is brighter because light from torch A can still reach it. The point in the overlap receives no direct light from either torch.
Frequently asked questions
Can two separate shadows form without an overlap?
Yes. The shadows can fall on different parts of the screen. Their positions depend on where the torches, object and screen are placed.
Should I explain the result using shadow size?
For this question, the decisive clue is which paths the object blocks. Draw straight lines from each torch to the chosen point.
Official information
MOE, 2023 Primary Science Teaching and Learning Syllabus, Primary Four Energy Forms and Uses (Light). Syllabus checked 11 November 2026. The examples are original.