Two setups can have the same gap between an object and a screen but produce different shadow sizes. With a small light source, the shadow gets larger when the object and screen are moved together closer to the source while keeping the gap between them fixed.
The MOE Primary Science syllabus includes investigating how the positions of the light source, object and screen affect shadows. This example keeps one distance fixed so you can explain the effect of the other.
- Name both distances in the setup.
- Keep the object and its orientation the same.
- Trace straight lines from the source past the object’s edges.
- Compare the distance between those lines on each screen.
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Set up a fair comparison
Imagine a small lamp, a card that blocks light and a white screen. Keep the card and screen upright and lined up with the lamp. Mark a gap of 20 cm from the card to the screen. When you move the card, move the screen by the same amount to preserve that gap.
From the lamp, sketch one straight line just past the top edge of the card and another just past the bottom edge. The space between the two lines on the screen shows the shadow height in this simple model.
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Worked example | Two setups with a 20 cm gap
In setup A, the card is 10 cm from the lamp and the screen is 30 cm from the lamp. In setup B, the card is 20 cm from the lamp and the screen is 40 cm from the lamp. Both card-to-screen gaps are 20 cm.
In setup A, the lines from the lamp to the card’s edges spread farther apart over the next 20 cm. The shadow is taller in setup A. State which distance changed when you explain why. The card is nearer the lamp in A, although the gap to the screen is the same.
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Try holding the card still instead
Return to setup B. Keep the lamp and card still and move the screen farther away. The shadow grows because the lines continue to spread after they pass the card. This time the card-to-screen gap changes while the lamp-to-card distance stays fixed.
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Frequently asked questions
What if the shadow edge looks fuzzy?
A large light source can produce a fuzzy edge. For this comparison, use a small source and measure both shadows by the same rule.
What should a parent ask during revision?
Ask the child to label both gaps and point to the one that was kept fixed. Then ask them to draw the two edge rays before predicting the larger shadow.