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Why does a shadow become larger, smaller, sharper, or wider? In this tutorial, we will use a three dimensional light, object, and screen simulation to investigate one powerful idea: light travels in straight lines, and a shadow is the part of the screen where light is blocked.

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On the screen, the yellow source is the light. The blue object blocks some of the light rays. The pale screen catches the shadow. The yellow boundary rays show which light just touches the object before reaching the screen.

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Before changing anything, choose a prediction. I will test this one: object closer to the light makes a larger shadow. This is a good scientific prediction because it can be checked with measurements.

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Now move the object closer to the light. Watch the shadow size measurement. As the object moves nearer to the light source, the boundary rays spread out more before they reach the screen, so the shadow becomes larger.

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Next, move the screen farther away. The object is blocking the same bundle of rays, but those rays continue to spread as they travel. The evidence is the larger shadow size and the larger scale value.

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Change the object from cube to sphere. The rule is the same: light still travels in straight lines, and the shadow is still formed where light is blocked. But the boundary rays now outline a rounder shadow.

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The key idea is simple and powerful. A shadow is not something made by the light. It is the region where light from the source cannot reach. Try your own fair test: change only one slider, read the evidence, and explain the result using straight line light rays.