How Shadows Form

Activities to try

1. 1

   Resize the shadow: a shadow is blocked light, not a copy

   Why it works: A shadow is the absence of light where an opaque object blocks light from reaching a surface; it is not a reflection, dark copy, or an object the thing carries around. Its size changes as the object moves between the light and the wall while the object itself stays the same size, which fits blocked light rather than a fixed copy of the object.

   1. Set up a light source, an opaque shape (e.g. a star or toy), and a wall or screen in a darkened space so a clear shadow is cast on the wall.
   2. Slide the object toward the light and watch the shadow on the wall grow larger; slide it back toward the wall and watch the shadow shrink — noting the whole time that the object itself has NOT changed size.
   3. State out loud: the object stayed the same size, but the shadow grew and shrank as the object moved between the light and the wall, so the shadow is not a fixed dark copy of the object — it is the part of the wall the object is keeping the light off.
   4. Keeping the object and wall in place, move the light from one side of the object (e.g. left) across to the other side (right) and watch the shadow on the wall move the opposite way (the shadow always falls on the side of the object away from the light).
   5. Swap the opaque shape for a translucent one and observe the shadow go pale, linking back to transparent/translucent/opaque materials.
2. 2

   Measured fair test: the distance–size pattern

   Why it works: Shadow size depends on the object's distance from the light source, not only on the size of the object; with a small nearby source (a torch), moving the object toward the light makes the shadow larger because it blocks a wider cone of spreading light.

   1. In a darkened room, use a torch, an opaque object, and a blank wall, with a ruler or tape measure to measure the shadow on the wall.
   2. Decide which single thing to vary — the object's distance from the torch — and keep the torch and wall fixed so it is a fair test.
   3. Before each trial, predict whether the shadow will be bigger or smaller, then measure the shadow and record object-distance against shadow-size in a results table.
   4. Complete a pattern sentence stem from the results, e.g. 'the closer the object is to the light source, the bigger the shadow'.
   5. Explain why: with the small nearby torch, closer to the source the object blocks a wider spread of the straight-travelling light, so the shadow grows.
3. 3

   Predict-explain-test: outline or copy?

   Why it works: The shadow can only be a featureless dark outline, because it is the region where the opaque object blocked light from reaching the wall — it cannot be a picture (face/colours) of the object. A clear/transparent object lets light through, so it does not cast a dark shadow like an opaque one; without a light source there is no shadow at all.

   1. Look at a toy standing in front of a lamp casting a shadow on the wall. Predict whether the shadow will show the toy's FRONT — its face and colours — or only its outline shape, and say why BEFORE testing.
   2. Also predict whether the shadow gets bigger or smaller when the toy slides toward the lamp, and say why.
   3. Test both predictions and explain what you see.
   4. Consider the counterexample: if the toy were made of clear glass instead, would it cast the same dark shadow? Explain your reasoning.
   5. Consider: could there be a shadow with no light at all in the room? What does a shadow actually need to exist?