Why does the shadow get bigger at all?

Because the light spreads out from one point. Rays that pass either side of the object keep going straight and keep getting further apart, so by the time they reach the wall they are wider apart than the object is tall. Bring the torch closer and they spread more steeply, and the shadow grows. This is worth saying out loud, because it is the bit a class is really finding: a shadow is bigger than the thing that made it because light comes from a point and spreads. Sunlight arrives in a beam that is near enough parallel, which is why a shadow outdoors is roughly the size of the thing casting it and does not change when you walk the torch, so to speak, closer.

Why is the object up on a stand?

So its bottom is level with the torch's lamp, which is what makes the bottom ray run straight and level to the wall and the shadow land in a neat band. It is also how the experiment is set up in a classroom: a card in a stand, a torch on its side and a screen behind. Light passes underneath the object, which is why the floor behind it and the bottom of the wall are not in shadow.

Does the light really come from one tiny point?

Here it does, and that is why there is one answer. A real torch has a lamp a centimetre or two across, and light leaves every part of it, so a real shadow has a dark middle and a soft grey border where only some of the lamp is blocked. Worth showing a Year 6 class with an actual torch after they have used this.

What happens if the shadow is bigger than the wall?

The wall goes completely dark and the board says the shadow is taller than the wall, which is the honest answer. Move the torch back and the top of the shadow comes into view again.

Does making the object taller make the shadow bigger?

Yes, but not by more times. A 20 cm object and a 40 cm object in the same place both have shadows the same number of times bigger than themselves. It is a good prediction to take a vote on.

Is the picture to scale?

Yes, both ways and at the same scale, which matters here: a diagram that stretched one direction would draw a shadow the wrong shape, and the shape of a shadow is half of what this is about.

Shadow Investigation

Science · Year 3–Year 6

A torch, an object and a wall. Move the torch and the shadow changes size.

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About the shadows

A torch on the floor, an object in a stand in front of it and a wall behind. The light is blocked by the object, and the shadow it leaves lands on the wall. Drag the torch along the floor and the shadow changes size in front of the class.

Year 3 asks children to find patterns in the way the size of shadows change, and a pattern needs more than one arrangement. Move the torch towards the object and the shadow grows. Move the object back towards the wall and it shrinks. The numbers are on screen, so a class can record them and look for the rule.

The rule is worth finding: the shadow is bigger than the object by the same amount that the wall is further away than the object. That is why making the object taller does not change how many times bigger its shadow is, which is a good thing to predict before trying.

The object sits in a stand with its bottom level with the torch's lamp, which is how a class sets it up on a table. Light gets underneath it, so the floor behind the object stays lit and the shadow is a band on the wall rather than a block reaching the ground.

The two dashed rays are the Year 6 half of it: light travels in straight lines, and the shadow is exactly where those straight lines land. Turn them off to ask a class where the top of the shadow will be, and back on to check.

Hide the numbers to estimate first. Everything is in whole centimetres and the picture is to scale, so a shadow twice as tall really is drawn twice as tall.

How to use it on your whiteboard

1Drag the torch along the floor, or use the Torch buttons to move it 5 cm at a time.
2Drag the object to move it, or drag the gold dot on top of it to change its height.
3Rays hides the two straight lines, for a class making a prediction.
4Numbers hides the measurements, so a class estimates before measuring.
5Tap Fullscreen to fill the board.

Questions

Why does the shadow get bigger at all?: Because the light spreads out from one point. Rays that pass either side of the object keep going straight and keep getting further apart, so by the time they reach the wall they are wider apart than the object is tall. Bring the torch closer and they spread more steeply, and the shadow grows. This is worth saying out loud, because it is the bit a class is really finding: a shadow is bigger than the thing that made it because light comes from a point and spreads. Sunlight arrives in a beam that is near enough parallel, which is why a shadow outdoors is roughly the size of the thing casting it and does not change when you walk the torch, so to speak, closer.
Why is the object up on a stand?: So its bottom is level with the torch's lamp, which is what makes the bottom ray run straight and level to the wall and the shadow land in a neat band. It is also how the experiment is set up in a classroom: a card in a stand, a torch on its side and a screen behind. Light passes underneath the object, which is why the floor behind it and the bottom of the wall are not in shadow.
Does the light really come from one tiny point?: Here it does, and that is why there is one answer. A real torch has a lamp a centimetre or two across, and light leaves every part of it, so a real shadow has a dark middle and a soft grey border where only some of the lamp is blocked. Worth showing a Year 6 class with an actual torch after they have used this.
What happens if the shadow is bigger than the wall?: The wall goes completely dark and the board says the shadow is taller than the wall, which is the honest answer. Move the torch back and the top of the shadow comes into view again.
Does making the object taller make the shadow bigger?: Yes, but not by more times. A 20 cm object and a 40 cm object in the same place both have shadows the same number of times bigger than themselves. It is a good prediction to take a vote on.
Is the picture to scale?: Yes, both ways and at the same scale, which matters here: a diagram that stretched one direction would draw a shadow the wrong shape, and the shape of a shadow is half of what this is about.