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Light and Shadows – Apply the correct rules

Before you read on, you might want to download this entire revision notes in PDF format to print it out for your child, or to read it later.

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These are scientific knowledge about light and shadows.

 

To make the shadow bigger, place the torch closer to the object OR place the screen further away from the torch OR place the object closer to the torch.

 

To make the shadow smaller, place the torch further away from the object OR place the screen closer to the torch OR place the object further away from the torch.

 

Do the rules apply to all situations?

No.

Let’s look at this example

Position A

Ali is walking along the pavement at night. He is at position A and is walking towards position B.

At position A, the light source is closest to Ali.

 

Wait a second.

 

I learnt that shadow is the biggest when the object is closest to the light source.

Why is the shadow shortest / smallest at position A?

 

Let’s visualise how light rays travel:

We learnt that a shadow is formed when light rays are / light is blocked by the person.

The light rays (as shown with the yellow coloured arrows) travel all the way to the ground. Some light rays are blocked by Ali. The light rays that are blocked by Ali will form a shadow underneath him.

Position B

Now, as Ali walks further away from the lamp post, the shadow gets longer.

Why is that so?

 

Let’s visualise how that works by looking at the diagram below:

At position B, the light rays travel above Ali’s head. As light travels in a straight line, the light rays reached the ground far away from Ali.

Other light rays are blocked by Ali. As a result, a shadow is formed in front of Ali as shown in the diagram above.

Conclusion

As Ali is walking along the ground from position A to B, more light rays are blocked by Ali and the shadow is longer.

OR

As Ali walks further away from the lamp post, more light rays are blocked by Ali and the shadow is longer.

Learning pointers

When you see a question with the following condition: the object is moving along the screen / the ground. We have to use a different set of rules.

Alternatively, you can always draw the light rays as I did on the diagrams.

That way, you can visualise how light travels and why is the shadow longer or shorter at different positions.

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Talk again soon!

Andy Ling

 

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