Ingredients

Two torches A couple of torches A window A window that is dark on both sides, with a friend on the othe side.

Instructions

You stand one side of the window and your friend the other, each ot you hold a torch.

Turn off all the lights so it is dark on both sides

Turn on your torch, what do you see in the window?

Turn off your torch and turn on theirs, what do you see?

Try turning on both.

Result

You should be able to see yourself when you turn your torch on, them when they have their torch on, and mostly them when you both have a torch on.

Explanation

This experiment works because whilst glass is very transparent it will also reflect back some light. So probably 95% of the light goes straight through and maybe 5% reflects back. You don't really notice this when both sides of the glass are bright as the light going straight through is so much brighter than the reflection.

Light hitting a window WIndow with both sides bright
When light hits glass most goes straight through, but some is reflected. If both sides of the glass are bright then the reflection is overwhelmed by the light coming straight through.

But if the far side is dark then there will be no light coming from there, so the only thing you can see is the reflection. Similarly if your side is dark then there is no light hitting the glass to form the reflection, so you can't see it.

When you are dim A window when they are dim
If your side is dark then there is no light to form a reflection But if ther far side is dark the only light coming from the window will be your reflection.

This is why you can't see out of windows at night, because it is brighter inside or into them during the day, as then it is brighter outside.

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