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Coin Experiment – Coin in Water

Did you know that water can play tricks on your eyes?

This experiment is incredibly simple and requires only a glass, water and a coin of your choice.

It demonstrates a special property of water, called light refraction.

If you put a coin inside a glass of water, you will be able to see that it appears in two places at once.

It’s as if a mirror were used to show the coin to both the front and the back of its surface at the same time.

It’s almost like you have doubled the money!

Try it. It’s a cool trick.

Refraction Optical Illusion | Double The Coin | Science experiment

Coin Experiment - Coin in Water

Prep Time: 10 minutes
Active Time: 20 minutes
Total Time: 30 minutes

This experiment makes use of the optical property of water and air.

Materials

  • water
  • a penny (or any coin)

Tools

  • a clear glass
  • adult supervision

Instructions

  1. Put a penny into an empty glass.
  2. Position your head so that you see the coin from the side of the glass (not from above).
  3. Slowly pour water into the glass until you see a second coin appear on the other side of the glass when you see from above.
  4. Move your head up and down and see how one penny becomes two and vice versa. penny at bottom of glass with water shows a reflection

Did you try this project?

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Why

Light bends when it passes from one medium (water) into another medium of a different density (air).

This bending of light, called refraction, causes the apparent position of the coin to change to a shallower position​1​ (see diagram).

The light bends when it passes from water to air on top of the water.

It does the same thing when it passes through the glass, making the coin appear to be closer to you.

As a result, you see two images of the coin.

An illustration of refraction of light from a coin to the eye, turning the image of 1 coin into 2 coins.
Glass of water with a penny inside, but you can see 2 pennies including the refracted image. Refraction Optical Illusion | Double The Coin | Science

References

  1. Jiang W, Chen RT, Lu X. Theory of light refraction at the surface of a photonic crystal. Phys Rev B. June 2005. doi:10.1103/physrevb.71.245115
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