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Make a Coin Disappear!


Introduce your kids to the magic of science with this easy, yet fascinating experiment! Make a coin disappear and learn about how light travels in waves.

disappear coin

What to get:  

  • Two coins
  • Two jars or glasses
  • Water
  • Tape

What You Do: 

  1. Place one jar (or glass) on top of one coin.
  2. Fill the glass with water and watch the coin disappear.
  3. Tape the other coin to the inside wall of the jar.
  4. Fill the glass with water. Can you see the coin? Does it look larger than before?
  5. Rotate the glass while looking at the coin from the side. Did it disappear?

The science behind the experiment:  

As light waves travel through different substances, their speed changes, which changes the light’s direction and the light is “bent” or refracted. (Think of how your speed changes when you try to move quickly in water, and the density of water slows your movements.) What we see when we look at an object is the light that is bounced back at us (reflected) as it bends (refracts) off the object.

The water slows down the light and changes the angle at which it bounces back to our eyes, making the coin under the glass invisible. The bending of light also magnifies things, so the coin taped inside the jar looks bigger. The jar and water act like a lens, bending the light rays inward to a focal point, where the light rays come together. But the light continues past the focal point, and the rays pass each other, so the light that was on the left side is now on the right, and vice versa.

Scientists ask questions:

  1. Does the size of the coin make a difference?
  2. What happens if the water is warmer? Colder?
  3. Does the size of the jar change the result?