Glass of water with a rock floating in it.Find out more about invisibility by hiding a glass jar in plain sight. Then, use the jar to create a clever illusion, making a rock appear to float mysteriously!

hot hazard iconSafety: This activity is for ages 8+. It involves glass in slippery conditions, so handle it carefully. Wear enclosed shoes so if you do accidentally break glass, you don’t step on the pieces! If there’s a break, ask an adult to help clean up immediately.

You will need

  • Drinking glass
  • Very small glass jar (e.g. from a tiny pot of jam, honey or anchovy paste, also available from some discount stores)
  • At least 200 mL glycerine, also called glycerol
  • Small rock
  • Funnel

What to do

  1. Put the small jar into the glass.
  2. Image of someone pouring glycerine from a bottle into a small jar inside a glass.Carefully pour glycerine into the small jar until it just starts to overflow into the glass. Have a look. Do you notice anything interesting?
  3. Glass jar inside a glass with some water in the glass, half submerging the jar.Fill the glass until the jar is half covered. Have a look. Do you notice anything interesting?
  4. A glass jar filled with water with an invisible glass jar inside.Add more glycerine until the jar is completely covered. Take a look. Can you see the jar anymore?!
  5. Glass jar half filled with water with what looks like a rock floating on the water.For a cool illusion, you can gently balance a heavy-looking object, such as a small rock, on top of the jar. It will look like the object is floating!


Clean up

Glycerine is non-toxic, but it’s not good to pour lots of it down the sink.

  1. Put the funnel in a container that you can throw out. If you used all the glycerine in the bottle, you can put the funnel in that bottle instead.
  2. Carefully take the jar out of the glass and empty the glycerine into the funnel. Then put the empty jar in the sink.
  3. Empty the glass into the funnel, and then put it in the sink.
  4. You can wash up the jar and glass as you would any kitchenware.
  5. Throw away the glycerine in the container. Or you could use some of the glycerine to make bubble mix!


What’s happening?

Glass is clear and colourless, so how do we see it? If you look through a glass, the background looks bent or distorted. It’s this bending that we see, not the glass itself. It’s the same thing with water or glycerine or any clear, colourless substance. You can’t actually see the thing itself, only what it does to the image of other things.

This is all to do with something called the refractive index. This number measures how slowly light moves through a substance – larger numbers mean light moves more slowly through the material. Air has a very small refractive index, water is higher, and glass even higher again. Gemstones such as diamonds have a very high refractive index, which is one reason they look so sparkly.

When light moves between two substances, it gets bent. The amount it gets bent is based on the difference between the refractive indexes of the object it’s leaving and the object it’s entering. So light going from air into water gets bent, but not as much as going from air into diamond.

In this activity, light goes from air to glass, and from glass to glycerine. There’s a big difference between the air and glass, so you can see the outside of the glass. The refractive indexes of glass and glycerine are very similar, so it’s much harder to see the edges of the jar inside them!

For more on hiding in plain sight, check out ‘Inventing invisibility’ in Issue 25 of Double Helix magazine!

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