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