Door 22: The disappearing glass

You need:

• A large glass bowl
• A bottle of rapeseed oil
• Some test tubes

Recipe:

1. Fill a bottle of rapeseed oil in a clear bowl
2. Take a test tube and fill it with rapeseed oil
3. Insert the test tube into the bowl

You do not see the test tube as the rapeseed oil and the glass have approximately the same refractive index. Refractive index is a measure of the material's ability to conduct light. Glass has a refractive index of approx. 1.5, i.e. when light passes through glass, the speed of light is 1.5 times slower than in vacuum. We put a whole test tube into the bowl with rapeseed oil. Then we broke a test tube and left the pieces in the bowl. That way, you can fool people, when you pick up the whole tube. It looks like you can do magic!