Here’s a really interesting activity you can do when studying light or materials (or both). In this activity, pupils investigate how light bends in different materials and how this can make some objects invisible. They will find out what happens when pyrex objects are placed in vegetable oil.

You will need

• 2 or more large Pyrex bowls
• 2 Small Pyrex bowls or beakers (or test tubes)
• Regular drinking glasses
• Water
• Vegetable Oil
• Newspaper, washing up liquid and paper towels for cleanup
• A funnel (if you want to get the oil back in the bottle to use again!)

 How to do it

1. Fill a large Pyrex bowl with vegetable oil, and another with water.
2. Submerge a smaller Pyrex bowl/beaker into the bowl of water. Can you still see it?
3. Repeat with the bowl of sunflower oil.
4. Which liquid makes the bowl invisible?

How does this work?

The Pyrex bowl will vanish in vegetable oil because it has the same refractive index as the oil. The light goes at the same speed in vegetable oil as in the pyrex. This means that the light isn’t reflected or refracted when it passes from the oil to the pyrex, so you have no way of seeing the bowl. There’s a better explanation here.

Note: This must be done with vegetable oil and Pyrex. Other oils or a regular glass have different refractive indexes, so they won’t be hidden. You can also use pyrex test tubes – these can work better than beakers as they don’t have any white lines marked on them!

As a challenge: Give pupils a range of objects and other liquids including lemonade, bubble bath, etc. They can then explore for themselves until they find the combination that makes the object disappear. Test different objects and liquids. Think about things like their material, colour, shape and size. Make notes about which work and which don’t.