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If you’ve ever heard the story of The Emperor’s New Clothes, you’ll remember the big twist ending: the Emperor’s expensive suit of clothes turns out to be nothing at all! Now, in the story, his tailors convinced him to pay a lot of money for “invisible” clothes and he gets humiliated as a result. In the real world, though, scientists are working to make things invisible! And no wonder. Whether it’s the Emperor’s clothes, the cloaking devices from Star Trek, or Harry Potter’s invisibility cloak, we’ve been fascinated with invisibility for a long time. With this experiment, you’ll make something invisible by using a simple trick of light!
Materials
- A large glass pitcher or bowl (make sure it’s clear, and Pyrex works best)
- Baby oil
- Water
- A small glass that can fit all the way inside the big one (make sure it’s clear, too, and Pyrex is best here, as well)
- A kitchen cloth or paper towels
Process
- Fill the large pitcher or bowl with water.
- Carefully place the small glass inside. Can you still see it through the clear water?
- Pour out the water and thoroughly dry both the bowl and the small glass.
- Fill the large pitcher or bowl up with baby oil until it’s not quite as high as the height of the small glass.
- Carefully place the small glass into the baby oil. Look through the side at it. How much of it can you see when the baby oil doesn’t fully cover it?
- Pour in enough baby oil to completely cover the small glass? What happens? How is it different from when the glass was covered in water?
- Look at it from lots of different angles. Are any better for seeing the small glass? Why might that be?
Summary
It’s pretty crazy how hard it can be to see the smaller glass when it’s completely submerged in baby oil. The water and the baby oil are very different, which you may have noticed if you accidentally got any of them on your hands. They’re both clear, though, so how come one makes the glass invisible and the other doesn’t? It all has to do with the way light moves.
Whenever light goes through something, it slows down and gets bent a little bit. This is called refraction, and you may have noticed it if you’ve ever put a straw into a glass of water: it looks bent or even broken. That bend in the straw is due to the bending of the light as it passes through the water. Different substances bend light different amounts, and the amount a given substance slows and bends the light is called the “index of refraction.” Water has a different index of refraction from glass, so the light bends three times: when it hits the big glass bowl, when it hits the water, and when it hits the small glass inside the water. That makes the glass inside very visible.
The baby oil and the glass have indices of refraction that are much closer. If you managed to find some Pyrex, they’re almost identical. When the light passes through, then, it bends once at the outer glass, once in the water, and then hardly at all when it hits the inside glass. Since the light pretty much continues straight through the water and the inside glass, it makes the inside glass look almost invisible.
Scientists who are working on making things invisible are working with a similar idea: if light doesn’t reflect or bounce off of something, before it hits your eyes then you can’t see that thing. Some scientists are using tiny video cameras and tiny screens to make clothes that show someone’s front on their back and vice versa, which simulates light passing right through them. Some cutting-edge scientists are figuring out ways to make light bend around things without even hitting them at all! Anything that the light flows around like this would be totally invisible, just like the Emperor’s clothes!
Okay, let’s be honest. He wasn’t wearing anything at all.
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