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Could Harry Potter’s Invisibility Cloak really make someone invisible?

I’ve been thinking lots of good science thoughts this week because I’m dreaming up a new and exciting project.

Science is pretty cool, and science for kids is even cooler. Many of you know I used to perform kids science shows as part of the Shell Questacon Science Circus and for Science on the Move. I’m also a past editor of Australia’s Scientriffic magazine, now Double Helix.

(If you want awesome science activities and news delivered to your inbox every week for free, subscribe to the FREE Science by Email…it’s brilliant).

But, back to the title of this post:

Could Harry Potter’s Invisibility Cloak really make someone invisible?

And the answer is…


If your eyes are open and the lights are on, you can see most things. Objects are visible because they reflect or bend the light that hits them.  If you can see something, it’s because light is bouncing from that something into your eyes.  Some things — like air or water or glass — are transparent or translucent, so instead of reflecting all the light that hits them, some of that light can shine right through them. Transparent things are harder to spot, but there is one giveaway: light usually bends as it shines through an object.

The speed of light in a particular material is constant.  However, when light moves between two different materials, it usually changes speed.  This change in speed causes the light to bend, and our eyes can detect the change in its direction.

For example, when light moves from the air into a raindrop and back out again, the light changes speed, which causes the light to bend.  Our eyes can see this bend in the light, which is why we can see raindrops falling in the sky.

Each material bends light by a particular amount.  We call this amount the refractive index of that material.  If the refractive index of rain was exactly the same as the refractive index of air, light wouldn’t bend as it went through a raindrop falling in the sky.  And if the light didn’t bend, our eyes couldn’t see the raindrops falling at all – they would be invisible!


If the refractive index of Harry’s cloak was exactly the same as the refractive index of the air in a room, the cloak would be invisible in that room.


Even if Harry’s cloak was invisible, we would still be able to see Harry underneath it!  Harry’s body would have a different refractive index to the air, and to his cloak, which means he would still be visible behind his cloak in the same way that you are still visible behind a glass window.  For the cloak to make Harry invisible, it would need to change the refractive index of Harry’s body to exactly match the refractive index of the air.

Impossible? Yes. But only in a world without magic 😉

Question to think about: Would Harry’s cloak still be invisible if you looked at it underwater?

And of course, an activity to try at home!

Matching refractive indices

This experiment demonstrates that if light passes through two media with equal refractive indices it will not bend nor reflect at the boundary.

  1. Put a small Pyrex bowl inside a larger Pyrex bowl.
  2. Pour Baby Oil into the small bowl till the oil overflows into the large bowl. The refractive index of Baby Oil is nearly equal to the refractive index of Pyrex glass. The small bowl should become practically invisible.
  3. If you have a glass eyedropper try putting it into the oil.  It will be easy to see because of the difference between the refractive indices of air and glass.
  4. Try sucking up the oil into the eyedropper.  The eyedropper should become almost invisible because the refractive index of the glass is nearly the same as that of the oil.  When light passes between the oil and the glass it is only bent a little, and the dropper appears invisible.

Note: If you don’t want to waste the baby oil, use the glass to pour it back into the bottle. Make sure you do this over the bowl so it doesn’t get everywhere.

This post is a revised version of my original article, first printed in The Helix magaazine.

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