Refractive Index: Ninja Fish

It's been a while since I promised to explain the Ninja Fish, and here it is, at long last. 

This is for you, sister. Sorry I couldn't get it animated.

You'll recall from our discussion of eyeglasses that light passing from a medium with one index of refraction into a medium with a different index will bend at the boundary. How much the light will bend is encapsulated in Snell's Law. 

That thing that looks like a scandinavian O is  the Greek letter theta, and it denotes the angle of the light as it enters and leaves a boundary. The n's denote the refractive index. If light passes from a high index to a low index material, it bends more, or away from the normal. If light passes from a low index to a high index, it bends less, or toward the normal.

Air has a refractive index nearly equal to 1 (vacuum has an index of exactly 1), while water has a refractive index of about 1.3. So any light bouncing off objects under water will have to bend more once it enters the air. 

Which brings us to the ninja fish. 

By which I don't really mean that the fish is a ninja, but I mean the classic gag about trying to catch  a fish with your bare hands (or a stick) by calm awesomeness. Like this scene in Mulan* (at 1:30)

Light, we wont bother with where it comes from for our purposes, bounces off the fish, which is under water, and bends, or refracts,  as it exits the water.

You, the observer, have a brain that does not account for changes in refractive index in the way it translates what the eye sees. It assumes whatever it sees is straight ahead, leading you to think that the fish is other than where it is.

So when you go to stab your delicious dinner, you end up stabbing the water instead, and catch nothing for your troubles.

One way to avoid this, if you could get the fish to stay still, would be to view it from directly above, as light that hits a boundary straight on will not bend.

That's it. This is the entire reason why stabbing where you think the fish is is a bad strategy. Its not that the fish moves. Its that the fish was  never there to begin with.

Next time, on Refractive Index: Fiber optics

*I know Mulan  takes place in China, and ninjas are Japanese. It was the most widely known example I could think of.