Law of Refraction

My husband’s late grandfather was a fascinating man and a brilliant engineer. When I first met him, he asked me if I knew about Snell’s law. At the time, I was still in college and hadn’t taken physics yet, but it sounded familiar. When he started to explain it to me, I recognized it as the law of refraction. I remember his eyes lighting up when I caught on. Later, I learned that it was one of the ways he judged a person, and he had deemed me worthy.

I bring up this physics term because it also relates to water.

The easiest way to explain Snell’s law is with a physical example.

Take a smooth, clear glass and fill it full of water. The glass or cup can’t have any ridges or funky shapes in the glass, or the demonstration might not turn out right. A pub-style pint glass works well. Next, find a straw or a pencil—I recommend an eco-friendly steel or biodegradable paper straw—and put it in the water. Now, spend some time looking at the glass from different angles. Look from above and at eye level with the water line, and move the straw around.

What you should see is that the straw doesn’t appear perfectly straight. Sometimes the submerged half of the straw seems slightly thicker. Sometimes the two ends don’t line up, and there may be a slight bend in the straw that isn’t actually there. From above the end of the straw might look a little curved to one side.

The law of refraction governs how light bends or refracts as it passes from one medium to another, like from the air to the water. This law explains why things are not always where they appear to be in the water as seen from the air.

Luckily for us, we have a straight forward equation we can use figure out the refracted angle, though you may need to do some more independent reading to understand why it works. Other creatures don’t have mathematical formulas to help them, though.

For example, an osprey flying over a body of water will have to learn how to accurately find its prey underwater or it will starve. From the air, a fish may appear to be in one spot but may actually be a few feet to the right. If the osprey misjudges where the fish is the first time and doesn’t catch it, then its chances of getting a fish the second time are greatly reduced because it lost the element of surprise. Birds of prey must learn to adjust to this optical illusion, a failed attempt at catching prey is a waste of energy.

Another example is the Australian archer fish, a fish that has developed the ability to spit jets of water at bugs on overhanging tree branches. Archer fish learn to do this because during the drought season, their normal food supply may become scarce, so they spit at bugs to try and knock them into the water to eat.

The same distortion exists for the archer fish below as it does for birds from above. The bug that the archer fish wants to knock off its branch may actually be three inches to the left instead of straight above the fish. So, through trial and error, archer fish learn to calculate where the bugs are above the water.

At the aquarium I volunteer at, one of my favorite activities is talking to guests about the archer fish’s ability to spit water at bugs on overhanging branches. Occasionally, we’re allowed to demonstrate this by getting a live cricket on a stick that we extend over the exhibit. Very quickly, little jets of water are arching out of the water as the fish try to knock the cricket into the water.

The fish that successfully hits the cricket isn’t always the one to eat the cricket. In fact, sometimes the pig-nose turtle in the tank gets the cricket. And sometimes, the volunteer (reads as: me) doing the demonstration gets spit in the face by the archer fish. But who can get mad at that? It actually made my day!

Sources:
https://www.physicsclassroom.com/class/refrn/Lesson-2/Snell-s-Law
https://www.math.ubc.ca/~cass/courses/m309-01a/chu/Fundamentals/snell.htm
https://www.britannica.com/science/Snells-law

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