Exploring Snell’s Law

“Look – the paddle has a kink!”

Admittedly, I don’t have a paddle for this blog post. But I have a pen. That’s basically the same when it comes to optics. And today I did not only prepare one, but TWO experiments for exploring Snell’s Law. And they are great for younger students because they both seem like magic. And thank you to Mirjam (@fascinocean_kiel) and her great idea for this post – without her niece I wouldn’t have thought of how fascinating this phenomenon can be even for younger children.

Is this pen really broken? Where does the kink come from?

Instead of a pen, every straight object could do. A spoon, a straw, a ruler. Or even a paddle. This makes the experiment so great, because you can even explore it outdoors. And even though the explanation is a little hard to grasp, even young children notice this phenomenon. It’s not only a great opportunity to learn about the physics content but also about observation and inquiry. Which I absolutely adore about experiments.

The reason for the seemingly broken pen is because the light is refracted. And here are several concepts coming into the game:

First, the idea of seeing itself. The light coming from the light source in the room is reflected by the object it hits. The reflection happens in every possible direction and some of the reflected light reaches our eyes. This is how we eventually see anything in the world.
When light reaches our eyes, we still need to convert the information from our retina into an image. This happens in the brain. You might have heard how our brain turns the image around so that we don’t see the world upside down, because this is how the image gets projected onto the retina. Keep that in mind, that we still need our brains to see anything.

The second concept is the refraction. Light moves with a certain speed (the speed of light, you might have heard about that) but in other materials than air, it moves slower. Water is one of those materials. And with this slower speed comes the refraction. The other way around works the same – light coming out of the water into the air gets refracted as well.

Now let’s put these together: The light being reflected from the pen gets refracted when it passes from the water to the air. And now something interesting happens. Our brains cannot put together this concept of refraction. In our mind, we lengthen the light rays backwards in a straight line. And that is why the underwater part of the pan looks a little closer to the surface and the pen looks kinked. It isn’t really broken, it just appears closer.

Black lines represent the way the light travels after being reflected by the object. The dottet lines is how the brain makes up the image of it.

There is another really great experiment showing this phenomenon. And it might be even more magical than the seemingly broken pen. Take a look at the video and note your observation. How does this fit together with the explanation I just gave for the kink in the pen?

The coin is fixed with tape so it won’t move when I pour water into the cup.

Let’s take a closer look at what happens:

The coin is fixed with tape in the middle of the cup.
Now let’s move the cup until we don’t see the coin anymore.

Remember, I taped the coin to the bottom, so it’s really not floating. You can try yourself at home if you like!

And suddenly we can see the coin again. This is another example of refraction and optical lift of the object in the water.

This is no magic but simple physics. You can draw the light rays the same way I did for the pen. It’s like getting the ability to look around a corner. Isn’t that just amazing?

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