Guest post: Alice shows magic tricks and explains refraction of light in water

My friend Alice Langhans runs a super cool science communication Instagram (@edu_al_ice), where she posts about her experiences as PhD student in physics education research. And there is a lot more going on on that Instagram than just pretty (but oh so pretty!) pictures. I make sure to read all her posts, because there are always interesting, motivating, inspiring thoughts hidden behind that “read more” button. And now she’s even started a new series of physics experiments on #experimentalfriday, and I am super excited that she wrote this guest post for me!

But now look at the picture below, and then read about some magic! :-)

Alice writes:

Magic! One of the arrows changes its direction and here is why:

Click for large picture. Picture by Alice Langhans.

First, the arrows are unchanged and visible through the glass.

Click for large picture. Picture by Alice Langhans.

Adding water to the glass, the image of the arrow gets bigger and appears mirrored!

Click for large picture. Picture by Alice Langhans.

With even more water even the second arrow appears bigger and mirrored.

Click for large picture. Picture by Alice Langhans.

The waterglass I used is round and the refraction of light in water is different than in air, which makes the water glass act like a positive (converging) lens. This is why the image of the arrow appears bigger and mirrored.

Think of the arrow as many points, each of which is the source of a divergent bundle of light. The light coming from the point that is the arrowhead on the right, is refracted through the waterglass and reaches our eye to the left. The light from the left end of the arrow refracts in such a way that it now enters our eye on the right side.

Notice, how you can also see how the upper arrow appears even bigger? The glass is more wide at that height, magnifying properties of the water glass lens are therefore increased.


Isn’t that a super nice demo? I love it! Thank you for writing this guest post, Alice! :-)

P.S.: Alice has just been interviewed for a podcast. Curious what she’s talking about on there? Me too, but that’s why I follow her Instagram (@edu_al_ice) — to never miss out on all the cool stuff she’s up to! :-)

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