## Flipping arrow – awesome refraction demonstration

Posted: April 26, 2014 in Institute of Physics, Thoughts and ideas
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When @CardiffScience posted (on Google+) a video demonstration of an arrow drawn on a piece of paper that flipped direction when seen through a glass of water I knew I had to try it myself and write this post. The video below shows the demonstration which is pretty neat, but carry on readying below the video for what I think is the explanation.

As some of you might know, I am one of the Editors of Talkphysics.org with David Cotton and he posted the below photo on this thread, which is think is a convincing explanation of what goes on in my video of the flipping arrow. If you trace the path of the three rays in the Dave’s photo you can see the ray that start from the top slit from the ray box ends up at the bottom on the multimeter. This is essentially what is happening in the video, so the light reflected by the right side of the arrow gets refracted by the water inside the glass and ends up on the left when it reaches the camera. Looking at the photo above though gave me an idea, i.e. “If I go close enough to the glass I should go beyond the focal point of the glass lens and see the arrow flipping again!” – WRONG! That didn’t actually happen. However, I just noticed that Dave’s liquid was Glycerine (at least if the name of his image file tells the truth), so I wondered whether the refractive index of glycerol was such to cause less bending inside the glass, but I was wrong again. In fact, water has a refractive index of 1.33 and glycerol of about 1.47, so there should be more bending of light inside the glass. I still haven’t figured out why I can’t flip the image again if I go close enough to the glass, but I still think it was worth posting this article and if you know the answer, please leave a comment! Thanks!

It is quite amazing what you can learn by a simple visit to your old school (well I am on a Secondment, so it is still my school…). And it is quite scary, because I got this really cool demonstration by the guy who is covering me for this year and I am starting to fear they will want to get rid of me to keep him :-S

His name is Jonathan Wallace and he is an NQT at Croesyceiliog School (Cwmbran in Sunny Wales) you can contact him at jonny.wallace@live.co.uk

Anyway, have you ever seen the trick of the jelly marbles disappearing in water? Well that happens because these marbles are superabsorbent polymers that get filled with water when the come in contact with it, so when you put them into water they seem to disappear, because, being filled with water they have the same refraction index as the water surrounding them, i.e. light goes straight through them without being refracted (bent)! There is a really nice explanation of this phenomenon on Steve Spangler’s blog and you can buy these jelly marbles quite cheaply here.

But what William (Oops, I meant Jonathan) showed me a really nice twist, especially because it uses items that are a bit more familiar to the kids than some superabsorbent polymers, although they are really cool! William (Blow! I’ve done it again, I meant Jonathan) pours glycerine in a Pirex beaker and an empty (and very clean) test tube inside.

At this point you can still see the test tube inside the beaker, because the air inside the tube refracts the light going through it! But what would happen if we add Glycerine inside the test tube too?

Magic! The test tube disappears in the Glycerine! So, has the Glycerine dissolved the glass of the test tube, is it real Magic, or just another wonder of Physics? What does really happen here?

The answer is quite simple and it is very similar to the jelly marbles. The Pirex and Glycerine have the same (or at least very similar) refraction index and, therefore, light is not refracted at their boundaries and carries on through its path undisturbed by refractive effects, which means that the test tube appears to be invisible!

Thanks to William Wallace (again? Sorry, I meant Jonathan; I know it’s not funny if you are not a member of staff at Croesy, but I have to take the mick) for this great demonstration!