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

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!


  1. This is a nice demonstration and is part of the physical basis behind the H.G Wells story “The Invisible Man” – see chapter 19:

    “And if you put a sheet of common white glass in water, still more if you put it in some denser liquid than water, it would vanish almost altogether, because light passing from water to glass is only slightly refracted or reflected or indeed affected in any way.”

    Of course by denser he mean optical density rather than mass density. This might be a good starting point for a discussion on invisibility and how it might be possible. If you were invisible in air how visible would you be in water? What would you need to do to make a non-transparent object invisible? Meta materials – here we come!

  2. Me again!

    Something else to consider is that because you can more easily see the test tube in some situations then this means that light is being reflected/refracted and lost from the “direct line of sight”. This could introduce the idea of an optical coating on a lens to improve performance (your camera lens will probably have a slight purple colour to it). I believe moths’ eyes have antireflective microstructures to achieve the same effect, although here it is to stop reflections giving them away to predators.

  3. Sally Divall says:

    We do this demonstration with a small beaker inside a large one rather than a test tube – although the beaker disappears you can still read the writing.

  4. William oops Jonathan Wallace says:

    I think you’ve been watching too much braveheart! Although I’m probably related to the guy some way back. My yr 10’s loved the demo today! another way to make science cool!

  5. Suresh Chandra says:

    A few years back a colleague took a test tube, asked a student to break it into pieces and put the broken tube into a magic liquid ( which was conola oil or peanut oil ) and then asked the class to chant some silly words to pray for the test tube to be repaired. After a couple of minutes he took forceps and took out an intact test tube. After a few minutes he took an other tube and immersed in oil to show them the test tube disappear as it went into the oil.

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