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 firstname.lastname@example.org
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!