## Now you see it, now you don’t!

Posted: September 4, 2010 in Institute of Physics, Thoughts and ideas
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I saw this “Magic trick” performed as a lesson starter by one of the best Student Teachers I have ever observed, Bethan Rowland-Jones, who was at the time a Student Teacher in Swansea University. The lesson was an introduction to light aimed at an audience of yr 8 pupils and, as you can see from the video of the trick I reproduced below, she grabbed the children’s attention right from the start. The pupils were just spellbound!

This was an excellent icebreaker, especially because it generated many questions and discussions. But what is actually happening here? Well, there are a number of things that your students will notice.

First of all, while the level of water is rising the children can see the effects of light refracting from water to air, because it looks as if the coin is lifting up. However, they know this is impossible because the coin is under the glass and not in the water at all!

Then, when the water level is high enough, the coin seems to disappear. This is the effect of total internal reflection of light inside the water. At this angle the light reflected by the coin hits the walls inside the glass at an angle greater than the critical angle and it gets totally internally reflected back inside the glass. That is why we don’t see the coin anymore! What we see (at that particular angle) is the reflection of the wooden board on which the glass and coin are standing.

I cannot think of a smarter and simpler starter for this topic and I thought the lesson was outstanding!

There have been a few people who were not convinced by the TIR explanation, so I have added the video below and you can see how it works in this great simulation. The video should convince anyone, or at least any Physicist, that this cannot be explained in any other way than TIR as you get two reflections of the coin inside the tall glass. If the disappearing coin were an effect of merely refraction, we wouldn’t see any reflection inside the glass!

I recently come across a really nice, simple and quick Web 2.0 Tool called vozMe. It is a fun application that turns whatever text you write in the text box provided into an electronic voice message. I used it a lot to freak out my classes as they came in and to build some rapport by having a laugh at the beginning of the lesson. It is also quite useful to share the lesson objectives in a fun way with your pupils and it works magic when you use it to rebuke a misbehaving child (only the first two or three times though), because they actually listen to the voice, more than they would listen to you, and do what it says!

You can choose a male or female voice and also the language you want to write on. This will change the sounds the computerized voice will make for given letters and syllables. I can see how this could be used in MFL lessons to get your pupils to check their pronunciation of foreign words, etc. But I believe it could be used in English Literature as well to show the importance of intonation and emphasis of words! Try writing one of Shakespeare poems in the text box and play it back to the children to see their responses! And if you have big school Drama Productions you might just need a robotic voice in some of your plays.

vozMe is very simple and many could think quite pointless, but sometimes the simplest ideas are great and can work wonders. Have a go with your classes and let me know how useful, or not, this tool has been to you!