Posts Tagged ‘normal’

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!

You must have guessed I have a soft spot for comics and Superheroes by now. So, when @russeltarr (look at his excellent website here) twitted a link to a video made by his 14-years old Historians (as he calls them) I got immediately interested in this new video creator with animated characters; xtranormal.com. Believe me, it is really good fun and very easy and quick to use. The below video took me about 15 min to make and it was a simple attempt to create an example for my Yr 10 pupils, so that they could also use this tool to create fun videos about an area of Physics we have studied (if you can’t see the video from this blog click here). Because we have been amused by the Physics of Superheroes in a number of lessons, I though they would like the video I created to understand and remember the difference between Speed and Velocity!

The intent, however, was never to use this video creator as a teaching tool, but rather as a lovely way to get my pupils engaged with Physics and to get them talking about processes by explaining them through unusual situations (a bit like the Marvel Comic on Momentum).

Unfortunately there were two problems I had not anticipated: