Fat films

Friday 23 January 2015

We're always talking about thin films but what about fat ones? At 5m thick, the film simulated in my latest GeoGebra adventure can hardly be called thin (actually it's variable thickness from 0 to 10m so 5m average). I thought this one would be easy but it turned out much more difficult than I thought (until I thought of using the built in reflection function). The problem is that the wave equations are based around the origin but the reflected wave originates from the reflector, as the reflector moves so does the origin of the wave. I tried to model this by fixing the phase of the reflected wave so that it changed as the reflector moved however it only gave the right result for multiples of λ. Then I thought of making a function for the wave after the reflector and reflecting it in the errr, reflector. It worked.

Today I worked out how to make the phase change to fit a moving mirror. Quite satisfying to solve the puzzle. In the end reverted to maths solving the equations such that both waves have the same phase at the mirror. A neater solution than using reflections and one that allows the sum of the reflections to be calculated.


Magic moment
24 Jan 2015
3D in GeoGebra
20 Jan 2015