More ideas about thin film interference

 

I might have worked out why the colours seem more like secondary colours with thin film interference. Here's the ray diagram:

I am sure that the red ray that reflects from the lower surface ends up parallel to the red ray that reflects from the top surface. That means that they can interfere. Now the blue ray should have the same but having had a bigger refraction inside the thin film. This must mean that the red and blue rays can interfere with each other. Some wavelengths of colour will cancel so that not all will constructively superpose so we don't get a white reflection but if a few colours do constructively superpose, that would account for the strange colours.