Help~total internal reflection

Which bit are you stuck on?
You know how reflection works? You know Snell's Law?
Or is it you don't know how adding water to the bottom makes the button reappear?

 

TIR depends on the ratio of the refractive indexes ... what happens to the refractive index when you add the water?

 

You can tell more than that. What happens to the ratio of the refractive indexes and what does that mean for the critical angle for TIR?

You need to draw a ray diagram of what is happening in the TIR case, and then repeat for if there is water under the beaker.

There are also youtube demos.