physics aqa as level help

Can anyone explain 7c to me please.

http://filestore2.aqa.org.uk/subjects/AQA-PHYA2-W-QP-JUN10.PDF

I get how the angle of incidence is 63degrees, but I don't understand how you can just assume this is the critical angle? It tells you it goes under total internal reflection at P, but why does this have to be the critical angle? All 63degrees tells us is that the critical angle is less than 63? But it doesn't tell you by how much. I know it's just a one mark question, but I would still like to understand the reasoning. Thanks

(Btw the answer is 63)

The question says "At this point... the ray is totally internally reflected at P for the first time". The fact that it's the first time shows us that this is the critical angle.

But if it was the critical angle wouldn't it produce a angle of refraction of 90 degrees?

Totally internally reflected means that the incident angle is at the critical angle, yes it would produce a refractive angle of 90 degrees which is just a line along the surface. 90 degrees FROM the normal remember

doesn't totally internally reflected mean that it has exceeded the critical angle?

doesn't totally internally reflected mean that it has exceeded the critical angle?