[علي أشرف محروس]

In problems concerning Prism to know the refractive index ; some books apply the following
n=sin phi one/sin theta one= sin theta two/sin phi two

but sometimes in the answers they apply this law or use n=sin phi two/sin theta two instead of it

why?
and according to what in the problem ? given that the problem give no information to reach this method


2- Chapter 4

what is the mechanism of the U-shaped tube balance ? when we add oil to the water then the oil pushes the water and we take the separating surface .....is this correct meaning?


thanks for ur help