# Maximum angle of the refracted light beam

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1. Jun 19, 2016

### Tardis Traveller

1. The problem statement, all variables and given/known data
A light ray falls from the air ($n_a=1$) into the center of the upper surface of a long cilindrical glass tube with an index of refraction $n_t=3/2$. The tube is submerged into water all the way to the upper edge and the waters index of refraction is $n_w=4/3$. What is the maximum entry angle of the light ray at which it will only be traveling inside the glass?
a)$arcsine(1/6)$
b)$arcsin(\sqrt{15}/6)$
c)$arcsin(\sqrt{17}/6)$
2. Relevant equations
3. The attempt at a solution
I gues the snells law must be used $n_1sin(x_1)=n_2sin(x_2)$ but i dont get the problem. What is meant by maximum entry angle at which the beam will be traveling inside the cylinder only and how do i get that? I dont know my starting points. Could you hint?

2. Jun 19, 2016

### haruspex

The light ray strikes the top of the cylinder at some angle. Refraction rotates it to a steeper angle within the glass. What might happen next?

3. Jun 19, 2016

### Tardis Traveller

I thought about it and i think the refracted ray inside the glass should come to the edge at a critical angle so that all of it gets reflected to the glass again and so on right?

4. Jun 19, 2016

### haruspex

Yes, though I'm not sure what you mean by "and so on" there.