# Critical angle/ last angle of refraction

1. Jan 28, 2009

### davidelete

1. The problem statement, all variables and given/known data
A piece of glass has the shape shown in the below diagram. The index of refraction of the glass is 1.45. Find the critical angle for the glass. Sketch the path of the light ray until it emerges from the side of the figure and find the last angle of refraction.

http://img179.imageshack.us/img179/6531/physicsimageez8.png [Broken]
Sorry if the image is a little ugly...
2. Relevant equations
Snell's law

$$n_1\sin\theta_1 = n_2\sin\theta_2\ .$$

3. The attempt at a solution
sin-1($$\frac{1}{1.45}$$)=43.6 degrees.

I have no idea on how to draw in the diagram. Could someone help me on drawing it? Mine just looks like a bunch of rays bouncing around inside an irregular pentagon on my paper when I try it.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

Last edited by a moderator: May 3, 2017
2. Jan 28, 2009

### astrorob

Are we told the angle at which the light is incident on the glass? Is it incident normally (at 90 degrees)?

3. Jan 28, 2009

### davidelete

That is truly everything. I would just have to assume 90 degrees based on the information given.

4. Jan 28, 2009

### astrorob

I don't understand how you're expected to solve that without the incident angle and also the angle at which the glass is sloped on the left hand side.

5. Jan 28, 2009

### davidelete

I suppose it is more of a best guess type of thing. What would your best guess be when drawing a light beam coming in? Where would it bounce to? Where would it exit?

6. Jan 28, 2009

### astrorob

If you don't know what angle its entering the medium relative to the normal then how are you supposed to know if it will internally reflect? It could either exit the medium on the second face or bounce around inside for a while...

As a best guess, and this seems so entirely unphysical that it should have a disclaimer, i'd assume it enters the medium normally, and assume that the angle of the slope is 45 degrees. Since this is a larger angle than critical, it's going to bounce off the lower face, then off the top face, then off the right face, then off the bottom face, and then exit normally out the face it came in through.

7. Jan 28, 2009

### davidelete

Thank you. That was exactly what my best guess looked like as well. Some of the questions on these homeworks are outrageous. I wish they gave more information.