# Light ray passing thru plastic block

1. Nov 26, 2012

### herbally

1. The problem statement, all variables and given/known data

A light ray passes downward into a block of transparent plastic with an angle of incidence of θ1 = 77.7°.

If total internal reflection is to occur when the light strikes the left edge of the block at interface B, what is the index of refraction of the plastic?

2. Relevant equations

Snell's Law - n1sin(θ1)=n2sin(θ2)

sin(θcrit)=n2/n1

3. The attempt at a solution

I thought that I could use the law of refraction at interface A and the equation for the critical value at interface B (since total internal refraction occurs there), and use those two equations to solve for n2. I've done all manners of substitutions, but nothing that yields a reasonable answer.

I would really appreciate a nudge in the right direction.

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2. Nov 26, 2012

### TSny

Hello.
Your outline of how to approach the problem sounds good. Can you show your work so we can see what you've done?

3. Nov 26, 2012

### herbally

I solved Snell's law for sin(θ2) which yielded n1sin(θ1)/n2.

I set that equal to n2/n1. Then I solved for n2 which gave me n2=+-n1√sin(θ1).

When I solve I get some value like .98755 which is less than one making it obviously incorrect.

4. Nov 26, 2012

### TSny

I think you're using the same symbol θ2 for two different angles.