# Maximum critical angle for total reflection?

1. Aug 26, 2014

### shangriphysics

1. The problem statement, all variables and given/known data
A beam of light is incident on the boundary between a piece of glass to air. The index of refraction of the piece of glass is 1.4. Find the critical angle.

It is quite simple to figure out this problem. Sinpheta critical = n2/n1 (as long as n1 is greater than n2)

However, let say I wanted to find pheta 2, with different pheta1s. Since I know the critical angle, I know that if pheta1 = critical angle, then pheta2 should be 90 degrees.

When I double check using snells law I do not seem to get 90 degrees. Can I even use snells law in these scenarios to find pheta2, for the total reflection angle, refraction angle?

I also know if pheta 1 is less than pheta2 refraction will occur, and if pheta1 is greater than the critical angle, total internal reflection will occur.

2. Aug 26, 2014

### ehild

What do you mean on pheta 2?

Show what you get for the angle of refraction when the angle of incidence in glass is equal to the critical angle.

ehild

3. Aug 27, 2014

### shangriphysics

Pheta 2 is the angle that light makes after it either reflects or refracts in respect to the normal line.

1.4Sin(45)=1Sin(?) Where 1.4 is n1 and some medium, and 1 is n2 which is the air.

I will get 81.86 degrees for angle 2(pheta2)

4. Aug 27, 2014

### ehild

45° is less than the critical angle. You get the critical angle θc if pheta2 =90°.

1.4 sinθc= sin90° = 1. What is θc?

ehild

5. Aug 27, 2014

### shangriphysics

Thank you Ehild! I see now. I can use snells law!

6. Aug 27, 2014

### ehild

Of course you can use it!

ehild