# Homework Help: Optics question

1. Nov 11, 2004

### ephdub

Here's the question:

http://www.geocities.com/rhack_/physics.jpg

I'm doing a CAPA (Computer Assisted Personalized Approach) - basically online assignments. So.. here's one of my questions.. I know that...
in order to have total internal reflection, the angle of incidence for the first internal reflection point must be greater than or equal to the critical angle for the glass. (As the initial angle of incidence increases, so does the angle of refraction, thus the angle of incidence for the internal reflection decreases.)

And that by using the given angles for the prism.. i should be able to calculate the angle of refraction.. and then calculate the intial angle.. I'm sucking it up and I can't get this one.. i should be able to do it with just snell's law..

(n1)[sin(theta1)]=(n2)[sin(theta2)]

any help is much appreciated..

2. Nov 11, 2004

### ephdub

i got up to..

(n1)[sin(theta1)]=(n2)[sin(theta2)]
where n1 = air
n2 = my index

(1)[sin(theta1)]=2.1[sin45]
but the only problem is.. i know that this is wrong.. since (2.1)(sin45) is > 1.. and theta will be messed up on it..

I know it has to do with something with the geometry inside.. but my brain's farting..