# Homework Help: Prism : what if incident angle is greater than apex angle?

1. Oct 6, 2014

• Warning: Problem statement should include a description of the problem; Solution attempt must be included.
1. The problem statement, all variables and given/known data
incident angle : 60 degree
apex angle : 30 degree
prism refraction index : 1,6
deviation angle : ?

2. Relevant equations

3. The attempt at a solution

2. Oct 6, 2014

### ehild

You mean the angle of the prism? Why do you think it makes a problem if the angle of incidence is greater than that? What is the question?

ehild

3. Oct 6, 2014

if i solve this equation to find first refraction angle (r1) :
sin(I1) = np sin (r1), which (I1) is incident angle =60 degree, and np is prism refraction index = 1,6
i find r1 = 32,76 degree.
if beta = r1 + I2 , which beta is apex angle = 30 degree, and I2 is second incident angle,
i find I2 = -2,76 degree
i cant quite understand whats the meaning of (-) in my answer :/

4. Oct 6, 2014

### ehild

It means that the second angle of incidence falls to the opposite side of the normal. See picture. You still can use $|I_2|=|\beta-r_1|$. The second refracted ray will travel upward, in the direction of the apex.
I suggest to draw the rays always before applying a formula.

ehild

5. Oct 8, 2014

so i can still use
Deviation angle = i1+r2-beta to find second refraction angle? (r2)
thank you so much that enlightens me so much about prism.
many thanks man

6. Oct 8, 2014

### ehild

You find r2 by applying Snell's Law for I2. And I1 and r2 and beta determine the deviation angle.

ehild

7. Oct 8, 2014