Snell optics problem

[Sith Lord 13]

Homework Statement

The block of glass n = 1.5 shown in cross section in the figure (Intro 1 figure) is surrounded by air. A ray of light enters the block at its left-hand face with incident angle Θ1 and reemerges into the air from the right-hand face directed parallel to the block's base.
Determine Θ1

http://session.masteringphysics.com/problemAsset/1090003/2/GIANCOLI.ch32.p46.jpg

Homework Equations

n1 sin Θ1 = n2 sin Θ2 = n1 sin Θ3

The Attempt at a Solution

3. n1 sin Θ1 =n2 sin Θ2 = n1 sin Θ3
n1 =1, n2=1.5, Θ3=45
1 sin 45 = 1.5 sin(x) x= 28.125
1.5 sin 28.125 = 1 sin(x) x = 45

 

[tiny-tim]

Welcome to PF!

n1 sin Θ1 =n2 sin Θ2 = n1 sin Θ3
n1 =1, n2=1.5, Θ3=45
1 sin 45 = 1.5 sin(x) x= 28.125
1.5 sin 28.125 = 1 sin(x) x = 45

Hi Sith Lord 13! Welcome to PF!

Don't forget that Θ2 on the left face isn't the same as Θ2 on the right face.

 

[Sith Lord 13]

Hi Sith Lord 13! Welcome to PF!

Don't forget that Θ2 on the left face isn't the same as Θ2 on the right face.

Thank you.

Why aren't they the same? And how do I determine what the left is then?

Thanks

 

[tiny-tim]

Why aren't they the same? And how do I determine what the left is then?

Because the left and right sides aren't parallel …

so the internal angle (to the normal) at which the same ray meets the left side isn't the same as the internal angle at which it meets the right side …

just use a bit of geometry to work out how they're connected.