Angle of incidence and total internal reflection

[ellieee]

Homework Statement

qn: explain why a ray of light that enters the glass through surface A cannot be totally internally reflected at surface B.

Relevant Equations

n = sin I / sin r

the angle of incidence at A is 40°, then why did the answer key say "angle of incidence at A is just less than 90°". it is obvious that 40° is quite far from 90° right?

 

[BvU]

Hi, and

!​

Breaks my neck to look at your picture

it is obvious that 40° is quite far from 90° right?

Yes, that is obvious. But to get a possible total internal refection at B, you need a maximum angle of refraction, and therefore a maximum angle of incidence. So they observe ( correctly !) that for a maximum angle of incidence at A

angle of incidence at A is just less than 90°

$\$

 

[ellieee]

you need a maximum angle of refraction, and therefore a maximum angle of incidence.

just to clarify, is it maximum angle of refraction and incidence at B ?

 

[BvU]

Can you think of a way to get a bigger angle of incidence at B ?