# Homework Help: Light reflected in rod (Snell's Law)

1. Nov 9, 2009

### Jstew

1. The problem statement, all variables and given/known data
Consider a solid glass rod of length 75cm and diameter 1.5cm with n=1.46.
Light enters the center of the end of the rod from the air. What is the maximum angle of incidence for which the light is totally reflected inside the rod?

2. Relevant equations
n1 sin (A1) = n2 sin (A2)

3. The attempt at a solution
I don't know where to start because I don't know what is meant by "totally reflected inside the rod". Any help would be greatly appreciated!

2. Nov 10, 2009

### indr0008

first u can try by drawing how the light enters the rod :)

3. Nov 10, 2009

### Jstew

Last edited by a moderator: May 4, 2017
4. Nov 10, 2009

### indr0008

as u see from ur picture, if the rod is very long, what will happen to that ray in the picture?

5. Nov 10, 2009

### Jstew

It will reflect or refract, depending on the angle. But what does it mean to "reflect completely"?

6. Nov 10, 2009

### indr0008

that means that if the angle that enters the rod is bigger than that, the ray will be refracted that's all

7. Nov 10, 2009

### rl.bhat

"totally reflected inside the rod". means the light is not cumming out of the second face. For that θ2 should be equal to or greater than the critical angle of the glass rod which is found by the equation
sinC = 1/μ.
Since the rod is so large compared to its diameter, this condition cannot be achieved. May be one of the dimension is not correct.