# Homework Help: Why there is no deviation of light ray when it enters a semi circular glass block?

1. Jan 26, 2012

### Mphisto

1. The problem statement, all variables and given/known data
Well, this is not really a homework question but something I wanted to find out. Basically, Why there is no deviation of light ray when it enters a semi circular glass block?

2. Relevant equations

3. The attempt at a solution
I thought light ray would not be deviated only when it enters another medium perpendicular to boundary. So why would it not be deviated when it enters the a semi circular glass block?

Thank you

2. Jan 26, 2012

### ehild

Re: Why there is no deviation of light ray when it enters a semi circular glass block

Hi, Mphisto,

The light ray will deviate if it enters at an angle with respect to the surface normal. Why do you think it will not?

ehild

3. Jan 26, 2012

### Mphisto

Re: Why there is no deviation of light ray when it enters a semi circular glass block

4. Jan 26, 2012

### Staff: Mentor

Re: Why there is no deviation of light ray when it enters a semi circular glass block

How is the light entering the block? What angle is it making with the normal to the surface?

5. Jan 26, 2012

### Mphisto

Re: Why there is no deviation of light ray when it enters a semi circular glass block

http://t2.gstatic.com/images?q=tbn:ANd9GcRhvx6-lH4b3Dpe1otCjmmVHn_bcsWOpLUnEf-_A3IhQQON_YG0 [Broken]
Taken from google, as you can see the incident ray passes straight through the glass block

Last edited by a moderator: May 5, 2017
6. Jan 26, 2012

### Staff: Mentor

Re: Why there is no deviation of light ray when it enters a semi circular glass block

At the first surface (air to glass), the angle with the normal is zero so it passes straight through. Is that what you mean? (The ray of light is directed along the radius, so it is normal to the surface.)

Of course it undergoes refraction when it leaves the block.

7. Jan 26, 2012

### Mphisto

Re: Why there is no deviation of light ray when it enters a semi circular glass block

"angle with the normal is zero"? shouldn't the light ray be refracted at the first surface? (air to glass)

8. Jan 26, 2012

### Staff: Mentor

Re: Why there is no deviation of light ray when it enters a semi circular glass block

That's correct.
What's the angle of incidence?

9. Jan 26, 2012

### Mphisto

Re: Why there is no deviation of light ray when it enters a semi circular glass block

I am not sure... since it's curved i don't know how the normal would be placed there

10. Jan 26, 2012

### Staff: Mentor

Re: Why there is no deviation of light ray when it enters a semi circular glass block

Note that the incoming light ray is aimed along the radius of the circle and is thus normal to the surface. (Imagine a tangent line to the semi-circle drawn at the point where the light enters--take that as the surface.)

11. Jan 26, 2012

### Mphisto

Re: Why there is no deviation of light ray when it enters a semi circular glass block

So basically the normal of first surface is like a tangent line to the semi-circle? am I right to say that?

12. Jan 26, 2012

### Staff: Mentor

Re: Why there is no deviation of light ray when it enters a semi circular glass block

No. The normal is perpendicular to the tangent line.

13. Jan 26, 2012

### Mphisto

Re: Why there is no deviation of light ray when it enters a semi circular glass block

Oh ok, I got it

Thank you!!!