# Refraction homework help

1. Nov 22, 2009

### ibysaiyan

1. The problem statement, all variables and given/known data
Hi all its me again.need help again =]
well basically i am stuck at b part (iii), no idea on how to find out theta 3 =/

2. Relevant equations
hmm 180- something =/? no idea

3. The attempt at a solution
well i though at first that it would be this way:
180=90+19.47 =/ but.. on the mark scheme the answer is way low/
Thanks.

2. Nov 22, 2009

### mgb_phys

Re: Refraction

if you got the first two parts right then you must know Snell's law
the only thing to be carefull about is that theta3 will be measured from the normal, and remember which way the light bends going from high n to low n

3. Nov 22, 2009

### ibysaiyan

Re: Refraction

yea i do but doesn't snells law work on ratio of indices between two mediums =/? as in this case theta 2 and 3 both are in the same medium?

4. Nov 22, 2009

### mgb_phys

Re: Refraction

looking at the diagram more closely - have you heard of total internal reflection?

5. Nov 22, 2009

### ibysaiyan

Re: Refraction

yeah which occurs when light propagates from dense medium to less dense and i got the critical angle, its just finding out theta 3 bit.

6. Nov 22, 2009

### mgb_phys

Re: Refraction

So having determined that it reflects at A what is the angle - remember that TIR is the same as normal reflection

7. Nov 22, 2009

### ibysaiyan

Re: Refraction

err... would it be 90degree or same as theta 2

8. Nov 22, 2009

### mgb_phys

Re: Refraction

imagine the bottom of the prism is a mirror draw the reflected ray from where the theta2 ray hits A
then angle theta 3 is between this reflected ray and the normal at A

9. Nov 22, 2009

### ibysaiyan

Re: Refraction

ooh.. i still don't get it sorry for being such a nuisance lol ,looking at the time its 1.20 am xD and i got college in less than 8 hours ,...wonder what am i missing here hmm..

10. Nov 23, 2009

### ibysaiyan

Re: Refraction

OO hi all back again and sorry for the late reply.I think i have figured it out.
what i did was basically finding all of the unknown angles lol yea.. ugh.. image hosting sites are blocked at my college , cant upload screenshot.this is what i did:
180-(135+19.47)=25.53.
=> so to get theta3
90-25.53=64.47 which is >critical angle , hence TIR.
Thanks alot mgb phys =]
i will be posting more threads soon xD.