Get Refraction Homework Help - Stuck on Theta 3? Let Us Help You!

[ibysaiyan]

Homework Statement

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 =/

Homework Equations

hmm 180- something =/? no idea

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.

 

[mgb_phys]

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

 

[ibysaiyan]

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

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?

 

[mgb_phys]

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

 

[ibysaiyan]

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.

 

[mgb_phys]

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

 

[ibysaiyan]

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

 

[mgb_phys]

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

 

[ibysaiyan]

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..

 

[ibysaiyan]

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 , can't 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 a lot mgb phys =]
i will be posting more threads soon xD.