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RosSin · Apr 9, 2009, 11:29 PM

Q. A ray of light passes from air (na) through a pane of glass (ng) and out the other side. If the angle of incidence is θ, and the pane of glass has parallel faces, then the angle in air on the other side of the glass is:

A θ/2
B sin-1((ng/na)sinθ)
C sin-1((na/ng)sinθ)
D 2θ
E θ

And another similar question:

Q. A ray of light passes from air (na) through a pane of glass (ng) and out the other side. If the angle of incidence is θ, and the pane of glass has parallel faces, then the angle of incidence on the far side of the glass (ie from glass to air) is:

A θ
B sin-1((ng/na)sinθ)
C 2θ
D sin-1((na/ng)sinθ)
E θ/2

If anyone knows the answer, could you please post it asap
Thank you

Capuchin · Apr 10, 2009, 01:15 AM

where are you having problems? Have you sketched the situation? Do you know the equation to use?

RosSin · Apr 10, 2009, 01:30 AM

I don't really understand the question.
I think the equation to use is snell's law, am I right?

Capuchin · Apr 10, 2009, 01:33 AM

Yes... which is.. :)

RosSin · Apr 10, 2009, 01:39 AM

sorry, it is n1sinθ1 = n2sinθ2

Capuchin · Apr 10, 2009, 01:42 AM

OK, so now replace θ1 and θ2 with θr and θi (=θ), and replace n1 and n2 with na and ng, (make sure you get these the right way around).

Then you need to realise that the question is really asking you for θr (because θi on the parallel edge forms a z-angle with θr on the first edge, making them the same angle).

So just solve for θr

RosSin · Apr 10, 2009, 01:48 AM

so:

na(sin)θr = ng(sin)θi
θr = ng(sin)θi/na(sin)

RosSin · Apr 10, 2009, 01:53 AM

So would the answer for the first question be C

Capuchin · Apr 10, 2009, 02:00 AM

Sorry, I was solving the second question. The first question should be easier.

RosSin · Apr 10, 2009, 02:04 AM

oh, OK so let's stick with the second question first.

so:

na(sin)θr = ng(sin)θi
θr = ng(sin)θi/na(sin)

Is the answer D?

Capuchin · Apr 10, 2009, 02:18 AM

You can't just divide by sin. Sin is a function, so you need to provide the reverse function

i.e

$sin^{-1}(sin(\theta)) = \theta$

RosSin · Apr 10, 2009, 02:29 AM

I'm a little confused now. Where did na and ng go?

RosSin · Apr 10, 2009, 02:34 AM

So is it just ng/na
The answer for Q2. Is B, right?

Capuchin · Apr 10, 2009, 03:12 AM

I was just giving an example of inverse functions, have you don'e trigonometry? You seem confused by sin()

RosSin · Apr 10, 2009, 03:20 AM

Yes I have. That's OK now, I seemed to have found the answer.

Capuchin · Apr 10, 2009, 03:24 AM

you've confused na and ng by the way, na goes with theta, and ng goes with the angle you're looking for.