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Refraction in a prism, Snell's law: Please help!

by ObviousManiac
Tags: prism, prisms, refraction, snell's law
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ObviousManiac
#1
Mar29-12, 12:00 AM
P: 37
1. The problem statement, all variables and given/known data
A horizontal beam of light enters a 45-90-45 prism at the center of it's long side, as shown below. The emerging ray moves in a direction that is 34˚ below the horizontal. What is the index of refraction for the prism?




2. Relevant equations
n[itex]_{1}[/itex]sinø[itex]_{1}[/itex] = n[itex]_{2}[/itex]sinø[itex]_{2}[/itex]


3. The attempt at a solution
n1 x sin(i) = n2 x sin(r1): (1st refrection)

and n2 x sin(r2) = n1 x sin(34˚): (2nd refraction)

^^Here n1 is the refractive index of air, n2 is the refractive index of prism, r1 and r2 are the angles of reflection at the two surfaces, and i is the incidence angle.

r1 + r2 = 45˚ or r2 = 45˚ - r1.
Substituting the above value of r2 in the equation(2),we get
n2 x sin(45 - r1) = 1 x sin(34˚)

...and then I'm stuck!

To simplify what I've figured out:
first refraction: sin45 = n sin a

second refraction: n sinb = sin34

and using geometry: a + b = 45

BUT HOW DO I FIND a and b?!?!?
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tiny-tim
#2
Mar29-12, 04:51 AM
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Hi ObviousManiac!
Quote Quote by ObviousManiac View Post
first refraction: sin45 = n sin a

second refraction: n sinb = sin34

and using geometry: a + b = 45

BUT HOW DO I FIND a and b?!?!?
Learn your trigonometric identities
sin(45° - a) = … ?
ObviousManiac
#3
Mar29-12, 05:29 AM
P: 37
Quote Quote by tiny-tim View Post
Hi ObviousManiac!


Learn your trigonometric identities
sin(45° - a) = … ?
okay so sin(45˚ - a) = sin(45)cos(a) - cos(45)sin(a)

but I'm not sure where to take it from there...
I tried setting up something like:

sin(45)/sin(34) = sin(a)/sin(b)
sin(45)/sin(34) = sin(a)/sin(45-a)
sin(45)/sin(34) = sin(a)/[sin(45)cos(a) - cos(45)sin(a)]

but I don't even know if any of that ^^ is right, I could be going in a totally wrong direction.

tiny-tim
#4
Mar29-12, 05:36 AM
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Refraction in a prism, Snell's law: Please help!

Hi ObviousManiac!
Quote Quote by ObviousManiac View Post
first refraction: sin45 = n sin a
Quote Quote by ObviousManiac View Post
okay so sin(45˚ - a) = sin(45)cos(a) - cos(45)sin(a)
Substitute from the first equation into the second.
ObviousManiac
#5
Mar29-12, 07:16 AM
P: 37
Quote Quote by tiny-tim View Post
Hi ObviousManiac!



Substitute from the first equation into the second.
Sin(45-a) = nsin(a)cos(a) - cos(45)sin(a)

...doesn't this just introduce a new variable? (n?)


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