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Critical Angle (refraction) watch

1. So, I was doing some synoptic revision, and came across something that confused me.

Sin θc = 1 / n according to the formula sheet (AQA)

as 1n2 = n2/n1,

sin θc = 1 /(n2/n1) = n1/n2

right?

However, I did a past paper, June '02, and in the mark scheme, it says you work it out using the formula "sin θc = n2/n1"

(n1 -> n2)
(1.50 -> 1.32)

1n2 = n1/n2

= 1.50/1.32
= 1.136

critical angle is inverse sin of 1/n

= 61.6

-----------

You always have to remember that Sin c must lie between 0 and 1, so therefore n will have to be greater than one - otherwise there will be no critical angle.

MdSalih
3. (Original post by MdSalih)
1n2 = n1/n2
Why not 1n2 = n2/n1 as usual?

It was 3) (b) (ii)
4. No it's n2/n1

critical angle is when θ2 = 90
sin 90 = 1...

sin θ1 / sin θ2 = n2/n1

therefore

sin θc = n2/n1

n2 represents the second refractive index
n1 represents the initial refractive index
5. (Original post by corkskrew)
No it's n2/n1

critical angle is when θ2 = 90
sin 90 = 1...

sin θ1 / sin θ2 = n2/n1

therefore

sin θc = n2/n1

n2 represents the second refractive index
n1 represents the initial refractive index
Ok, thanks, I suppose the formula sheet just gives sin θc = 1/n to be confusing then?
6. Actually that equations ok. Remember that "n" (the refractive index) is the speed of a wave relative to that wave in a vacuum. As air is condered to have n=1, for a wave travelling from glass into air...

sin θ1 / sin θ2 = n2/n1

therefore

sin θc = n2/n1

When going from glass to air, air is n2, so n2=1
n1 is glass, so n1 =1.3 or whatever

therefore

sin θc = 1/n1
7. I always get confused with the n1/n2's and which to use, this info has been helpful thanks!

Also, Mdsalih did you used to hand around in planetarion irc? sure ive seen you somewhere....

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