# Snells Law

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#1
Light is incident upon a piece of Crown Glass at an angle of 50°, what is the angle of refraction when it passes to air?

I keep getting error when I type it on my calculator is this a trick question?
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5 months ago
#2
(Original post by onionnn)
Light is incident upon a piece of Crown Glass at an angle of 50°, what is the angle of refraction when it passes to air?

I keep getting error when I type it on my calculator is this a trick question?
You need to provide the refrective index values and direction of propagation (air to glass or glass toair) for someone to be able to help you properly. Have you come across total internal reflection BTW ? If you get an unphysical angle for the refracted ray then maybe it didnt refract but had to do something else .
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#3
its from the crown glass to air
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5 months ago
#4
(Original post by onionnn)
Light is incident upon a piece of Crown Glass at an angle of 50°, what is the angle of refraction when it passes to air?

I keep getting error when I type it on my calculator is this a trick question?
What have you done so far?

n1sinø1 = n2sinø2

Refractive index :

air -- n=1.0
crown glass -- n = 1.52
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#5
the refractive index for crown glass I was given was 1.51
n1sinø1 = n2sinø2
1.51xsin(50)=1xsinø2
sin-1=1.51xsin(50)/1
error
Last edited by onionnn; 5 months ago
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5 months ago
#6
(Original post by onionnn)
the refractive index for crown glass I was given was 1.51
n1sinø1 = n2sinø2
1.51xsin(50)=1xsinø2
sin-1=1.51xsin(50)
error
So what is total internal reflection and when does it happen ?
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#7
yes that is what the question is asking the light is passing from crown glass to air
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5 months ago
#8
(Original post by onionnn)
yes that is what the question is asking the light is passing from crown glass to air
So when the maths gives you an unphysical answer, it means one of two things, (1) you did the maths wrong or (2) the maths does not apply to that physical situation and "breaks". Which one is it here. Have you done "critical angle" ?
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#9
(Original post by Mr Wednesday)
So what is total internal reflection and when does it happen ?
TIR happens when n1>n2 and i>c
does that make the angle of refraction 90? I'm not sure
anyhow I've already tried to type that answer into the answer box but its still saying incorrect
Last edited by onionnn; 5 months ago
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5 months ago
#10
(Original post by onionnn)
TIR happens when n1>n2 and i>c
does that make the angle of refraction 90? I'm not sure
anyhow I've already tried to type that answer into the answer box but its still saying incorrect
Ok, for your situation you clearly have n1 > n2. Next you need to calculate the critical angle and see if it is bigger or smaller than the incidence angle. BTW for problems like this its ALWAYS a good idea to show a diagram to make sure you defined the angles in the right way relative to the normal.
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#11
(Original post by Mr Wednesday)
Ok, for your situation you clearly have n1 > n2. Next you need to calculate the critical angle and see if it is bigger or smaller than the incidence angle. BTW for problems like this its ALWAYS a good idea to show a diagram to make sure you defined the angles in the right way
relative to the normal.
sinøc=n2/n1 sin-1=1/1.51 =41.47

I have tried to answer the question to two significant figures which gives 42 but its still saying incorrect
Last edited by onionnn; 5 months ago
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5 months ago
#12
(Original post by onionnn)
sinøc=n2/n1 sin-1=1/1.51 =41.47
Ok, two thing here, if you want to type a "power", best to use a ^ sign so its clear that sin^-1 is inverse sin, and not sin minus 1. If your input angle is > critical angle then what happens to the light ray, does it refract ?
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#13
(Original post by onionnn)
sinøc=n2/n1 sin-1=1/1.51 =41.47
t by onionnn)
(Original post by Mr Wednesday)
Ok, two thing here, if you want to type a "power", best to use a ^ sign so its clear that sin^-1 is inverse sin, and not sin minus 1. If your input angle is > critical angle then what happens to the light ray, does it refract ?
does that mean TIR occurs and doesn't refract?
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#14
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5 months ago
#15
(Original post by onionnn)
does that mean TIR occurs and doesn't refract?
Yup, it cannot refract so has to reflect instead to obey energy and momentum conservation - we just invented the optical fibre - YAY !
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#16
(Original post by Mr Wednesday)
Yup, it cannot refract so has to reflect instead to obey energy and momentum conservation - we just invented the optical fibre - YAY !
unfortunately, I have to type in an answer and I've tried 0.00 degrees since it requires a two significant figures answers and it asks me to try again, and this came up: An angle of 0∘ means that the light would be travelling perpendicularly away from the interface. Is that what you meant? If it is, then take another look at your equation, you may have got some values wrong.

i have also posted a picture of the question above for you to check if I misunderstood the question and worded it wrong
Last edited by onionnn; 5 months ago
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5 months ago
#17
(Original post by onionnn)
does that mean TIR occurs and doesn't refract?
Yup, it cannot refract so has to reflect instead to obey energy and momentum conservation - we just invented the optical fibre - YAY !

(Original post by onionnn)
unfortunately, I have to type in an answer and I've tried 0.00 degrees since it requires a two significant figures answers and it asks me to try again, and this came up: An angle of 0∘ means that the light would be travelling perpendicularly away from the interface. Is that what you meant? If it is, then take another look at your equation, you may have got some values wrong.
Ok, try an angle relative to the surface normal that represents the ray reflecting back into the glass in that case. You might end up with a negative angle if so, thats fine physically.
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#18
(Original post by Mr Wednesday)
Yup, it cannot refract so has to reflect instead to obey energy and momentum conservation - we just invented the optical fibre - YAY !

Ok, try an angle relative to the surface normal that represents the ray reflecting back into the glass in that case. You might end up with a negative angle if so, thats fine physically.
and what might that be? could you give me an example?
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5 months ago
#19
(Original post by onionnn)
and what might that be? could you give me an example?
Optical fibre uses TIR to guide light over long distances - pretty much the entire internet is carried that way.

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#20
(Original post by Mr Wednesday)
Optical fibre uses TIR to guide light over long distances - pretty much the entire internet is carried that way.

i have an idea how optical fibre works i meant could you give me an answer that I could try typing into the answer box thanks in advance
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