# Critical angle

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Thread starter 6 years ago
#1
I had done:
1.96/2.03 = 0.97 = refractive index
since 1/sinC = refractive index, I tried to find the critical angle but the maximum value of sin is 1 so it didn't work!

How should I go about this question?? Can someone please explain it to me in simple terms. Furthermore, why is 1/sinC = refractive index??

The equatioin is refractive index = sin i/sin r
Since at the critical angle, sin r is 90, shouldn't it be sin i = refractive index?
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6 years ago
#2
To find the critical angle of a material, you have to go from the denser medium, so i is actually r and r is actually i. So you can either swap them around or do sin(r)/sin(i) = sin(c)
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Thread starter 6 years ago
#3
(Original post by mclean_tom_)
To find the critical angle of a material, you have to go from the denser medium, so i is actually r and r is actually i. So you can either swap them around or do sin(r)/sin(i) = sin(c)
Is that always the way to find critical angle?
And when it says "refractive index of glass", which one out of the incident angle and refractive angle is the angle fir glass? This always confuses me

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6 years ago
#4
If you are doing the critical angle, you have to go from the denser medium. So in this one, you have to do

Refractive index = 2.03/1.96 = 1.036

If you think about it, the refractive index has to be bigger than 1 because the light is slowing down.

Then, you can use sin(i) = 1 / 1.03

i = sin-1(1/1.03) = 75º

Remember, in critical angles you have to go from the denser medium and that sin(r) = 1 because r = 90.
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Thread starter 6 years ago
#5
(Original post by mclean_tom_)
If you are doing the critical angle, you have to go from the denser medium. So in this one, you have to do

Refractive index = 2.03/1.96 = 1.036

If you think about it, the refractive index has to be bigger than 1 because the light is slowing down.

Then, you can use sin(i) = 1 / 1.03

i = sin-1(1/1.03) = 75º

Remember, in critical angles you have to go from the denser medium and that sin(r) = 1 because r = 90.
Yes, i understood that, thanks.

But in other questions when it says "the refractive angle of glass", is glass the 2nd medium? Like, in the equation sin i/sin r, is r the angle to the normal for the glass?

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6 years ago
#6
(Original post by jtbteddy)
Yes, i understood that, thanks.

But in other questions when it says "the refractive angle of glass", is glass the 2nd medium? Like, in the equation sin i/sin r, is r the angle to the normal for the glass?

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Yes

If you get a number bigger than 1 for refractive index, you are most likely correct
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6 years ago
#7
When you're just given the refractive index, it's the index from a vacuum into that material. Also the same as the speed of light in a vacuum divided by the speed of light in that material. Critical angle is the incident angle required to give a 90 degree refraction (and thus go along the normal)
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Thread starter 6 years ago
#8
(Original post by mclean_tom_)
Yes

If you get a number bigger than 1 for refractive index, you are most likely correct
(Original post by lerjj)
When you're just given the refractive index, it's the index from a vacuum into that material. Also the same as the speed of light in a vacuum divided by the speed of light in that material. Critical angle is the incident angle required to give a 90 degree refraction (and thus go along the normal)

So in this example, medium 1 is air, and medium 2 is the plastic layer and it's called "the refractive index of the plastic layer"?
My question is, when it says "the refractive index of x", x is always the 2nd medium right?

In this question, could you help me draw the line outside the prism? I know that for a denser solution, the line inside the prism will be closer to the normal, but how is it meant to be drawn out of the prism?

Thanks for your help
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6 years ago
#9
(Original post by jtbteddy)
So in this example, medium 1 is air, and medium 2 is the plastic layer and it's called "the refractive index of the plastic layer"?
My question is, when it says "the refractive index of x", x is always the 2nd medium right?

In this question, could you help me draw the line outside the prism? I know that for a denser solution, the line inside the prism will be closer to the normal, but how is it meant to be drawn out of the prism?

Thanks for your help
Do you know Snell's law? If you do you should be able to work this out by yourself. You can make up numbers for the refractive index and draw out the answers if you want as long as the denser medium (high sugar conc.) has a larger refractive index than the lower medium (low sugar conc.)

If you're still stuck I'll help you later
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Thread starter 6 years ago
#10
(Original post by lerjj)
Do you know Snell's law? If you do you should be able to work this out by yourself. You can make up numbers for the refractive index and draw out the answers if you want as long as the denser medium (high sugar conc.) has a larger refractive index than the lower medium (low sugar conc.)

If you're still stuck I'll help you later
Yeah i do, but i dont know how a more concentrated solution links with velocity or how big/small the angle if refraction is.. And i know that inside the prism the more concentrated solutions path will be towards the normal, as i was taught this at school, but what about outsude the prism?

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6 years ago
#11
The more concentrated solution has a higher refractive index lower speed of light.

Does that help? I can probably go through it for you, but it would be better if you went through it using Snell's law to see how that different angles change.
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Thread starter 6 years ago
#12
(Original post by lerjj)
The more concentrated solution has a higher refractive index lower speed of light.

Does that help? I can probably go through it for you, but it would be better if you went through it using Snell's law to see how that different angles change.
Refractive index is a measure of how close the path is to the normal right? So the more concentrated solution will have a path closer to the normal?
Since refractive index = v1/v2, that explains why it has a lower speed?
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6 years ago
#13
(Original post by jtbteddy)
Refractive index is a measure of how close the path is to the normal right? So the more concentrated solution will have a path closer to the normal?
Since refractive index = v1/v2, that explains why it has a lower speed?
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Thread starter 6 years ago
#14
(Original post by lerjj)
Kkay, thanks. But is refractive index a measure of how close the path is to the normal?
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6 years ago
#15
(Original post by jtbteddy)
Kkay, thanks. But is refractive index a measure of how close the path is to the normal?
No, not really, it's a measure of how much refraction takes place. Less dense to more dense--> towards the normal

more dense to less dense--> away from normal.

So to answer the original question with the prism, the refraction will be greater both entering and exiting, which means that it will first move more towards and later more away from the normal. Does that make sense?

P.s. check that using snell's law because I haven't drawn out the diagram personally.
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Thread starter 6 years ago
#16
(Original post by lerjj)
No, not really, it's a measure of how much refraction takes place. Less dense to more dense--> towards the normal

more dense to less dense--> away from normal.

So to answer the original question with the prism, the refraction will be greater both entering and exiting, which means that it will first move more towards and later more away from the normal. Does that make sense?

P.s. check that using snell's law because I haven't drawn out the diagram personally.
Yup, it does. Thank you very much

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Thread starter 6 years ago
#17
(Original post by lerjj)
No, not really, it's a measure of how much refraction takes place. Less dense to more dense--> towards the normal

more dense to less dense--> away from normal.

So to answer the original question with the prism, the refraction will be greater both entering and exiting, which means that it will first move more towards and later more away from the normal. Does that make sense?

P.s. check that using snell's law because I haven't drawn out the diagram personally.
Nvm. Got it. Thanks
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6 years ago
#18
(Original post by jtbteddy)
Nvm. Got it. Thanks
Do you want help with that question as well? I think violet light refracts more strongly than red light.
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Thread starter 6 years ago
#19
(Original post by lerjj)
Do you want help with that question as well? I think violet light refracts more strongly than red light.
Yupp, youre right:

Violet refracts the most, i.e. Its angle of refraction is the smallest.

Got it thanks!

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6 years ago
#20
(Original post by jtbteddy)
Yupp, youre right:

Violet refracts the most, i.e. Its angle of refraction is the smallest.

Got it thanks!

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It's angle of refraction is the largest you mean? It's 4.61 degrees more.
But yeah, that diagram is perfect so just use that for all future queries re refraction.
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