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# The Physics PHYA2 thread! 5th June 2013 Watch

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1. (Original post by mejdoub.wassim)
What would the angle look like? Is it the same as when it hits the boundary? Also, where would the dotted line begin, exactly at the point at which the ray makes contact with the boundary?
I don't think you know the reflected angle exactly so an estimated guess (roughly the reflection angle should suffice).

Yes

(Correct me If I'm wrong guys)
2. (Original post by StalkeR47)
But how do you know that if the incident substance has larger index than the other. And, incident angle is smaller than critical angle. TIR does not occur. How do you know this? Any explanations?
If the refractive index of n1 is less than n2, in the equation to work out the critical angle (sin thetac = n2/n1) , sin theta would be greater than 1, which is impossible
3. (Original post by x-Sophie-x)
Stalker, I'm not sure what you're trying to say exactly but I think you should accept that those guys are right haha
Sophie, I have accepted them. But, I am confused about the fact that, why TIR does not occur if the incident substance has larger refractive index than the emergent substance. I mean, you are only meeting 1 criteria (for TIR) but not the other. Why would it be refraction and not reflection.
If the refractive index of n1 is less than n2, in the equation to work out the critical angle (sin thetac = n2/n1) , sin theta would be greater than 1, which is impossible
exactly

Hi guys can you help me with something..

I was wondering what you do for questions about slits and diffraction that say show what you see on the screen. Do you draw the intensity graph type thing or just band or dots

Also when you diffract white light through a grating how do you show that the inner side is violet and the outer is red. Is this done on the intensity graph?.
6. (Original post by StalkeR47)
Alright! Thanks man. Not really smart. I was just trying to clarify what I thought as in the exam paper question. And, I am still not sure. I know the rules properly but I need an explanations to why is that. Thanks.
Explain to me why it is better for the signal to be in the core not the cladding?
7. (Original post by StalkeR47)
Amm sorry. I did not get you. Can you answer this please? Anyone please? why TIR does not occur if incident substance has larger refractive index than the other? Ca
otherwise sine of critical angle would exceed 1 which is not possible...
8. (Original post by StalkeR47)
Sophie, I have accepted them. But, I am confused about the fact that, why TIR does not occur if the incident substance has larger refractive index than the emergent substance. I mean, you are only meeting 1 criteria (for TIR) but not the other. Why would it be refraction and not reflection.
okay can i ask what you mean by incident substance, if you mean where the ray came from​ then what i have said earlier is correct.
If the refractive index of n1 is less than n2, in the equation to work out the critical angle (sin thetac = n2/n1) , sin theta would be greater than 1, which is impossible
Yes I know that. But in this case, n1 is not less than n2 is it?
10. (Original post by StalkeR47)
Sophie, I have accepted them. But, I am confused about the fact that, why TIR does not occur if the incident substance has larger refractive index than the emergent substance. I mean, you are only meeting 1 criteria (for TIR) but not the other. Why would it be refraction and not reflection.
Because the angle of incidence doesn't exceed the critical angle?
I still don't understand the point you are trying to make entirely; I shall leave it to the experts :L
11. (Original post by StalkeR47)
Sophie, I have accepted them. But, I am confused about the fact that, why TIR does not occur if the incident substance has larger refractive index than the emergent substance. I mean, you are only meeting 1 criteria (for TIR) but not the other. Why would it be refraction and not reflection.
You've just stated it. You're only meeting 1 criteria

How do you work out critical angle? And what's the definition of critical angle?
12. (Original post by kinglj81)

Hi guys can you help me with something..

I was wondering what you do for questions about slits and diffraction that say show what you see on the screen. Do you draw the intensity graph type thing or just band or dots

Also when you diffract white light through a grating how do you show that the inner side is violet and the outer is red. Is this done on the intensity graph?.
I'm not sure how, but I doubt you use intensity graphs to show that.
13. (Original post by masryboy94)
okay can i ask what you mean by incident substance, if you mean where the ray came from​ then what i have said earlier is correct.
Incident substance is the core.
14. (Original post by StalkeR47)
No! All I was saying that why TIR does not occur if incident substance has larger refractive index than the other? Can you answer this? HUH?
Total internal reflection can happen only when a beam of light travelling through a dense medium crosses the interface with a rarer medium.
For example, through a glass piece to air.

When such a beam reaches an interface it makes an angle (called the angle of incidence) with the perpendicular at that point. When the beam exits the interface into the rarer medium. it makes a larger angle(called the angle of refraction) with the same perpendicular.

As the angle of incidence increases, so does the angle of refraction. There is one value of the angle of incidence for which the angle of refraction is 90 degrees and the emerging ray is tangential to the interface. This is called the critical angle.
For all angles of incidence greater than the critical angle the incident ray will not emerge into the rarer medium at all. In stead it gets reflected back into the denser medium itself.
15. (Original post by StalkeR47)
Incident substance is the core.
How do you work out critical angle? And what's the definition of critical angle?
16. (Original post by x-Sophie-x)
I'm not sure how, but I doubt you use intensity graphs to show that.
Do you have to show that? And wouldn't it be based on wavelengths?
17. The grade boundaries on Unit 2 seem to be a lot lower than those of Unit 1.
18. (Original post by Qari)
You've just stated it. You're only meeting 1 criteria

How do you work out critical angle? And what's the definition of critical angle?
Alright then! sini(critical)=n2/n1. n2 is 1.45 and n1 is 1.55. critical angle is therefore 69.3 deg. So, 1 (criteria) is met and the other isn't. Then, why refraction and not reflection.?
19. (Original post by kinglj81)

Hi guys can you help me with something..

I was wondering what you do for questions about slits and diffraction that say show what you see on the screen. Do you draw the intensity graph type thing or just band or dots

Also when you diffract white light through a grating how do you show that the inner side is violet and the outer is red. Is this done on the intensity graph?.
You would just show the screen how you would see it if you were looking at it. Meaning you would just draw a rectangle/square/your favourite shape and show the maxima as either the same width or half the width of the central maxima (depending on whether its double slit or single).

As for indicating the violet colour near to the central maxima, you would just label it saying something like 'the maxima closer to the central maxima have a violet tinge while the maxima further away take on an increasingly darker red tinge'.

(Original post by IWantSomeMushu)
The grade boundaries on Unit 2 seem to be a lot lower than those of Unit 1.
Yeah tell me about it! On average, you can drop 9 marks and still walk away with full UMS. Seems like a golden opportunity but I just know I'm going to throw away marks somewhere tomorrow
20. does anyone understand wavelets and how the waves passing through a single slit actually manages to interfere with itself to create minimas?

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