Physics question diffraction help please

Hi,
Please could I have help on 4c? I’m not sure why p remains in the same exposition but x and y will rotate 90 degrees?
Here is the paper: https://pmt.physicsandmathstutor.com/download/Physics/A-level/Past-Papers/CAIE/Paper-2/QP/June%202010%20(v1)%20QP.pdf
Page 12
Thanks!

Reply 1

jnsmith

3

Original post by anonymous56754

Hi,
Please could I have help on 4c? I’m not sure why p remains in the same exposition but x and y will rotate 90 degrees?
Here is the paper: https://pmt.physicsandmathstutor.com/download/Physics/A-level/Past-Papers/CAIE/Paper-2/QP/June%202010%20(v1)%20QP.pdf
Page 12
Thanks!

consider the position of p relative to the axis of rotation (the beam of light). How is this different from the position of x and y? If this doesn't help, I can offer more help, I just don't want to completely answer the question for you

Reply 2

anonymous56754

OP

11

Original post by jnsmith

consider the position of p relative to the axis of rotation (the beam of light). How is this different from the position of x and y? If this doesn't help, I can offer more help, I just don't want to completely answer the question for you

I see why P would remain in the same position but why would X and Y not? I don't get how X and Y rotate through 90 degrees?

Reply 3

Al345

10

Original post by anonymous56754

I see why P would remain in the same position but why would X and Y not? I don't get how X and Y rotate through 90 degrees?

Well think about it, the vertical lines were diffracting the light sideways, so horizontal lines should diffract light up and down. I spent a while thinking about how to explain it online but tbh I really couldn't add anything to what I have already said.

Reply 4

anonymous56754

OP

11

Original post by Al345

Well think about it, the vertical lines were diffracting the light sideways, so horizontal lines should diffract light up and down. I spent a while thinking about how to explain it online but tbh I really couldn't add anything to what I have already said.

wait but if the grating was rotated 90 degrees and x and y rotate 90 degrees, why doesn't P do that too?
thanks

Reply 5

Al345

10

Original post by anonymous56754

wait but if the grating was rotated 90 degrees and x and y rotate 90 degrees, why doesn't P do that too?
thanks

Because P is right in the middle, so it doesn't move. P is where the light doesn't diffract and just goes straight through, the zero order (P in this case) always goes straight through. However I am not a teacher and I am doing a different exam board to you so just double check what the key words that you need to include are

(edited 1 month ago)

Reply 6

anonymous56754

OP

11

Original post by Al345

Because P is right in the middle, so it doesn't move. P is where the light doesn't diffract and just goes straight through, the zero order (P in this case) always goes straight through. However I am not a teacher and I am doing a different exam board to you so just double check what the key words that you need to include are

ok, thank you!

Reply 7

Al345

10

Original post by anonymous56754

ok, thank you!

No problem, hopefully you understood properly so you don't forget

Original post by Al345

Because P is right in the middle, so it doesn't move. P is where the light doesn't diffract and just goes straight through, the zero order (P in this case) always goes straight through. However I am not a teacher and I am doing a different exam board to you so just double check what the key words that you need to include are

I disagree with this explanation or reasoning for the question raised by OP.

It is not about which exam boards, it is about the physics.

Original post by anonymous56754

Hi,
Please could I have help on 4c? I’m not sure why p remains in the same exposition but x and y will rotate 90 degrees?
Here is the paper: https://pmt.physicsandmathstutor.com/download/Physics/A-level/Past-Papers/CAIE/Paper-2/QP/June%202010%20(v1)%20QP.pdf
Page 12
Thanks!

When we “discuss” diffraction grating pattern at A level, we would use “vertical interference patterns” as shown below when light passes through a diffraction grating with vertical slits (similar to the left diagram in Fig. 4.2 in the question).

If we consider a spot P on the central maximum m=0 and spots on m=1 and m=-1 respectively based on the diffraction pattern from vertical slits, we can imagine that spots P, X and Y are on the x-axis (usual cartesian coordinate system) and P is at the origin (0,0).

When the diffraction grating is rotated 90°, the diffraction pattern would also rotate but the origin (0,0) does not “really” rotate (imagine rotating the x-y coordinate system around the origin).

Reply 10

Al345

10

Original post by Eimmanuel

I disagree with this explanation or reasoning for the question raised by OP.
It is not about which exam boards, it is about the physics.

Sorry if I got it wrong, just the way I've always remembered it. Can you tell me exactly where I was wrong so I don't make the same mistake again

Original post by Al345

Because P is right in the middle, so it doesn't move. P is where the light doesn't diffract and just goes straight through, the zero order (P in this case) always goes straight through. However I am not a teacher and I am doing a different exam board to you so just double check what the key words that you need to include are

Original post by Al345

Sorry if I got it wrong, just the way I've always remembered it. Can you tell me exactly where I was wrong so I don't make the same mistake again

This statement “P is where the light doesn't diffract and just goes straight through, the zero order (P in this case) always goes straight through.” is wrong.

I'm not sure what physics you are trying to explain.
Light passes through the diffraction grating will diffract if the wavelength of the light is comparable to the slit width. Next, the light from different slits meet at P to interfere constructively.

Put it in simply: no diffraction, no interference no bright and dark band.

Physics question diffraction help please

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