PointeShoes-x
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I'm really stuck on this question:

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In particular I don't know why slit S should be narrow and S1 and S2 act as coherent sources. I've looked in my textbook but it's really not very helpful, I can't really understand its explanations. Can anyone help me? Thanks.
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808
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To produce significant diffraction at the aperture need to be approximately the same size as the wavelength of the source, the light needs to spread out by diffraction before encountering the two other slits. The two slits act as coherent sources, because the light starts from the same source and is in phase at the double slit. Fringes are produced by the interference of the two waves propagating from the slits, the superposition of the two waves causes contructive and destructive interference where the waves are in phase and antiphase. I'm not sure if that's a good answer.
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AdotK
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If the light needs to be spread out by diffraction before it illuminates the double slit then why do we see an interference pattern when laser light is incident (without the use of the first single slit) upon the double slit because laser light is highly monochromatic and it forms an almost perfectly parallel beam of light ?
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LearningMath
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a=aperature
y=wavelength

y << a = no diffraction
y < a = little diffraction
y ~= a = diffraction (Cant find correct symbol :P)
y > a = much diffraction

To produce an interference pattern we want as much diffraction as possible, i assume having S before S1+S2 gives more diffraction... I think they could've just had S1+S2 though, without S.
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LearningMath
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(Original post by AdotK)
If the light needs to be spread out by diffraction before it illuminates the double slit then why do we see an interference pattern when laser light is incident (without the use of the first single slit) upon the double slit because laser light is highly monochromatic and it forms an almost perfectly parallel beam of light ?
We dont 'need' S at all. We'd see a pattern without it... i guess it serves to increase the fringe spacing because diffraction is happening twice.
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AdotK
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oh okay thanks
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808
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(Original post by AdotK)
If the light needs to be spread out by diffraction before it illuminates the double slit then why do we see an interference pattern when laser light is incident (without the use of the first single slit) upon the double slit because laser light is highly monochromatic and it forms an almost perfectly parallel beam of light ?
Ok I don't know how that's supposed to work. You diffract the first beam of laser light so you can get a coherent source of waves to pass through equidistant slits that are sufficiently parted to observe the diffraction patterns. If you were to not use this first slit, how would the waves be spread far enough apart to pass through sufficiently spaced apertures. They'd either have to be very close to eachother or the source 'almost perfectly parallel' light would not pass through them. The diffraction is also required initially so that the initial slit, S can act as a point source of waves, these will be coherent at the second obstactle.
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M_E_X
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(Original post by PointeShoes-x)
I'm really stuck on this question:

Name:  Physics 006.JPG
Views: 455
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In particular I don't know why slit S should be narrow and S1 and S2 act as coherent sources. I've looked in my textbook but it's really not very helpful, I can't really understand its explanations. Can anyone help me? Thanks.
(Original post by LearningMath)
We dont 'need' S at all. We'd see a pattern without it... i guess it serves to increase the fringe spacing because diffraction is happening twice.

This is wrong!!
We do need S! Without it the light would not be a coherent source.

With S, we know the light will arrive at S1 and S2 at the same time, in phase. This is important to produce the diffraction pattern.

Without S there, the source would not be coherent.
S should be narrow to make the source as coherent as possible - ie to eliminate the different paths the light could take from the light source to S1 and S2. The smaller the range of paths --> the more coherent the source.

Slit S is important.
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LearningMath
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(Original post by M_E_X)
This is wrong!!
We do need S! Without it the light would not be a coherent source.

With S, we know the light will arrive at S1 and S2 at the same time, in phase. This is important to produce the diffraction pattern.

Without S there, the source would not be coherent.
S should be narrow to make the source as coherent as possible - ie to eliminate the different paths the light could take from the light source to S1 and S2. The smaller the range of paths --> the more coherent the source.

Slit S is important.
Uuuh are you sure? My textbooks contradict you, none of the diagrams have S, only two slits with a monochromatic source... Laser light shone directly at S1 and S2 is sufficient because they are the two sources which need to be coherent to produce clear interference patterns. Anything which happens before S1+S2 doesnt matter...

I'm perfectly willing to believe you lol! But the exam is in 1.25 hours and textbook has quite clear diagrams and descriptions...
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M_E_X
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(Original post by LearningMath)
Uuuh are you sure? My textbooks contradict you, none of the diagrams have S, only two slits with a monochromatic source... Laser light shone directly at S1 and S2 is sufficient because they are the two sources which need to be coherent to produce clear interference patterns. Anything which happens before S1+S2 doesnt matter...

I'm perfectly willing to believe you lol! But the exam is in 1.25 hours and textbook has quite clear diagrams and descriptions...
Lasers are coherent, you don't need slit S for them.

But in the question, it doesn't mention a laser! It just says "monochromatic light source", which aren't always coherent. Monochromatic just means all light is the same wavelength / colour. A light bulb is not monochromatic, for example.

Slit S is to do with coherence, not whether the light is monochromatic or not.

Good luck in your exam anyway mate!
Just remember to read each question carefully and write as many (correct) points as you can

James
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AdotK
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i thought that the double slits act as a coherent source of waves because a wavefront of light from a non-laser light source passes through one slit a given constant time before it passes through the other slit so the transmitted wave fronts have a constant phase relationship and are therefore coherent.
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LearningMath
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Well i'm off to the exam now, good luck all. :p:
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teachercol
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Its a good question - but you dont need to know for A level.
Spatial and temperol coherence arent on any A level syllabus.

You need the single slit to get coherence with 'normal' light sources. eg light bulb or sodium light
You dont need it if using a laser.
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inowantusernam:(
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When light diffracts (through the single slit) there is only light where waves interfere constructively. Therefore, any light that reaches the two slits is in phase, having interfered constructively.
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Eimmanuel
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(Original post by inowantusernam:()
When light diffracts (through the single slit) there is only light where waves interfere constructively. Therefore, any light that reaches the two slits is in phase, having interfered constructively.
You want to help the more recent questions posted in the physics forum rather this post that is dated 21-May-2009.

Cheers.
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