# Double slit diffraction confusion

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#1
This is from the A Level CGP Textbook:

"In order to get clear interference patterns, the waves from the two sources must be monochromatic and coherent"

Then it describes Young's Double-Slit experiment:

"If you were to use a white light source instead of a coherent monochromatic laser, the diffraction pattern would be less intense, with wider maxima."

Can someone explain why you can use a white light source when it clearly says the double slit experiment requires monochromatic light?

And I'm also confused why the diffraction pattern would have less intense and wider maxima?

Also, what is the reason that the central maximum is white for white light? Does it simply pass through?
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4 years ago
#2
This is how I understand it.

The experiment requires you to have monochromatic light instead of white light because "the diffraction pattern would be less intense, with wider maxima." - This basically means you CAN use white light, but it's much more difficult to accurately measure the fringe spacing, so using monochromatic light is better. Monochromatic light is light of a single frequency [OCR textbook definition] (E.g. Light from a laser which has a specific colour, it has a specific colour because it has a specific frequency)
Using a laser to demonstrate young's double slit is great because the fringe spacing is really clear and the dots are small - you can accurately measure them and you percentage uncertainty for measurement is far less.

For an answer to why white light has a diffraction pattern has less intense and wider maxima, i have found a link for you, it explains it much better than I can:

http://www.kelsohighschool.org.uk/si...%20Grating.pdf

Just go down to where it says 'grating and white light'

Hope this helps
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4 years ago
#3
(Original post by Teenage Hype)
This is from the A Level CGP Textbook:

"In order to get clear interference patterns, the waves from the two sources must be monochromatic and coherent"

Then it describes Young's Double-Slit experiment:

"If you were to use a white light source instead of a coherent monochromatic laser, the diffraction pattern would be less intense, with wider maxima."

Can someone explain why you can use a white light source when it clearly says the double slit experiment requires monochromatic light?

And I'm also confused why the diffraction pattern would have less intense and wider maxima?

Also, what is the reason that the central maximum is white for white light? Does it simply pass through?
You can use light that isn't monochromatic, but it isn't as clear

The middle fringe is white because every colour goes to that point in the centre and contributes to the white colour.

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#4
(Original post by Kyx)
You can use light that isn't monochromatic, but it isn't as clear

The middle fringe is white because every colour goes to that point in the centre and contributes to the white colour.

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Thank you.

For the central maxima, does the light pass straight through, since the whole of the middle fringe is white?

If that's not the case I still don't understand why the central is white, I would have expected it to be more violet in the centre and then more red on the edges of the middle fringe? We were taught this very badly soo
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4 years ago
#5
(Original post by Teenage Hype)
Thank you.

For the central maxima, does the light pass straight through, since the whole of the middle fringe is white?

If that's not the case I still don't understand why the central is white, I would have expected it to be more violet in the centre and then more red on the edges of the middle fringe? We were taught this very badly soo

I just read it straight from the book

I don't think it goes straight through though

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#6
(Original post by Kyx)

I just read it straight from the book

I don't think it goes straight through though

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Hey, at least you don't have to do the new spec AQA Physics A Level! I actually have to revise AS (and A2 ofc) properly as a result, the exams are damn hard...
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4 years ago
#7
(Original post by Teenage Hype)
Hey, at least you don't have to do the new spec AQA Physics A Level! I actually have to revise AS (and A2 ofc) properly as a result, the exams are damn hard...
Yeah

But now uni is going to be so difficult, maxwell's equations and **** like that

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#8
(Original post by Kyx)
Yeah

But now uni is going to be so difficult, maxwell's equations and **** like that

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gl with that my man,

I'm not doing physics at degree level! Doing economics instead
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4 years ago
#9
(Original post by Teenage Hype)
gl with that my man,

I'm not doing physics at degree level! Doing economics instead
Good luck

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0
4 years ago
#10
(Original post by Teenage Hype)

.....
Also, what is the reason that the central maximum is white for white light? Does it simply pass through?

White light is composed of many different wavelength of visible light. When the white light passes through the slits, you can imagine that the “individual wavelength” of light “pass through” the slit simultaneously. Individual wavelength of light will have its interference pattern. Light of wavelength (600 nm) will have an interference pattern different from that light of wavelength 300 nm as shown in the diagram below.

The zero order fringe width for 300 nm will be smaller than that of the 600 nm. However, at zero order maxima, all the different wavelength of the visible light will then superimpose to give the white light.

You can see interference pattern for a white light in the following website.
http://www.skinners-physics.co.uk/Ye...n_3_Waves.html
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