# Diffraction grating help

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#1
Do maxima on a diffraction grating have different colours of light?
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6 years ago
#2
If the light source is monochromatic, ie. a laser, the maxima will all be the same colour, so if the laser is red the maxima are all read. However if you use a source of white light, the zeroth order maximum will be white but the other maxima will be spectra (all the colours of light will be observed). The indigo blue colours will always be on the outer side of a spectrum (futher away from the zeroth order) and red will be on the inner side of a spectrum (closer to the zeroth order), and this is because sinθ is proportional to the wavelength in the equation d sinθ = nλ .
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#3
(Original post by PLM98)
If the light source is monochromatic, ie. a laser, the maxima will all be the same colour, so if the laser is red the maxima are all read. However if you use a source of white light, the zeroth order maximum will be white but the other maxima will be spectra (all the colours of light will be observed). The indigo blue colours will always be on the outer side of a spectrum (futher away from the zeroth order) and red will be on the inner side of a spectrum (closer to the zeroth order), and this is because sinθ is proportional to the wavelength in the equation d sinθ = nλ .
Thank you.
May I also ask why in the single and double slit experiments, the maxima on either side of the central maximum have lower intensity?
0
6 years ago
#4
Because at the zeroth order all the light waves travel the same distance and arrive in phase with each other, and hence they interfere constructively, forming a very bright maximum. However if you go away from the centre, not all of the waves arrive in phase with each other, as some travel a different distance than others, and there is a degree of destructive interference due to a percentage of the waves arriving in antiphase with each other. This degree of destructive interference increases the further away you go from the zeroth order, causing the intensity of the maxima to decrease the further you go from the centre. Note that both first order maxima will have the same intensity, and both the second order maxima will have the same intensity, and so on, as they are symmetrically positioned across the zeroth order.
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#5
(Original post by PLM98)
Because at the zeroth order all the light waves travel the same distance and arrive in phase with each other, and hence they interfere constructively, forming a very bright maximum. However if you go away from the centre, not all of the waves arrive in phase with each other, as some travel a different distance than others, and there is a degree of destructive interference due to a percentage of the waves arriving in antiphase with each other. This degree of destructive interference increases the further away you go from the zeroth order, causing the intensity of the maxima to decrease the further you go from the centre. Note that both first order maxima will have the same intensity, and both the second order maxima will have the same intensity, and so on, as they are symmetrically positioned across the zeroth order.
Thanks. Can I also ask why a diffraction grating has brighter maxima than doublé slits?
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#6
(Original post by PLM98)
Because at the zeroth order all the light waves travel the same distance and arrive in phase with each other, and hence they interfere constructively, forming a very bright maximum. However if you go away from the centre, not all of the waves arrive in phase with each other, as some travel a different distance than others, and there is a degree of destructive interference due to a percentage of the waves arriving in antiphase with each other. This degree of destructive interference increases the further away you go from the zeroth order, causing the intensity of the maxima to decrease the further you go from the centre. Note that both first order maxima will have the same intensity, and both the second order maxima will have the same intensity, and so on, as they are symmetrically positioned across the zeroth order.
According to this source, they're all the same intensity. I'm a little confused :/

http://hyperphysics.phy-astr.gsu.edu...opt/slits.html
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6 years ago
#7
A diffraction grating has brighter maxima because when the light is diffracted, each single slit acts as a coherent source of waves. Therefore in a double slit, you have 2 sources of light while in a grating, where you have much more slits, you have hundreds of sources of light. This means that at maxima in a double-slit experiment, there is only light waves arriving in phase from 2 different sources, therefore the intensity is not very high. However in a diffraction grating, at each maxima there are light waves arriving in phase from many hundreds of slits, which means that the maxima will be much brighter. It may sound strange, if you don't understand what i am trying to say please tell me and I will explain it again in another way

I don't want to say your source is wrong, but in my physics textbook there is a picture like this one, and this is what i have been taught:

Hope it helps!!
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