A level Physics diffraction confusion

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Thread starter 8 months ago
#1
If different wavelengths of light are diffracted by different amounts/at different angles then how come when a white light is shone through single/double slits or a diffraction grating, the bright fringes apart from the central one are rainbow - I mean I do understand the theory behind this - but I don’t get why there isn’t a fringe of only red, a fringe of only violet etc - because shouldn’t all the light of the same wavelength be diffracted at the same angle?
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8 months ago
#2
(Original post by Mavs04)
If different wavelengths of light are diffracted by different amounts/at different angles then how come when a white light is shone through single/double slits or a diffraction grating, the bright fringes apart from the central one are rainbow - I mean I do understand the theory behind this - but I don’t get why there isn’t a fringe of only red, a fringe of only violet etc - because shouldn’t all the light of the same wavelength be diffracted at the same angle?
You've touched on the answer but I think the picture in your mind is slightly unclear. What happens if you shine monochromatic light though a grating? Do you expect just a single diffraction? No! You have constructive interference at integer wavelength spacing. The same is true here.

Shine white light through a grating and you see the rainbow pattern occurring at integer wavelength spacings. The central fringe is still white since it is not diffracted. The first rainbows (m= +/-1) show the diffracted pattern - each colour appears at a slightly different place as the wavelength of the light is different (eg: green and red light have different wavelength). This pattern then continues for m = +/- 2, you see two more rainbows where the light has been diffracted.

"I don’t get why there isn’t a fringe of only red, a fringe of only violet etc - because shouldn’t all the light of the same wavelength be diffracted at the same angle?"
There is - but within each rainbow! But white light contains all the colours hence you see a rainbow. I think you have forgotten the condition for constructive interference so perhaps read over that for peace of mind.
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Thread starter 8 months ago
#3
(Original post by 98matt)
You've touched on the answer but I think the picture in your mind is slightly unclear. What happens if you shine monochromatic light though a grating? Do you expect just a single diffraction? No! You have constructive interference at integer wavelength spacing. The same is true here.

Shine white light through a grating and you see the rainbow pattern occurring at integer wavelength spacings. The central fringe is still white since it is not diffracted. The first rainbows (m= +/-1) show the diffracted pattern - each colour appears at a slightly different place as the wavelength of the light is different (eg: green and red light have different wavelength). This pattern then continues for m = +/- 2, you see two more rainbows where the light has been diffracted.

"I don’t get why there isn’t a fringe of only red, a fringe of only violet etc - because shouldn’t all the light of the same wavelength be diffracted at the same angle?"
There is - but within each rainbow! But white light contains all the colours hence you see a rainbow. I think you have forgotten the condition for constructive interference so perhaps read over that for peace of mind.
Ahhh that makes sense yep thanks I wasn’t thinking about the path difference and interference properly - so it’s like a rainbow because all the different wavelengths have their own sort of interference pattern?
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8 months ago
#4
(Original post by Mavs04)
Ahhh that makes sense yep thanks I wasn’t thinking about the path difference and interference properly - so it’s like a rainbow because all the different wavelengths have their own sort of interference pattern?
Yes. It may be easier to think of white light as all the colours separately. Then they all get diffracted separately according to their wavelengths, hence the pattern. I'm glad you understand now.
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