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# Diffraction Grating Confusion watch

1. For the zeroth-order maximum, the light wave coming through the middle slit travels in a straight line to the screen, for convenience-sake we'll say it takes a whole number of wavelengths to get there.

Now at the slit one lower than the middle the laser is diffracted again, one specific wave travels with a path difference equal to one wavelength.

This means that the meet at the mid point at the exact same point in their wave-cycle and interfere constructively making a bright maximum.

If you move a short distance above the zeroth maximum however the the two waves from the two slits that go to that point have waves that interfere destructively and it is dark.

My question is this:

//What would happen if you made the slit separation larger?//

Would there no longer be a zeroth maximum?

Because the wave that goes straight there will still take a whole number of wavelengths to get there, for it nothing has changed. But for the waves the originate from the slits either side of it the distance has just increased, so the waves now no longer interfere constructively and there is no zeroth maximum?

OR

Would the zeroth maximum stay there except it no longer has a wave that originates from the middle slit?

Will the slits either side of it still have the a path difference that is a whole number of wavelengths so they still interfere constructively?

Any and all help is greatly appreciated, thank-you.
2. (Original post by Retsek)
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3. (Original post by Retsek)

For the zeroth-order maximum, the light wave coming through the middle slit travels in a straight line to the screen, for convenience-sake we'll say it takes a whole number of wavelengths to get there.

Now at the slit one lower than the middle the laser is diffracted again, one specific wave travels with a path difference equal to one wavelength.

This means that the meet at the mid point at the exact same point in their wave-cycle and interfere constructively making a bright maximum.

If you move a short distance above the zeroth maximum however the the two waves from the two slits that go to that point have waves that interfere destructively and it is dark.

My question is this:

//What would happen if you made the slit separation larger?//

Would there no longer be a zeroth maximum?

Because the wave that goes straight there will still take a whole number of wavelengths to get there, for it nothing has changed. But for the waves the originate from the slits either side of it the distance has just increased, so the waves now no longer interfere constructively and there is no zeroth maximum?

OR

Would the zeroth maximum stay there except it no longer has a wave that originates from the middle slit?

Will the slits either side of it still have the a path difference that is a whole number of wavelengths so they still interfere constructively?

Any and all help is greatly appreciated, thank-you.
You just need to think about the *path difference* - don't worry about the number of wavelengths distance between slit and screen at all... you can't control that distance to a fraction of a wavelength of visible light and you don't need to.

also note that that diagram is *very not to scale*.

grating to screen distance is a few meters
distance between lines in the grating... probably 2x10-6 m or something like that.

The angle from one line on the grating to a maxima is effectively the same as the angle from the grating line to the same maxima
4. (Original post by Joinedup)
You just need to think about the *path difference* - don't worry about the number of wavelengths distance between slit and screen at all... you can't control that distance to a fraction of a wavelength of visible light and you don't need to.

also note that that diagram is *very not to scale*.

grating to screen distance is a few meters
distance between lines in the grating... probably 2x10-6 m or something like that.

The angle from one line on the grating to a maxima is effectively the same as the angle from the grating line to the same maxima
Thanks for replying, which line is the grating line? Is that the line perpendicular to the grating?
5. (Original post by Retsek)
Thanks for replying, which line is the grating line? Is that the line perpendicular to the grating?
the diffraction grating is a series of parallel opaque lines etched onto a piece of glass - the distance between these lines is usually expressed as number of lines/mm
typical values might be between 200 l/mm and 1000 l/mm

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