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diffraction pattern question help

laser light of wavelength 640nm is incident normally at a diffraction grating. the separation between adjacent slits (lines) is 3.3 x 10^-6 m.

what is the total number of bright spots that can be observed in the diffraction pattern?
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ak1237
5
Using the diffraction grating equation d sin(θ) = n λ, you can rearrange to get n = (d sin(θ))/λ. As the angle increases, more and more bright spots are visible. This occurs up until θ = 90 degrees at which point the light would be perpendicular to the incident light. Therefore, no light is visible at θ > 90 degrees. So setting θ = 90 in the equation (so sin(θ) = 1) gets n = 5.15625. This means that 5 total spots are visible (as the 6th spot is going to be after 90 degrees).

Hope this helps
Original post by ak1237
Using the diffraction grating equation d sin(θ) = n λ, you can rearrange to get n = (d sin(θ))/λ. As the angle increases, more and more bright spots are visible. This occurs up until θ = 90 degrees at which point the light would be perpendicular to the incident light. Therefore, no light is visible at θ > 90 degrees. So setting θ = 90 in the equation (so sin(θ) = 1) gets n = 5.15625. This means that 5 total spots are visible (as the 6th spot is going to be after 90 degrees).

Hope this helps

thank you for explaining this to me !!! :smile: you really help
5 either side of the central (n=0) so the correct answer is 11
why
Original post by therevenger778
why

The n calculated is the order of maxima. There are 2 first order, 2 second order so on and so forth. If qn were to ask max order, the answer would be 5. But this qn ask for the total no. of spots. So answer would have bee 11 (theres only 1 zeroth order, the straight thru).
That's correct but there are 5 on each side of the central maxima (n=0) so including the central spot, there are 11 in total.

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