Diffraction pattern from a grating

[techsingularity2042]

Homework Statement

Monochromatic light is incident on a diffraction grating. The diffraction pattern from the diffraction grating is then formed on a screen.
Only the central maximum and the first-order maxima can be observed on the screen.
What change will allow the second-order maxima to be observed on the screen?

Relevant Equations

d sin θ = n λ

A. Decrease the distance between the diffraction grating and the source of light

B. Increase the distance between the diffraction grating and the screen

C. Increase the wavelength of the monochromatic light

D. Reduce the number of lines per unit length of the diffraction grating


I chose A and got it wrong. I initially thought the second-order maxima were not visible due to their low intensity and assumed that decreasing the distance between the grating and the source would increase the intensity, making the second-order maxima observable.

However, the correct answer was D. This implies that the visibility issue was not a matter of intensity (as decreasing the number of lines/slits decreases the intensity of the bright fringes).

What I assumed after looking at the answer - the mark scheme does not provide any explanation, just the answer - was when there are lots of lines in a diffraction grating, only a small angular movement away from the center of a bright fringe will be needed before there is a pair of slits that have a phase difference of π. Because there are way too many lines/slits in the grating, destructive interference occurs instead. Hence, reducing the number of lines would help the constructive interference to take place at second-order maxima.

Is my assumption correct?
Also, shouldn't A also be correct?

 

[PeroK]

Only the central maximum and the first-order maxima can be observed on the screen.

Perhaps that means that the second-order maxima are too widely spaced to fit on the screen. Not that they are too faint to see?

 

[techsingularity2042]

Only the central maximum and the first-order maxima can be observed on the screen.

Perhaps that means that the second-order maxima are too widely spaced to fit on the screen. Not that they are too faint to see?

Thanks, I overlooked the part 'per unit length'. Reducing the number of lines per unit length increases the distance between slits. This leads to decrease in the distance between the neighboring bright fringes. So the second maxima would be captured on the screen.

 

[PeroK]

Thanks, I overlooked the part 'per unit length'. Reducing the number of lines per unit length increases the distance between slits. This leads to decrease in the distance between the neighboring bright fringes. So the second maxima would be captured on the screen.

I think that's what they mean.