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**Looking at diffraction gratings.....**

Ok, so I got a question in my assignment.

Monochromatic light of wavelength l is incident normally on a diffraction grating consisting of alternate opaque strips of width a and transparent strips of width b. The angle between the emerging zero order and second order spectra is given by?

So I assumed that the slit separation is given by

*a*since transparent strips (i assumed) is like those empty slits and slit separation is opaque.

So I tried dsinθ=nλ. But my ans is sin^-1 2l/a.

But the answer given is sin^-1 2l / (a+b)

Is there a problem in my understanding of the question?

EDIT: Is it because slit separation is calculated from the centre of each slit. Thus, the two 1/2 b will add together to get b, leading to slit separation a+b???

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