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???