How many lines per millimetre should the grating have if for the first order, observed at a wavelength λ = 500 nm, the reflected beam is observed at 10 deg from the normal, the incident angle being 60 deg? I've done the question and just want to know if I've done anything wrong, as my answer seems too big. mλ = d(sin i + sin θ) d = mλ/(sin i + sin θ) d = (1 x 500 x 10^-9)/(sin 60 + sin 10) d = 4.809 x 10^-7 m (distance between slits) No. of lines per m = 1/(4.809 x 10^-7) => No. of lines per mm = 1/(4.809 x 10^-7 x 10^-3) = 2.079 x 10^9 Thanks.