A diffraction grating has 600 lines per millimetre and is illuminated by light of wavelength 589 nm. What is the angle between the central maximum and the first-order maximum, and can the small-angle approximation reasonably be used for this situation?
a) About 6.9∘; the approximation is justified
b) About 42∘; the approximation is not justified
c) About 21∘; the approximation is not justified
d) About 21∘; the approximation is justified