Originally Posted by **engchunh** Does anyone know the explanation of this question? |

The diffraction formula is

d (sin α + sin θ) = m λ

d: Groove spacing

α: Angle between the incoming light and the normal of the grating

θ: Angle between the difracted light and the normal of the grating

m: Order

λ: Wavelength

If the angle of incidence is perpendicular to the grating, the |θ| must be the same for m=-1 and m=+1.

If θ differs for these two orders (i.e. m=-1 and m=+1), which is the case here, that means the angle of incidence is not perpendicular to the grating.

So the answer is B.