Light of wavelength $\lambda$ is incident at normal incidence on a diffraction grating. The angle between the second-order maximum and the normal to the grating is $\theta$. The graph shows how $\lambda$ varies with $\sin\theta$. How many lines per millimetre are there on the diffraction grating?
- A$400\,\text{mm}^{-1}$
- B$625\,\text{mm}^{-1}$
- C$800\,\text{mm}^{-1}$
- D$1250\,\text{mm}^{-1}$