A parallel beam of monochromatic light with wavelength $\lambda$ falls normally on a diffraction grating $G$. As shown, the angle between the directions of the two second-order diffracted beams at $P_1$ and $P_2$ is $\alpha$. What is the grating line spacing?
- A$\dfrac{2\lambda}{\sin \alpha}$
- B$\dfrac{\lambda}{\sin \alpha}$
- C$\dfrac{2\lambda}{\sin (\alpha/2)}$
- D$\dfrac{\lambda}{\sin (\alpha/2)}$