A beam of light with wavelength $5.5 \times 10^{-7}\,\text{m}$ falls normally on a diffraction grating. The separation between the second-order diffraction maxima is $80^\circ$, as illustrated. Determine the number of lines per metre of the diffraction grating.
- A$5.8 \times 10^{5}$ lines per metre
- B$9.0 \times 10^{5}$ lines per metre
- C$1.2 \times 10^{6}$ lines per metre
- D$2.3 \times 10^{6}$ lines per metre