When monochromatic light of wavelength 6.0 × 10-7 m is incident normally on a diffraction grating, the third-order maximum is observed at an angle of 45∘45^\circ45∘.
What is the number of lines per mm on the diffraction grating?
3.9×1023.9 \times 10^{2}3.9×102
3.9×1053.9 \times 10^{5}3.9×105
1.2×1031.2 \times 10^{3}1.2×103
1.2×1061.2 \times 10^{6}1.2×106
