The mean kinetic energy per atom in a sodium vapour lamp is 0.33 eV. Given that the mass of sodium atom is approximately 22.5 × 109eV, the ratio of the Doppler width of an optical line to its central frequency is
1
7 × 10-7
2
6 × 10-6
3
5 × 10-5
4
4 × 10-4