The wave emitted by any atom or molecule must have some finite total length which is known as the coherence length. For sodium light, this length is 2.4 cm. The number of oscillations in this length will be:

4.068 × 105 4.068 × 106 4.068 × 107 4.068 × 108

4.068 × 106

No. of oscillations in coherence length = $\frac{1}{\lambda}=\frac{0.024}{5.9 \times 10^{-7}}$ = 4.068 × 106 Hz