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 2.4cm.The coherence time will be:

8 x 10–8s    8 x 10–9s  8 x 10–10s  8 x 10–11s

8 x 10–11s

$\text{ The coherence time t } = \frac{l}{c} = \frac{0.024}{3\times 10^8} = 8\times 10^{-11}s$