Discussion
The coherence length of laser light is :
Answer: B
The coherence length is inversely proportional to the width of the spectral line.
The coherence time of a source is roughly the inverse of its frequency bandwidth, and the coherence length is the distance light travels in that time:
\[ \begin{aligned} L_{c} &= c\,\tau_{c} \approx \frac{c}{\Delta\nu} \\ &= \frac{\lambda^{2}}{\Delta\lambda} \end{aligned} \]
A laser's very narrow line therefore gives a long coherence length, metres or more, compared with micrometres for white light.
Direct proportionality to the line width has the relation upside down. The length of the active medium affects gain and mode spacing, not the coherence length directly, so neither option involving it is correct.
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