Solution:

A particular laser line is emitted at a very specific wavelength corresponding to one mode (light path) in the laser's cavity.

Over time this wavelength varies somewhat around a center wavelength (the amount of variation is the linewidth).

Coherence Length:

The length of time that coherence is maintained is called the “coherence time”. The length that the signal could travel in a vacuum during that time is called the coherence length.

Coherence Time:

The coherence time for an LED is of the order of half a picosecond and its coherence length is around 15 microns.

A simple laser may have a the coherence time of perhaps half of a nanosecond and a coherence length of perhaps 15 centimeters.

A very good quality narrow linewidth laser could have a coherence time of perhaps a microsecond and a coherence length of up to 200 meters.

It is relatively obvious that the range of variation of the signal (the linewidth) should be inversely related to the coherence length (and time).

$ \text { Length }_{\text {coherence }}=c \times \text { Time }_{\text {coherence }}=\frac{\lambda^2}{\Delta \lambda} $

Where $\Delta \lambda=$ Linewidth and $\lambda=$ Centre Wavelength.