I am currently studying the textbook Laser Systems Engineering by Keith Kasunic. Chapter 1.2.2 Spatial coherence says the following:
The concepts of spatial coherence and beam quality arise because there may be phase variations $\ge \lambda/4$ across the diameter of each mirror in any laser cavity.
The value of $\lambda/4$ seems quite specific, but, at the same time, the author seems to introduce it out of nowhere. Where does this value of $\ge \lambda/4$ come from? Is it derived from some physical property related to laser cavities?