Why does light from a source like sodium lamp undergo abrupt changes in phase? 
"If we use two sodium
lamps illuminating two pinholes (Fig. 10.11) we will not observe any
interference fringes. This is because of the fact that the light wave emitted
from an ordinary source (like a sodium lamp) undergoes abrupt phase changes in times of the order of $10^{-10}$ secs"


This was a point written in my HS textbook on wave optics, but what could be an explanation given to the above point on why that sudden phase change happens?
My current understand is that a sodium lamp works by taking in electrical energy to excite the sodium atoms and then release that energy as light. So, I can't see where the point of phase change could come here.
 A: I will take a stab at this.
Sodium vapor lamps work by forcing electrical flow through an ionized cloud of sodium vapor. This heats the vapor and kicks sodium electrons temporarily into higher energy orbitals. After a very short (but unpredictable) time, the electrons fall back into their original orbitals and emit a photon in a random direction. The process then repeats.
Now remember that the vapor consists of individual sodium atoms of order ~10^10 to 10^20 in number and at any instant a large (but unpredictable) number of them will be emitting photons at rapid (but unpredictable) intervals in random directions.
This guarantees that the phase relationship between any two photons given off by that cloud of glowing vapor in any particular portion of it at any particular instant in time will be similarly random i.e., uncorrelated.
I do not know how the author arrived at the specification of random phase shifts occurring on timescales of order ~10^-10 seconds, but it seems reasonable.
A: Here's the difference between the Na lamp's output and the output of a laser.
The laser generates output (photons) via stimulated emission, a process which produces photons which are in phase with the stimulating (input) photons.   However, the Na lamp generates photons, albeit from the same electron  quantum gap (between two levels) via the standard statistical random timing.  These photons, then, have a random relative phase because there is very little, if any, stimulated emission occurring.
A: The light from these two sources forms an incoherent not a coherent superposition. Perhaps this is meant by "abrupt phase changes".
