Wave Optics

For two light sources to be coherent, they must emit waves having the same wavelength and maintain a constant phase difference over time.
This ensures stable and observable interference patterns.
Amplitude, speed, or intensity can differ, but coherence strictly requires a fixed phase relationship.

Only transverse waves can be polarized, because polarization involves restricting the vibrations to a single plane, which is only possible if the direction of vibration is perpendicular to the direction of propagation.
In gases, longitudinal waves (like sound) involve compressions and rarefactions along the direction of propagation and cannot be polarized.
Transverse waves (though uncommon in gases) can, in principle, be polarized if they exist.

In Young's double slit experiment, the central point on the screen corresponds to the point where the path difference between the two interfering waves is zero.
When the path difference is zero: Therefore, a bright fringe is formed at the central point.