What wave phenomena best describes the reason for the dark and light bands that appear on the optical screen during the double slit experiment?

Interference

Explanation:

Interference is the phenomenon in which two waves superimpose to form a wave with smaller, larger or same amplitude.

There are two types of interference namely, constructive interference and destructive interference.

Constructive interference occurs when the waves are in phase and destructive interference occurs when the waves are out of phase.

In a double slit experiment, the two slits acts as sources of light and thus the waves combine to produce interference patterns. When the waves are in phase, that is the angle between them is 0°, they form a constructive interference pattern which gives rise to a light band. When the waves are out of phase, that is the angle between them is 180°, they form a destructive interference pattern which gives rise to a dark band.

Note also that diffraction patterns are also observed for wide slits but can be neglected for smaller slits.