How do two light waves create destructive interference?

In the study of optics, destructive interference occurs when two light waves overlap such that they are out of phase with each other. Specifically, this happens when the crest of one wave coincides with the valley (or trough) of another.12

When these waves are out of phase by a half-integral number of wavelengths—such as $\lambda /2$, $3\lambda /2$, or $5\lambda /2$—the electric and magnetic fields of the waves point in opposite directions. If the individual waves have equal amplitude, their superposition results in complete cancellation, meaning the net disturbance to the medium is zero.3412

This phenomenon is a fundamental consequence of the superposition principle, which dictates that the resultant disturbance of overlapping waves is the algebraic sum of their individual disturbances. In experiments like Young’s double-slit experiment, these regions of destructive interference appear as dark bands where the light waves have effectively extinguished one another.123