Wave optics necessary when wavelength is of size or smaller than object. Superposition, allows waves to add or negate each other. Called constructive and destructive interference.
Light is of one wavelength and comes from one source that gives a consistent phase.
Incoherent if phase changes rapidly and randomly - the case for most light sources. When recombined, can be constructive or destructive. Camera lenses have coatings to reduce reflections. Best for green, so reflected light is purple. Happens also for soap bubles and oil films.