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. Work this backwards to get the wavelength of light. White light fringes show that red wavelength is longer than blue. Interference of many coherent sources makes pattern brighter.