Enhancement and cancellation
In a linear wave description, contributions add as amplitudes. Matching phases enhance a signal; opposing contributions can reduce or cancel it locally. For light, adding the separate beam intensities misses interference because the detected intensity depends on the square of the combined field.
References: [1]
Double slits and path difference
Illuminating two narrow slits from a common source can maintain a stable phase relationship. Different paths to points on a screen produce alternating bright and dark fringes. Their spacing depends on wavelength, slit separation and observation geometry, linking the pattern to both source and apparatus.
References: [1]
Why ordinary lamps rarely make stable fringes
Stable fringes require suitable coherence: relative phase must not fluctuate too rapidly during observation. Independent ordinary lamps generally lack this stability. Reflections from a thin film can instead interfere because their shared origin and path difference preserve useful relationships, producing thickness-dependent colours.
Interference in quantum experiments
Individual detections of very weak light or electrons can be localized, yet repeated events accumulate into interference distributions. Quantum calculations combine probability amplitudes before deriving probabilities. Interference therefore applies not only to familiar macroscopic waves but also to the statistical outcomes of particle experiments.
References: [3]