How do two light waves create constructive interference?

In the context of optics, constructive interference occurs when two light waves overlap in such a way that their individual disturbances reinforce one another. This phenomenon is governed by the superposition principle, which states that the resultant amplitude of overlapping waves is the algebraic sum of their individual amplitudes.12

For constructive interference to occur, the following conditions must be met:

• Phase Alignment: The waves must be in phase, meaning their crests coincide with one another.12
• Path Difference: When waves originate from the same source (such as in a double-slit experiment), constructive interference occurs at points where the difference in the paths traveled by the two waves is an integral number of wavelengths ($0,\lambda ,2\lambda ,\dots$). This ensures that the crests of both waves arrive at the same location simultaneously.34
• Coherence: To observe a stable interference pattern, the light sources must be coherent, meaning they maintain a constant phase relationship over time.3

When these conditions are satisfied, the combined wave exhibits an increased intensity, appearing as a bright region or fringe.134