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Phase describes the position of a point within a wave cycle, expressed as an angle from 0 to 2π. Phase difference between two waves determines whether they interfere constructively (same phase) or destructively (π radians out of phase). A path difference of λ/2 corresponds to a phase difference of π radians.
You already know how to describe a wave using its wavelength, frequency, and speed. Those quantities tell you the wave's shape and how fast it repeats in space and time. But to understand what happens when two waves meet — whether they add up or cancel — you need one more concept: phase. Phase tells you where a particular point on a wave is within its cycle at a given moment, expressed as an angle between 0 and 2π radians (or equivalently, 0° to 360°).
Imagine two ocean waves approaching the same spot. If the crest of one wave arrives at exactly the same moment as the crest of the other, the two waves are in phase (phase difference = 0). Their amplitudes add — the combined crest is twice as tall. This is constructive interference. Now imagine one wave arrives exactly half a cycle late — while one has a crest arriving, the other delivers a trough. The phase difference is π radians (180°). Crest and trough cancel exactly, leaving calm water. This is destructive interference. Every intermediate phase difference produces a result between these two extremes.
The connection between path difference and phase difference is the key tool for spatial problems. Two waves travel the same frequency but arrive at a point via paths of different lengths. If one path is longer by a full wavelength λ, the wave that traveled the longer path has completed exactly one extra cycle — it arrives at the same phase as if the paths were equal. Phase difference = 0, constructive interference. If the path difference is λ/2, that wave has gone through half an extra cycle — arriving exactly out of phase. The conversion is: phase difference (in radians) = (2π/λ) × path difference. This formula is what allows you to predict interference patterns from geometry alone, which is the foundation for analyzing double-slit experiments, diffraction gratings, and thin-film problems.
A helpful clock analogy: think of phase as the position of a clock's hand. Two clocks running at the same speed are "in phase" if both hands point in the same direction at every moment. They are "out of phase by π" if one shows 12:00 while the other shows 6:00 — always pointing in opposite directions. The path difference tells you how many full clock rotations one wave has gained on the other; the phase difference tells you what angle separates the two hands after those extra rotations.