Interference (definition)
● VERBATIM — examined word-for-wordInterference is the superposition of two or more coherent waves to give a resultant wave whose amplitude is given by the principle of superposition, producing a fixed pattern of points of maximum and minimum intensity.
What this actually means
The word coherent is compulsory. Superposition happens whenever any two waves meet; interference is the name for the steady, observable pattern you get only when the sources hold a constant phase relationship.
The second compulsory idea is that the pattern is fixed in space. If the phase relationship drifts, the maxima and minima keep moving and the eye (or the detector) averages them into uniform brightness.
Constructive interference at a point means the waves arrive in phase and the resultant amplitude is the sum. Destructive interference means they arrive in antiphase and the resultant amplitude is the difference.
Interference does not create or destroy energy. It redistributes it: energy that does not appear at the minima turns up at the maxima, so the total across the pattern is unchanged.
Do not describe interference as waves cancelling. Say waves superpose to give a resultant of larger or smaller amplitude. Cancelling implies destruction and reads badly to a marker.
Defining interference without the word coherent, which loses the mark because it then describes ordinary superposition.
Prove it — watch it be true
- Open the two-source field with coherence on and note the fixed pattern of maxima and minima.
- Turn on the randomise-phase toggle and watch the screen intensity profile flatten to uniform brightness.
- Turn coherence back on and confirm the same fringe positions return.