3.3.2.1
Path difference determines whether interference is constructive or destructive
Interference & Diffraction — AQA A-Level Physics
Key Definition
Path difference — The difference in distance travelled by two waves from their sources to the point where they meet.
- Path differenceThe difference in distance travelled by two waves from their sources to a given point. Determines whether constructive or destructive interference occurs. is expressed in multiples of wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m)..
- It is the difference in the two distances, not the distance between the two paths.
Conditions for interference
- Constructive interference (maxima): path $difference = n \lambda$, where n = 0, 1, 2, 3...
- Destructive interference (minima): path $difference = (n + 1/2) \lambda$, where n = 0, 1, 2, 3...
$$\text{Constructive: path differenceThe difference in distance travelled by two waves from their sources to a given point. Determines whether constructive or destructive interference occurs.} = n\lambda$$
$$\text{Destructive: path differenceThe difference in distance travelled by two waves from their sources to a given point. Determines whether constructive or destructive interference occurs.} = \left(n + \tfrac{1}{2}\right)\lambda$$
Examiner Tips and Tricks
- Path difference is specifically how much longer (or shorter) one path is than the other.
- Do not confuse it with the distance between the two sources.