Principle of superposition
Waves - OCR A-Level Physics
Key Definition
Principle of superposition
When two or more waves meet at a point, the resultant displacement equals the vector sum of the individual displacements at that point.
When two or more waves meet at a point, the resultant displacement equals the vector sum of the individual displacements at that point.
- Constructive interferenceWhen two waves meet in phase and their displacements add together, producing a wave of greater amplitude. occurs when waves arrive in phase (path $difference = n\lambda). The displacements add$ to give a larger resultant.
- Destructive interferenceWhen two waves meet in antiphase and their displacements cancel, producing a wave of smaller (or zero) amplitude. occurs when waves arrive in antiphase (path $difference = (n + ½)\lambda). The displacements cancel$.
- After superposition, each wave continues unchanged - waves pass through each other.
Key Definition
Path difference
The difference in the distances travelled by two waves from their sources to a given point. Determines whether interference is constructive or destructive.
The difference in the distances travelled by two waves from their sources to a given point. Determines whether interference is constructive or destructive.
Key Definition
Coherence
Two sources are coherent if they emit waves with a constant phase difference. This requires the same frequency (and ideally the same amplitude for clear fringes).
Two sources are coherent if they emit waves with a constant phase difference. This requires the same frequency (and ideally the same amplitude for clear fringes).
- Coherent sources are needed for a stable, observable interference pattern.
- Two separate light bulbs are NOT coherent - they emit light at random phases.
- A single source split into two (e.g. Young's double slit) produces coherent beams.
- Lasers produce coherent, monochromatic light - ideal for interference experiments.
Examiner Tips and Tricks
When explaining conditions for visible interference fringes, state three things: (1) coherent sources, (2) same frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). (monochromatic light), (3) similar amplitudes for clear contrast between maxima and minima.