Interference, path difference and coherence
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Question
What is superposition?
Answer
Where two (or more) waves overlap, you **add their displacements** at every point to get the resultant.
Question
What is constructive interference?
Answer
Waves arrive **in step** (in phase) so their **amplitudes add**, giving a bigger wave (bright / loud).
Question
What is destructive interference?
Answer
Waves arrive **half a cycle out of step** (antiphase); equal **amplitudes cancel**, giving zero (dark / quiet).
Question
What is the path difference?
Answer
The **extra distance** one wave travels compared with the other to reach a point, in metres.
Question
Path difference for constructive interference?
Answer
**nλ** — a whole number of wavelengths (0, λ, 2λ, …). **Given** in the data booklet.
Question
Path difference for destructive interference?
Answer
**(n + ½)λ** — a whole number of wavelengths plus a half. **Given** in the data booklet.
Question
What does 'coherent' mean?
Answer
The sources keep a **constant phase difference** (same wavelength, fixed step). Needed for a steady, observable pattern.
Question
Why must the sources be coherent?
Answer
So the constructive and destructive points **stay in fixed places**; a drifting phase difference would smear the pattern away.
Question
Two equal waves, path difference = 1.5λ — resultant amplitude?
Answer
**Zero.** 1.5λ = (1 + ½)λ is destructive, and equal amplitudes cancel completely.
Question
Two equal waves of amplitude A meet in phase — resultant amplitude?
Answer
**2A** — in step, so the amplitudes add.
Question
When is destructive cancellation complete (zero)?
Answer
Only when the two **amplitudes are equal**; otherwise just part of one wave cancels.
Question
How do you tell constructive from destructive from a path difference?
Answer
Divide by λ: a **whole number** → constructive (nλ); a whole number **+ ½** → destructive ((n+½)λ).
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Topic 3.3 hub
Wave phenomena
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