Interference & Coherence
Interference & Coherence
- When overlapping waves repeatedly add and cancel, they make an interference pattern: fixed regions of large and small intensity.
- A stable pattern needs sources that keep the same timing relationship. These sources are called coherent.
Why some places are strong and others weak
- Constructive interference: crest meets crest, or trough meets trough. The waves are in phase and the resultant amplitude is larger, so the intensity is greater.
- Destructive interference: crest meets trough. The waves are in antiphase and the resultant amplitude is smaller; equal amplitudes cancel completely, giving zero intensity.
φ = 90°·resultant amplitude = 2A cos(φ/2) = 1.41·A·in between
Definition
1 markWhat is meant by interference?
Model answer: The superposition of two or more waves to produce a resultant wave of greater or smaller amplitude.
Use path difference to predict the result
Path difference is the extra distance travelled by one wave to reach the same point.
Symbols
- = path difference (m)
- = any whole number: 0, 1, 2, ... ()
- = wavelength (m)
Symbols
- = path difference (m)
- = any whole number: 0, 1, 2, ... ()
- = wavelength (m)
Whole wavelengths arrive in phase. A whole number plus half a wavelength arrives in antiphase.
Coherence keeps the pattern still
- Two sources are coherent when they have a constant phase difference. They must therefore have the same frequency.
- Equal amplitude is not part of the definition. It only makes dark points reach zero intensity when cancellation is complete.
Definition
2 marksWhat is meant by coherent sources?
Model answer: Sources that produce waves with a constant phase difference and the same frequency.
Two light sources have slightly different frequencies. Explain why they do not produce a stable interference pattern. [2] (9702/22/O/N/25 Q4(c))
Show worked answer
Their phase difference changes with time, so the sources are not coherent. The bright and dark positions therefore move instead of forming a stable pattern.