Interference and coherence Edexcel International A Level Physics revision
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In plain words
Two sets of ripples crossing on a pond make a pattern: in some places they pile up, and in others they cancel out. For the pattern to hold still, the two sources must keep in step with each other. Light does this too, and it is the proof that light is a wave.
5 things to know
- Where two waves arrive in phase they reinforce each other (constructive interference). Where they arrive in antiphase they cancel (destructive interference).
- Coherent sources have the same frequency and a constant phase difference. A steady interference pattern needs coherent sources.
- Path difference is the difference between the distances from the two sources to a point. For sources that are in phase, a whole number of wavelengths gives constructive interference, and an odd number of half wavelengths gives destructive interference.
- Path difference and phase difference are linked: a path difference of one wavelength is a phase difference of 360° (2π radians).
- Two-source interference can be shown with water waves (two dippers on one bar in a ripple tank), sound (two loudspeakers on one signal generator), microwaves (two slits in a metal plate) and light (two slits lit by one laser).
Worked example
Two loudspeakers connected to the same signal generator give out sound of wavelength 0.40 m, in phase. A point P is 3.0 m from one loudspeaker and 3.6 m from the other. Is the sound at P loud or quiet?
- Path difference = 3.6 − 3.0 = 0.60 m.
- In wavelengths that is 0.60 ÷ 0.40 = 1.5.
- One and a half wavelengths is an odd number of half wavelengths, so the waves cancel: P is quiet.
Tips and tricks
- Two separate lamps never give a pattern, because they aren't coherent. One source split in two, such as one laser lighting two slits, is.
- Work out the path difference in wavelengths first. A whole number means loud or bright; a half number means quiet or dark.
It lands in your notebook with its questions as flashcards.
Interference and coherence: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
For two sources in phase, what path difference gives constructive interference?
The waves arrive in phase.
What is needed for two sources to be coherent?
Without that the pattern keeps shifting and washes out.
Two waves of equal amplitude arrive at a point in antiphase. What is the resultant amplitude?
They cancel completely.
Two sources are in phase. What kind of interference is there where the path difference is three wavelengths?
Three is a whole number of wavelengths.
Why do two separate filament lamps not give an interference pattern?
The phase difference between them changes all the time.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 6 marks. Write your answers on paper, then check them.
Interference and coherence
Edexcel International A Level Physics WPH · 6 marks · papermunch.org
Name ______________________________ Date ______________
State what is meant by coherent sources.[2]
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Sources with the same frequency and a constant phase difference.
Two microwave sources emit waves of wavelength 1.0 cm in phase. At a point the path difference is 2.5 cm. State and explain whether there is a maximum or a minimum there.[2]
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A minimum. The path difference is 2.5 wavelengths, an odd number of half wavelengths, so the waves arrive in antiphase.
Find the phase difference that goes with a path difference of a quarter of a wavelength.[2]
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90° (π/2 radians).
Answers: Interference and coherence
- 1. Sources with the same frequency and a constant phase difference.
- 2. A minimum. The path difference is 2.5 wavelengths, an odd number of half wavelengths, so the waves arrive in antiphase.
- 3. 90° (π/2 radians).