Edexcel A Level Physics:復習筆記5.8 Phase & Path Difference
Phase & Path Difference
- Waves are said to be?coherent?if they have:
- The same?frequency
- A?constant phase difference
Phase Difference
- Two points on a wave, or on different waves, are in phase when they are the same point in their wave cycle
- The angle between their wave cycles is the phase difference
Path Difference
- The type of interference occurring at a given point (i.e. constructive or destructive) depends on the?path difference?of the overlapping waves
- Path difference is defined as:
The difference in distance travelled by two waves from their sources to the point where they meet
- Path difference is generally expressed in multiples of?wavelength
At point P2?the waves have a path difference of a whole number of wavelengths resulting in constructive interference. At point P1?the waves have a path difference of an odd number of half wavelengths resulting in destructive interference
- In the diagram above, the number of wavelengths between:
- S1?? P1?= 6λ
- S2?? P1?= 6.5λ
- S1?? P2?= 7λ
- S2?? P2?= 6λ
- The path difference at point?P1?is 6.5λ – 6λ =?λ / 2
- The path difference at point?P2?is 7λ – 6λ =?λ
- In general:
- The condition for constructive interference is a path difference of?nλ
- The condition for destructive interference is a path difference of?(n + ?)λ
- In this case, n is an integer i.e. 0, 1, 2, 3...
- Hence:
- Destructive interference?occurs?at?point?P1
- Constructive interference?occurs?at?point?P2
At point P?the waves have a path difference of a whole number of wavelengths resulting in constructive interference
- Another way to represent waves spreading out from two sources is shown in the diagram above
- At point?P, the number of?crests?from:
- Source S1?= 4λ
- Source S2?= 6λ
- The path difference at?P?is 6λ – 4λ =?2λ
- This is a whole number of wavelengths, hence?constructive interference?occurs at point?P
Worked Example
The diagram shows the interferences of coherent waves from two point sources.
Which row in the table correctly identifies the type of interference at points?X,?Y?and?Z.
? ? ? ANSWER:?B
- At point?X:
- Both peaks of the waves are overlapping
- Path difference = 5.5λ – 4.5λ = λ
- This is?constructive?interference and rules out options?C?and?D
- At point?Y:
- Both troughs are overlapping
- Path difference = 3.5λ – 3.5λ = 0
- Therefore?constructive?interference occurs
- At point?Z:
- A peak of one of the waves meets the trough of the other
- Path difference = 4λ – 3.5λ = λ / 2
- This is?destructive?interference
Exam Tip
Phase difference and path difference are easy to confuse because the names sound similar. However they are very different concepts.
Phase difference tells us how far apart the waves are when?comparing their phases?(you can think of this as their peaks and troughs).
Path difference is how much further along one wave is than another. Think of it as how much?further along the path?it has travelled.
轉載自savemyexams
以上就是關于【Edexcel A Level Physics:復習筆記5.8 Phase & Path Difference】的解答，如需了解學校/賽事/課程動態，可至翰林教育官網獲取更多信息。
往期文章閱讀推薦：
全網破防！ALevel CIE數學M1疑似錯題？經濟P2難度飆升？5月6日大考考情分析必看！
A-Level CIE就大規模泄題發布最嚴處罰！哪些考生必須重考？你的成績怎么辦？
翰林AMC8視頻課重磅上線！
國際競賽真題資源免費領取
