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# Physics past papers question

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1. Two coherent sources emit waves of wavelength lamda which are in phase. The two waves meet at a point , having travelled slightly different distances . The waves now have a phase difference of 180 degrees. Which of the following could be the path difference at this point?
2. (Original post by Rosie1999)
Two coherent sources emit waves of wavelength lamda which are in phase. The two waves meet at a point , having travelled slightly different distances . The waves now have a phase difference of 180 degrees. Which of the following could be the path difference at this point?
What are the following possibilities?
3. i never studied this.. hopefully will on next few onths
4. A) lamda/ 4
b) lamda / 2
c) 3lamda/4
4) lamda
5. (Original post by Dowel)
What are the following possibilities?
A) lamda/ 4
b) lamda / 2
c) 3lamda/4
d) lamda
6. (Original post by Rosie1999)
Two coherent sources emit waves of wavelength lamda which are in phase. The two waves meet at a point , having travelled slightly different distances . The waves now have a phase difference of 180 degrees. Which of the following could be the path difference at this point?
If they are 180 degrees out of phase there is destructive interference. for there to be destructive interference the path difference must be half a whole number of wavelength. n* lamba / 2

7. yup that's correct answer thanks for the explanation but isn't the destructive interference has a path difference ( n + 1/2 )lamda ??
8. (Original post by Rosie1999)
yup that's correct answer thanks for the explanation but isn't the destructive interference has a path difference ( n + 1/2 )lamda ??
Both and are valid expressions for the path difference when destructive interference occurs - in the first case, can be 0, but not in the second case (because then the path difference would be 0).
9. (Original post by Rosie1999)
yup that's correct answer thanks for the explanation but isn't the destructive interference has a path difference ( n + 1/2 )lamda ??
Destructive interference is when the path difference is half any whole number of wave lengths. Let the whole number be n and lambda be wavelength. Then we can say that (n *lambda)/2 is destructive interference.
(n + 1/2) lambda is the same but an unnecessarily complicated version. I personally use the the first one.
10. Oh , didn't know that , thank you so much )
11. Thank you so much for your reply and expalantion< it really helped

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