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The Huygens–Fresnel principle

  1. Oct 28, 2016 #1
    1. The problem statement, all variables and given/known data
    3229193775_1519209300_574_574.jpg
    I don't understand about some words of this problem. Does "over time" mean? Why constructive interference (CI) appear?
    2. Relevant equations
    I think we use The Huygens–Fresnel principle

    3. The attempt at a solution
    I choose answer 3-c
    3229194544_1358055013_574_574.jpg
    But I don't know the physical phenomenon.
     
  2. jcsd
  3. Oct 28, 2016 #2

    andrevdh

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    The incident wave and reflected wave interferes with each other and produces regions (crests) of inteference that travels in a certain direction.
    The question wants you to answer in which direction these constructive interference regions would travel.
     
  4. Oct 28, 2016 #3
    What is the correct answer?
     
  5. Oct 28, 2016 #4

    andrevdh

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    Have a look at the concept of reflection in a mirror and combine this with what you know about the interference pattern of a double slit.
     
  6. Oct 28, 2016 #5
    The correct answer is 1-a.
    There are many double slits not one . And the interference image is total of many double slits. How to know the direction of interference?
     
  7. Oct 28, 2016 #6
    Help me ??? Thanks
     
  8. Oct 28, 2016 #7

    TSny

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    Do any of the marked points below represent a point of constructive interference between the incident and reflected waves? If so, which?
    upload_2016-10-28_16-30-37.png
     
  9. Oct 28, 2016 #8
    I think a, b and c. because they are in constructive interference area between the incident and reflected waves.
     
  10. Oct 28, 2016 #9
    If I seem the souce S is very far from wall. The constructive interference area between the incident and reflected waves will be rectilineal. And I take the S' image, we have a double slits SS'. The waves go to P before going to Q. So 1-a is the right direction. Is OK ?
    3229243376_204113474_574_574.jpg
     
    Last edited: Oct 28, 2016
  11. Oct 29, 2016 #10

    TSny

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    The question asks about crests of constructive interference. In post #7 I should have been more specific and asked which of the 4 points is located on a crest of constructive interference. Only one of the points a, b, c, d is such a point.
     
  12. Oct 29, 2016 #11
    i think it is b. I dont know how the constructive interference appears? Because if constructive interference appears. Must have double slits or d2-d1=k lambda. I dont see any double slits. And cant calculate d2-d1. Does my idea in #9 correct?
     
  13. Oct 29, 2016 #12

    TSny

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    Interference occurs whenever two or more waves overlap. The double slit experiment is just one example of a case where waves interfere. In the situation that you are dealing with here, the reflected waves overlap with the incoming waves. So, they will interfere.

    A crest of constructive interference occurs where the crest of one wave overlaps the crest of another wave. So, if you look at the point a, b, c, d in post #7, only one of those points is a place where the crest of an incoming wave overlaps the crest of a reflected wave. (The blue lines represent wave crests.)
     
  14. Oct 29, 2016 #13
    Is c right ?Intially I think your blue lines is the direction of the incident. so I choose b
     
  15. Oct 29, 2016 #14

    TSny

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    upload_2016-10-29_11-43-15.png
    The blues lines are the wave crests. The lighter blue lines are wave crests of the incident wave. The green arrows show the direction of propagation of these incident wave crests. The darker blue lines are wave crests of the reflected wave. The orange arrows show the direction of propagation of these reflected wave crests.

    At which of the points a b c d does a wave crest of the incident wave overlap with a wave crest of the reflected wave?
     

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  16. Oct 29, 2016 #15
    I think that is c. But how this connect with the direction of constructive interference appear ?
     
  17. Oct 29, 2016 #16
    upload_2016-10-30_0-0-4.png
    The red points will be the crest of the incident wave overlap with a wave crest of the reflected wave.
     

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  18. Oct 29, 2016 #17

    TSny

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    Yes, point c is a point where crests are constructively interfering. You want to find the direction in which point c moves as the waves propagate. Draw the figure as shown. On the same figure, draw the waves as they would appear a little later in time.
     
  19. Oct 29, 2016 #18
    I understanded. Thanks upload_2016-10-30_0-9-50.png
     
  20. Oct 29, 2016 #19

    TSny

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    OK. Good.
     
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