1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Newton's Rings and Gap Size - See Attachment

  1. Nov 6, 2012 #1
    1. The problem statement, all variables and given/known data

    A shallowly curved piece of glass
    is placed on a flat one. When
    viewed from above, concentric
    circles appear that are called
    Newton’s rings. In order to see
    bright rings, the gap can be:

    1. 0
    2. 1/4λ
    3. 1/2λ
    4. λ
    2. Relevant equations

    1/2 λ occurs when n2>n1

    3. The attempt at a solution

    Why isn't the answer 1/2λ? For ray 1, when air travels into glass there is a 180° phase change. When glass travels back into air, there is no phase change. For Ray 2, it looks like air travels into air (although through glass) back into air so I didn't think there would be a phase change for that.
     

    Attached Files:

  2. jcsd
  3. Nov 6, 2012 #2

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Your wording has me a bit confused. The glass and the air are not traveling, just the light is traveling. Also, I'm not sure which ray you are calling Ray 1. Anyway, what's important are the two reflections that are labeled by B and C.

    Is there a phase change at B? At C?
     
  4. Nov 6, 2012 #3
    Sorry about that. I meant to say light is traveling.

    Phase change at B - Yes.
    Phase chance at C - No
     
  5. Nov 6, 2012 #4

    TSny

    User Avatar
    Homework Helper
    Gold Member

    That's not quite right. Can you state the general rule for deciding whether or not there is a phase change?
     
  6. Nov 6, 2012 #5

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Sorry, I see you already did state the rule! So, consider reflection B. Which medium is n1 and which is n2?
     
  7. Nov 6, 2012 #6
    I think my main problem is deciding the values of n2 and n1.

    n1 = air = 1.00
    n2 = glass ≈ 1.5?

    point c

    n1 = air = 1
    n2 = air = 1

    I'm unsure about part "C" because I know light travels through the glass but ends up in air.
     
  8. Nov 6, 2012 #7

    TSny

    User Avatar
    Homework Helper
    Gold Member

    At a point of reflection (say B), n1 is the medium in which the light is traveling just before it strikes the reflecting surface and n2 is the medium that the light would have traveled into if it had not reflected.
     
  9. Nov 6, 2012 #8
    Ohhhh, ok. That would mean n1 = glass ≈ 1.5. n2 = air = 1. n2 < n1 no phase change
     
  10. Nov 6, 2012 #9

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Right.
     
  11. Nov 6, 2012 #10
    Since that is the case, at point "C" there is a phase change. I'm still trying to understand why the answer is 1/4λ.
     
  12. Nov 6, 2012 #11

    TSny

    User Avatar
    Homework Helper
    Gold Member

    OK, good. So, the two reflections together amount to a half-wavelength phase difference. In order to get them back in phase, what extra distance does one wave have to travel compared to the other wave (in terms of the wavelength)?
     
  13. Nov 6, 2012 #12
    Does the other wave have to go an extra "1/2" because that way both waves will be in phase and lead to constructive interference?
     
  14. Nov 6, 2012 #13

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Exactly! So, how would you express the extra distance traveled by one of the waves in terms of the gap distance?
     
  15. Nov 6, 2012 #14
    I'm going to pretend I don't know what the answer is, haha.

    Instinctively I would add them to get λ but I'm trying to look for an equation so I can algebraically see why the answer is 1/4λ.
     
  16. Nov 6, 2012 #15

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Can you visualize the "extra distance" traveled by the wave that reflected at C?
     
  17. Nov 6, 2012 #16
    Yes. Would it make sense if I thought about it like "light has to travel the wedge distance twice?"
     
  18. Nov 6, 2012 #17

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Yes.
     
  19. Nov 6, 2012 #18
  20. Nov 6, 2012 #19

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Yahoo!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Newton's Rings and Gap Size - See Attachment
  1. Newton's Rings (Replies: 1)

  2. Newton's Rings (Replies: 1)

  3. Newtons Rings (Replies: 3)

Loading...