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Homework Help: Double Slit Question with Thin Lens concept

  1. Apr 20, 2016 #1
    1. The problem statement, all variables and given/known data
    Consider a double slit experiment with a light source of 540nm. Slit separation is 4mm. A thin glass plate is placed in front of one of the slit,, resulting in each bright fringe moving to the location of the adjacent bright fringe. Find the minimum thickness of the glass.

    Refractive index of the glass is 1.56. Ignore reflections caused by the glass.

    2. Relevant equations
    2nt= +- mλ (bright fringe)
    2nt= +- m+½λ (dark fringe)

    3. The attempt at a solution
    I think I should be using the thin lens formula since they emphasised the thin glass plate as "thin". Bright fringe moves the distance of a dark fringe to take the position of the adjacent bright fringe.

    It seems too simple to just use

    t= (m+½λ)/2n

    Is there something I am missing?
  2. jcsd
  3. Apr 20, 2016 #2


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    Hello John,

    No lens. But I don't see what minimum thickness you propose. m, t, n ?

    By the way, I'd like to see a drawing of the situation before and after placing the glass plate. I have difficulty understanding what exactly happens with the 1st maximum (does it move to where the 2nd was before?) Then how can the 2nd maximum move to where the 3rd was instead of to where the 4th was ?
  4. Apr 20, 2016 #3

    It's a conceptual question though.

  5. Apr 20, 2016 #4
    I merely used:

    Thickness= λ/4n

    to obtain 8.65×10^-8 m
  6. Apr 20, 2016 #5


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    Bit thin for a glass plate, I would say...

    What is the agrgument for your ##\lambda/(4n) ## ?

    PS I initially misread a part. Perhaps this helps to compensate ...
  7. Apr 20, 2016 #6
    Hmm but I don't have the distance between the slits and the wall.
  8. Apr 20, 2016 #7
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