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Homework Help: Newton's Rings (Fringes)

  1. Apr 23, 2007 #1
    Figure 35-46a shows a lens with radius of curvature R lying on a flat glass plate and illuminated from above by light with wavelength . Figure 35-46b, a photograph taken from above the lens, shows that circular interference fringes (called "Newton's rings") appear, associated with the variable thickness d of the air film between the lens and the plate. The radius of curvature R of the lens is 5.0 m and the lens diameter is 17 mm.

    (a) How many bright rings are produced? Assume that = 555 nm.

    (b) How many bright rings would be produced if the arrangment were immersed in water (n = 1.33)?

    I used the equation: m=r^2/R*lambda - .5

    Definetly not getting the right answer
  2. jcsd
  3. Apr 23, 2007 #2


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    Can you show more of your calculation?
    Last edited: Apr 23, 2007
  4. Apr 23, 2007 #3
    m=r^2/R*lambda - .5

    m=.017^2/(5*555*10^-9) - .5

    m=(1.33).017^2/(5*555*10^-9) - .5

    I've tried 103,104, and 105 for a

    and done the same for b

    but the method cant be right because in the book r=20mm, R=5m lambda=589nm and the answers are a. 34 and b. 46
  5. Apr 23, 2007 #4


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    17 mm is the diameter, not the radius.

    I don't understand, these are given in the answers?
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