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Interference of light Intensity

  1. Jan 12, 2016 #1
    An Interference pattern is produced by light of wavelength 490nm from a distant source incident on two identical parallel slits separated by a distance (between centers) of d=0.630mm.
    a) If the slits were very narrow what would be the angular position theta1 and theta2 of the first and second lateral interference maxima?
    b)Let the slits have width a=0.42 mm. In terms of the intensity I0 at the center of the central maximum, what is the intensity at each of the angular positions theta1 and theta2

    2. Relevant equations
    Intensity of light during diffraction:
    I=Im (sinx/x)^2 where Im is the max intensity and x=phase difference/2 = (pi*a*sin(theta))/wavelength
    Intensity of light during interference
    I=4I0cos^2(z) where z = phasedifference/2 = (pi*d*sin(theta))/wavelength

    Intensity of light during double slit diffraction

    3. The attempt at a solution
    I have solved for theta1 and theta2 and got:
    theta1=7.78 x 10^(-4) rd
    theta2= 1.55 x 10^(-3) rd

    What I cant seem to understand is how to find part the intensities in part b. the question does not mention whether we should take the Effects of diffraction into consideration and I don't know what formula to use. what should I do?
  2. jcsd
  3. Jan 12, 2016 #2


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    Staff: Mentor

    You have to take them into account, otherwise there won't be an intensity change. This diffraction is an interference effect as well, by the way.
    The one you have posted.
  4. Jan 12, 2016 #3
    I have posted 3 equations.. does that mean I have to consider the equation with both the interference and the diffraction factors? thanks in advance.
  5. Jan 12, 2016 #4


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    Staff: Mentor

    Well, you know the result of the "interference formula" already: you are calculating the intensity of a maximum.
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