1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Thin film interference

  1. Dec 29, 2009 #1
    1. The problem statement, all variables and given/known data

    Light of wavelength [tex]\lambda[/tex] = 500nm, produced by an extended source, is incident at an angle of [tex]\phi[/tex]= 30 degrees from the normal upon a dielectric film of refractive index, n=2, supported on a solid planar substrate. Reflectivity minima are observed to have an angular separation of 0.05 radians. What is the thickness, d, of the film?

    2. Relevant equations

    Phase difference [tex]\delta[/tex] = 2ndcos[tex]\theta[/tex]*(2*pi/[tex]\lambda[/tex]) where sin[tex]\phi[/tex]=nsin[tex]\theta[/tex]

    Intensity proportional to sin2([tex]\delta[/tex]/2)

    3. The attempt at a solution

    0th and first minima occur when the [tex]\delta[/tex]=0 and [tex]\delta[/tex]=2 pi respectively.

    2ndcos[tex]\theta[/tex]*(2*pi/[tex]\lambda[/tex]) = 2pi

    which gives d=258nm

    which seems very small and I didn't have to use the angular seperation to get my answer which leads me to believe I'm doing something wrong.

    I think my mistake is to do with the fact I don't now how to involve the angular separation of 0.05 radians into the solution?

    Help appreciated
  2. jcsd
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted