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For more information, please see full course syllabus of AP Physics B
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Thin Film Interference

  • Change of phase upon reflection: When light, traveling in a medium of index of refraction n_1, is reflected from a medium of index of refraction n_2 > n_1, a phase change of 180 degrees takes place.
  • The light waves reflected from a thin film of index of refraction n and thickness t, interfere destructively (i.e., no reflection takes place, the film looks dark) if

    2 n t = m * lambda

    Where lambda is the wavelength of the light and m = 0, 1, 2,...

    Constructive interference occurs when 2 n t = (m + 1/2)*lambda.

Thin Film Interference

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Change of Phase Due to Reflection 0:37
    • Plane Mirror
    • Object Produces Virtual Image
    • Consider a Screen and Point
    • Path Difference
    • Constructive Interferences
    • Destructive Interference
    • Two Media N1, N2
    • N2>N1 Changes in Phase 180 Degrees
  • Thin Film Interference 18:50
    • Air and Film and Air Film of Thickness
    • Angle of Incident is Very Small
    • Two Waves are Destructive
    • Path Difference
    • If Delta=1, 2, 3 No Change in Phase
    • Destructive Interference
    • Constructive Interferences
  • Example: Soap Bubbles 33:34
    • Air, Soap, Air
    • Thickness Results in Constructive Interference
  • Example: Non-Reflective Coating For Solar Cells 38:05
    • Sending Light
    • Destructive Interference
  • Extra Example 1: Spaced Plates Separation
  • Extra Example 2: Oil Film