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The thinnest film in which the reflected light will be a max

  • Thread starter Any Help
  • Start date
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1. Homework Statement
Monochromatic light, at normal incidence, strikes a thin film in air. If λ denotes the wavelength in the film, what is the thinnest film in which the reflected light will be a maximum?


  • A

    λ/4



  • B
    3λ/4



  • C
    λ


  • D

    much less than λ



  • E

    λ/2

2. Homework Equations
n air ∠ n glass
so at the first reflection it will cuase a 180° ray out of phase while the second ray refected from the inside it will stay inphase because n air ∠ n glass
so for constructive interference: 2L=odd number*λ/2
3. The Attempt at a Solution
least odd number is one , so L=λ/4 ?? why it is incorrect
 
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Answers and Replies

TSny
Homework Helper
Gold Member
12,158
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Your answer of λ/4 is correct. The problem statement does not mention glass.
 
703
272
Well all I can do is sympathize. 1/4 should have been correct. Perhaps they have in mind a low index film on a high index substrate? Or perhaps the problem setter forgot about the 180 phase shift front to back and just got it wrong.
 
NascentOxygen
Staff Emeritus
Science Advisor
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1,067

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