Physics-
General
Easy

Question

White light is incident normally on a thin film which has n = 1.5 and a thickness of 5000 Å. For what wavelengths in the visible spectrum (4000 – 7000 Å) will the intensity of the reflected light be a maximum?

  1. 5000 Å, 6000 Å    
  2. 5260 Å, 6000 Å    
  3. 4285 Å, 6000 Å    
  4. 4500 Å, 6500 Å    

The correct answer is: 4285 Å, 6000 Å


    Constructive interference occurs for
    OPD = 2nt = k +fraction numerator lambda over denominator 2 end fraction,
    where t is the thickness of the film, k an integer. Hence for maximum reflection the wavelengths are
     =fraction numerator 4 n t over denominator 2 k plus 1 end fraction=open curly brackets table row cell 6000 Å f o r k equals 2 end cell row cell 4285 Å f o r k equals 3. end cell end table close

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