Physics-
General
Easy

Question

In the adjacent diagram, C P represents a wavefront and A O and B P, the corresponding two rays. Find the condition on theta for constructive interference at P between the ray B Pand reflected ray O P

  1. cos invisible function application theta equals fraction numerator 3 lambda over denominator 2 d end fraction    
  2. cos invisible function application theta equals fraction numerator lambda over denominator 4 d end fraction    
  3. sec invisible function application theta minus cos invisible function application theta equals fraction numerator lambda over denominator d end fraction    
  4. sec invisible function application theta minus cos invisible function application theta equals fraction numerator 4 lambda over denominator d end fraction    

The correct answer is: cos invisible function application theta equals fraction numerator lambda over denominator 4 d end fraction


    P R equals d
    P O equals d sec invisible function application theta
    And C O equals P O cot invisible function application blank 2 theta equals d sec invisible function application theta cos invisible function application 2 theta
    Path difference between the two rays is,
    increment x equals C O plus P O
    equals left parenthesis d sec invisible function application theta plus blank d sec invisible function application theta cos invisible function application 2 theta right parenthesis
    Phase difference between the two rays is
    increment ϕ equals pi (one is reflected, while another is direct)
    Therefore condition for constructive interference should be

    capital delta x equals fraction numerator lambda over denominator 2 end fraction comma fraction numerator 3 lambda over denominator 2 end fraction horizontal ellipsis.
    Or d sec invisible function application theta open parentheses 1 plus cos invisible function application 2 theta close parentheses equals fraction numerator lambda over denominator 2 end fraction
    Or open parentheses fraction numerator d over denominator cos invisible function application theta end fraction close parentheses open parentheses 2 cos to the power of 2 end exponent invisible function application theta close parentheses equals fraction numerator lambda over denominator 2 end fraction
    Or cos invisible function application theta equals fraction numerator lambda over denominator 4 d end fraction

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