Physics-
General
Easy

Question

In the figure, an air lens of radii of curvature 10 cm (R subscript 1 end subscript = R subscript 2 end subscript = 10 cm) is cut in a cylinder of glassleft parenthesis mu equals 1.5 right parenthesis. The focal length and the nature of the lens is

  1. 15 cm, concave    
  2. 15 cm, convex    
  3. infinity, neither concave nor convex    
  4. 0, concave    

The correct answer is: 15 cm, concave


    fraction numerator 1 over denominator f end fraction equals open parentheses g mu subscript a end subscript minus 1 close parentheses open parentheses fraction numerator 1 over denominator R subscript 1 end subscript end fraction minus fraction numerator 1 over denominator R subscript 2 end subscript end fraction close parentheses equals open parentheses fraction numerator 2 over denominator 3 end fraction minus 1 close parentheses open parentheses fraction numerator 2 over denominator 10 end fraction close parentheses
    rightwards double arrow f equals negative 15 c m, so behaves as concave lens.

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