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AFMC AFMC Solved Paper-2002

  • question_answer
    A paper, with two marks having separation d, is held normal to the line of sight of an observer a[ distance of 50 m. The diameter of the eye lens of [he observer is 2 mm. Light wavelength 5000\[\overset{0}{\mathop{A}}\,\] is made to fall on the lens. The least value of d, so that the marks can be seen as separate, will be :

    A) 2.125 cm                             

    B) 1.525 cm

    C) 0.125cm                              

    D) 1.225 cm

    Correct Answer: B

    Solution :

    Key Idea: Resolving power is the ability of lens to measure the angular separation of images that are close together. The resolving power of a lens is limited by diffraction effects. The lens aperture is a hole that is analogous to a two dimensional  version of the single slit experiment. For an ideal lens the resolution is \[d=\frac{1.22\lambda \times D}{a}\] \[\therefore \]     \[d=\frac{1.22\times 5000\times {{10}^{-10}}\times 50}{2\times {{10}^{-3}}}\]               = 1.525 cm


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