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JEE Main & Advanced Physics Ray Optics Question Bank Mock Test - Geometrical Optics

  • question_answer
    A point object is placed at a distance of 25 cm from a convex lens of focal length 20 cm. If a glass slab of thickness t and refractive index 1.5 is inserted between the lens and the object, the image is formed at infinity. The thickness t is

    A) 10 cm  

    B) 5 cm

    C) 20 cm  

    D) 15 cm

    Correct Answer: D

    Solution :

    [d] Image will be formed at infinity if object is placed at focus of the lens, i.e., at 20 cm from the lens. Hence, Shift = 25 - 20\[=\left( 1-\frac{1}{\mu } \right)\mu \] Or \[5=\left( 1-\frac{1}{1.5} \right)t\]or \[t=\frac{5\times 1.5}{0.5}=15cm\]


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