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JEE Main & Advanced Physics Ray Optics Question Bank Self Evaluation Test - Ray Optics and Optical Instruments

  • question_answer
    A container is filled with water \[\left( \mu =1.33 \right)\] up to a height of 33.25 cm. A concave mirror is placed 15 cm above the water level and the image of an object placed at the bottom is formed 25 cm below the water level. Focal length of the mirror is

    A) 15 cm

    B) 20cm

    C) -18.31cm

    D) 10cm

    Correct Answer: C

    Solution :

    [c] The image I' for first refraction (i.e., when the ray comes out of liquid) is at a depth of \[=\frac{33.25}{1.33}=25cm\left[ \therefore \text{Apparent depth=}\frac{\text{Real depth}}{\mu } \right]\]Now, reflection will occur at concave mirror. For this F behaves as an object. Where \[\frac{25}{1.33}\] is the real depth of the image?  Using mirror formula we get \[\frac{1}{f}=\frac{1}{v}+\frac{1}{u},f=-18.31cm\]


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