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JEE Main & Advanced Physics Ray Optics Question Bank Refraction at Curved Surface

  • question_answer
    \[{{f}_{v}}\]and \[{{f}_{r}}\] are the focal lengths of a convex lens for violet and red light respectively and \[{{F}_{v}}\] and \[{{F}_{r}}\]are the focal lengths of a concave lens for violet and red light respectively, then                                                        [CBSE PMT 1996]

    A) \[{{f}_{v}}<{{f}_{r}}\] and \[{{F}_{v}}>{{F}_{r}}\]     

    B) \[{{f}_{v}}<{{f}_{R}}\] and \[{{F}_{v}}<{{F}_{r}}\]

    C) \[{{f}_{c}}>{{f}_{r}}\] and \[{{F}_{v}}>{{F}_{r}}\]      

    D) \[{{f}_{v}}>{{f}_{r}}\] and \[{{F}_{v}}<{{F}_{r}}\]

    Correct Answer: B

    Solution :

    According to lens makers formula                    \[\frac{1}{f}=(\mu -1)\ \left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\Rightarrow \frac{1}{f}\propto (\mu -1)\]                    Since \[{{\mu }_{Red}}<{{\mu }_{violet}}\] \[\Rightarrow {{f}_{v}}<{{f}_{r}}\]and \[{{F}_{v}}<{{F}_{r}}\]            Always keep in mind that whenever you are asked to compare (greater than or less than) u, v or f you must not apply sign conventions for comparison.


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