If fV and fR are the focal lengths of a convex lens for violet and… | NEET 1996 (AIPMT)

Q93NEET 1996 · AIPMTMedium

Prism, Dispersion & Scattering

If $f_V$ 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 we must have

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Answer: (b)

$\dfrac{1}{f} = (\mu - 1)\left(\dfrac{1}{R_1} - \dfrac{1}{R_2}\right)$ and $\mu_V > \mu_R$ , so $|f_V| < |f_R|$ for any lens.
Convex lens (positive $f$ ): $f_V < f_R$ .
Concave lens (negative $F$ ): $|F_V| < |F_R|$ , so $F_V$ is less negative, i.e. $F_V > F_R$ .
So the answer is (b) $f_V < f_R$ and $F_V > F_R$ .

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Prism, Dispersion & Scattering question from NEET 1996 (AIPMT)

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