Ask Questions, Get Answers

Want to ask us a question? Click here
Browse Questions
0 votes

A system of lenses is formed by fitting a plans convex lens, whose radius of curvature is R and $\mu_1$,the refractive index of plano convex lens is $\large\frac{3}{2} $ and $\mu_2$ the refractive index of Plano concave lens is $\large\frac{4}{3}$. Then the focal length of combination is

$(a)\;2R \\ (b)\;\frac{R}{2} \\ (c)\;6R \\ (d)\;\frac{R}{6} $

Can you answer this question?

1 Answer

0 votes
For a Plano convex lens
$\large\frac{1}{f_1}=(\mu_1-1) \bigg[ \large\frac{1}{\infty} -\frac{1}{-R}\bigg]$
$\large\frac{1}{f_2}=(\mu_2-1) \bigg[ \large\frac{1}{-R} -\frac{1}{\infty}\bigg]$
For the combination of lens
$\large\frac{1}{F}=\frac{1}{f_1} +\frac{1}{f_2}$
$\qquad= \large\frac{\mu_1 -\mu_2}{R}$
$F= \large\frac{R}{\mu_1 -\mu_2}$
$\quad= \large\frac{R}{\Large\frac{3}{2}-\frac{4}{3} }$
$\quad = \large\frac{R}{\large\frac{1}{6} }$
$F= 6R$
Hence c is the correct answer.
answered Jan 22, 2014 by meena.p

Related questions

Ask Question
student study plans
JEE MAIN, CBSE, AIPMT Mobile and Tablet App
The ultimate mobile app to help you crack your examinations
Get the Android App