info@clay6.com
+91-9566306857  (or)  +91-9176170648
logo

Ask Questions, Get Answers

X
 

The focal length of a Plano convex lens when curved surface is silvered and object is in front of plane surface is :-

$(a)\;\frac{R}{\mu} \\ (b)\;\frac{R}{2\mu} \\ (c)\;\frac{R}{3\mu} \\ (d)\;\frac{3R}{\mu} $

1 Answer

In this case
$\large\frac{1}{f_L}=(\mu-1) \bigg[ \large\frac{1}{\infty}-\frac{1}{-R} \bigg]$
$\qquad= \large\frac{\mu-1}{R} $ and $f_m =\large\frac{(-R)}{2}$
So, $P_L= \large\frac{1}{f_L}=\frac{(\mu-1)}{R}$
and $P_M=\large\frac{-1}{f_m} =\frac{2}{R}$
and hence power of system
$P=P_L+P_m+P_L$
$\quad=2P_L +P_M$
$P= \large\frac{2( \mu-1)}{R} +\frac{2}{R}$
$\quad= \large\frac{2 \mu}{R}$
$\therefore F= \large\frac{-1}{P}=\large\frac{-R}{2\mu}$
ie the lens will be equivalent to a converging mirror of focal length $\bigg( \large\frac{R}{2y}\bigg)$
Hence b is the correct answer.

 

answered Feb 13, 2014 by meena.p
edited Jul 22, 2014 by thagee.vedartham
the figure is wrong, which may confuse the understanding.
curved surface is silvered
 

Related questions

...
close