Question

A thin convex lens made from crown glass (n = 3) has focal length f. When it is measured in two different liquids
having refractive indices it has the focal length fy and f2 respectively. The correct relation between the
focal lengths is :
4 5
and
3 3'
(1) f2>fand fy becomes negative
(2) f1 and f2 both become negative
(3) f = f cf
(4) f1 > fand f2 become negative​

Answer 1

Using lens makers formula and taking ratios,

$\frac{f_m}{f} = \frac{(μ−1)}{( \frac{μ}{μ_m} −1)} \\ </p><p></p><p></p><p>$

$⇒ \frac{( \frac{3}{2} - 1)}{ \frac{3}{4} \div \frac{2}{3} - 1 }$

$⇒f_1=4f$

$\frac{f_2}{f} = \frac{( \frac{3}{2} - 1) }{( \frac{3}{5} \div \frac{2}{3} - 1)} = - 5$

$= f_2 < 0$