Optics Question 652
Question: A double convex thin lens made of glass of refractive index 1.6 has radii of curvature 15 cm each. The focal length of this lens when immersed in a liquid of refractive index 1.63 is
[UPSEAT 2000; Pb. PET 2004]
Options:
A) - 407 cm
B) 250 cm
C) 125 cm
D) 25 cm
Show Answer
Answer:
Correct Answer: A
Solution:
$ \frac{f _{l}}{f _{a}}=\frac{{( _{a}}{\mu _{g}}-1)}{{( _{l}}{\mu _{g}}-1)} $ ; $ f _{a}=\frac{R}{2({\mu _{g}}-1)}=\frac{15}{2(1.6-1)}=12.5 $
$ \Rightarrow $ $ \frac{f _{l}}{12.5}=\frac{(1.6-1)}{( \frac{1.6}{1.63}-1 )}\Rightarrow f _{l}=-407.5 $ $ cm $
