Ray Optics And Optical Instruments Ques 48

48. Focal length of a convex lens of refractive index $1.5$ is $2$ $ cm$. Focal length of the lens when immersed in a liquid of refractive index of $1.25$ will be

[1988]

(a) $10 $ $cm$

(b) $2.5 $ $cm$

(c) $5 $ $cm$

(d) $7.5 $ $cm$

Show Answer

Answer:

Correct Answer: 48.(c)

Solution:

  1. (c) $\frac{f_a}{f_l}=\frac{(\frac{\mu_g}{\mu_l}-1)}{(\mu_g-1)}=\frac{(\frac{1.5}{1.25}-1)}{1.5-1}=\frac{\frac{1}{5}}{\frac{1}{2}}=\frac{2}{5}$

$f_l=\frac{5}{2} f_a=\frac{5}{2} \times 2=5$ $ cm$

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