Ray Optics and Optical Instruments - Result Question 49
51. 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) $7.5 cm$
Topic 4: Prism & Dispersion of Light
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Answer:
Correct Answer: 51. (c)
Solution:
(c) $\frac{f_a}{f_l}=\frac{(\frac{\mu_g}{\mu_l}-1)}{(\mu_g-1)}=\frac{(\frac{1.5}{1.25}-1)}{1.5-1}=\frac{\frac{1}{5}}{\frac{1}{2}}=\frac{2}{5}$
$f_l=\frac{5}{2} f_a=\frac{5}{2} \times 2=5 cm$
