A double convex lens of focal length 20 cm is made of glass of refractive index 3/2. When placed completely in water a (aµϖ 4/3), its focal length will be
Answer & explanation
Correct answer: option 1
$\frac{f_w}{f_a}=\frac{\left(\mu_g-1\right)}{\left(\frac{\mu_g}{\mu_w}-1\right)} \Rightarrow \frac{f_w}{f_a}=4$
∴ fw = 4 × fa = 4 × 20 = 80 cms.