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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 |
80 cm 15 cm 17.7 cm 22.5 cm |
80 cm |
$\frac{f_w}{f_a}=\frac{\left(\mu_g-1\right)}{\left(\frac{\mu_g}{\mu_w}-1\right)} \Rightarrow \frac{f_w}{f_a}=4$ ∴ fw = 4 × fa = 4 × 20 = 80 cms. |
