First slide

The lens

Question

A convex lens of focal length 12 cm is made of glass of $μ = \frac{3}{2}$ . What will be its focal length when immersed in liquid of $μ = \frac{5}{4}$

Moderate

A

6 cm
B

12 cm
C

24 cm
D

30 cm

Solution

$\frac{f_{l}}{f_{a}} = \frac{(_{a} μ_{g} - 1)}{(μ_{g} - 1)} = \frac{(\frac{3}{2} - 1)}{(\frac{3 / 2}{5 / 4} - 1)} = \frac{5}{2}$
$∴ f_{h} = f_{a} (\frac{5}{2}) = \frac{12 \times 5}{2} = 30 cm$

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