Question

A biconvex lens of refractive index 1.5 has a focal length of 20 cm in air. Its focal length when immersed in a liquid of refractive index 1.6 will be:

• A. -160 cm
• B. 160 cm
• C. 16 cm
• D. -16 cm

Answer 1

Answer: (A)

-160 cm

Answer 2

Step 1: Given Data: - Refractive index of the lens μ l = 1.5 - Refractive index of the medium (liquid) μ m = 1.6 - Focal length in air f a = 20 cm

Step 2: Use the Lens Formula in Different Mediums: - The relationship between the focal length in air f a and the focal length in the medium f m is given by:

$\frac{f_m}{f_a} = \frac{\mu_l - 1}{\mu_l - \mu_m}$

Step 3: Substitute the Values:

$\frac{f_m}{20} = \frac{(1.5 - 1)}{(1.5 - 1.6)}$

$\frac{f_m}{20} = \frac{0.5}{-0.1}$

$f_m = 20 \times -5 = -160 \, \text{cm}$

So, the correct answer is: -160 cm