Question

The focal length of double concave lens having refractive index 1.5 kept in air with two spherical surfaces of radii \( R_1 = 20 \, \text{cm} \) and \( R_2 = 80 \, \text{cm} \) is

• A. 16 cm
• B. 24 cm
• C. 32 cm
• D. 48 cm

Hint:

For a concave lens, use the lensmaker's formula to calculate the focal length based on the radii of curvature and refractive index.

Answer 1

The Correct Option is B

The focal length of a double concave lens can be calculated using the lensmaker's formula. Given that the refractive index is \( n = 1.5 \) and the radii of curvature are \( R_1 = 20 \, \text{cm} \) and \( R_2 = 80 \, \text{cm} \), the focal length is calculated to be 24 cm. Thus, the correct answer is option (2).