Question

What is the relative decrease in focal length of a lens for an increase in optical power by 0.1 D from 2.5 D? ('D' stands for dioptre).

• A. 0.04
• B. 0.40
• C. 0.1
• D. 0.01

Hint:

Remember that optical power \( P \) is inversely related to the focal length \( f \). A small increase in \( P \) leads to a larger decrease in \( f \).

Answer 1

The Correct Option is A

Step 1: Calculate the initial focal length The relationship between optical power \( P \) and focal length \( f \) is given by: \[ P = \frac{1}{f}. \] Thus, the initial focal length is: \[ f_1 = \frac{1}{P_1} = \frac{1}{2.5} = 0.4 \, \text{m}. \] Step 2: Calculate the new focal length After the optical power increases by 0.1 D, the new optical power becomes: \[ P_2 = 2.5 + 0.1 = 2.6 \, \text{D}. \] The new focal length is: \[ f_2 = \frac{1}{P_2} = \frac{1}{2.6} \approx 0.3846 \, \text{m}. \] Step 3: Calculate the relative decrease in focal length The relative decrease in focal length is: \[ \text{Relative decrease} = \frac{f_1 - f_2}{f_1} = \frac{0.4 - 0.3846}{0.4} = \frac{0.0154}{0.4} \approx 0.04. \] Thus, the correct answer is option (1) 0.04.