Question

The power of a lens is +2 dioptre. What will be its focal length in cm?

• A. +50
• B. -50
• C. +25
• D. -25

Hint:

For lenses: \( P = \frac{100}{f(\text{cm})} \). A positive power means a convex lens, and a negative power means a concave lens.

Answer 1

The Correct Option is A

Step 1: Recall the formula for power of a lens.
The power of a lens is defined as the reciprocal of its focal length in meters: \[ P = \frac{1}{f(\text{in meters})} \] Step 2: Substitute the given value.
Given, \( P = +2 \, D \) \[ f = \frac{1}{P} = \frac{1}{2} = 0.5 \, \text{m} \] Step 3: Convert to centimeters.
\[ 0.5 \, \text{m} = 0.5 \times 100 = 50 \, \text{cm} \] Step 4: Sign convention.
Since the power is positive, the lens is a convex lens, and therefore the focal length is positive.
Step 5: Conclusion.
The focal length of the lens is +50 cm.