Question

Power of a convex lens is 2 dioptre. Its focal length will be

• A. 20 cm
• B. 50 cm
• C. 40 cm
• D. 60 cm

Hint:

The focal length is the reciprocal of the power, i.e., \( f = \frac{1}{P} \), where \( P \) is in dioptres.

Answer 1

The Correct Option is B

Step 1: Formula for focal length and power.
The relationship between power (\(P\)) and focal length (\(f\)) is given by the formula: \[ P = \frac{1}{f} \quad \text{(in meters)} \] where \( P \) is the power of the lens in dioptres and \( f \) is the focal length in meters. Step 2: Given values.
We are given that the power of the convex lens is 2 dioptres, so: \[ P = 2 \, \text{dioptres} \] Using the formula, we can find the focal length: \[ f = \frac{1}{P} = \frac{1}{2} \, \text{m} = 0.5 \, \text{m} = 50 \, \text{cm} \] Step 3: Conclusion.
Thus, the focal length of the lens is 50 cm.
\[ \boxed{50 \, \text{cm}} \]