Question

A person uses a convex lens of focal length 20 cm in spectacles, the power of lens will be :

• A. -5 D
• B. +5 D
• C. +2 D
• D. -2 D

Hint:

Always remember to convert the focal length into meters before calculating power.
- Convex lens \(\to\) Positive focal length \(\to\) Positive power.
- Concave lens \(\to\) Negative focal length \(\to\) Negative power.

Answer 1

The Correct Option is B

Step 1: Note the type and sign of the lens.
A convex lens used in spectacles is converging, so by sign convention its focal length is positive, $f = +20\text{ cm}$.
Step 2: Convert to metres before using the power formula. \[ f = 20\text{ cm} = \frac{20}{100}\text{ m} = 0.2\text{ m} \]
Step 3: Apply the power formula. \[ P = \frac{1}{f} = \frac{1}{0.2} = 5\text{ D} \]
Step 4: Attach the correct sign.
Since the lens is convex, the power carries a positive sign.
\[ \boxed{+5\text{ D}} \]