Question

An optician prescribes a corrective lens of power \(+2D\). Then the focal length of the required convex lens is

• A. \(10\ \text{cm}\)
• B. \(50\ \text{cm}\)
• C. \(10\ \text{m}\)
• D. \(50\ \text{m}\)

Hint:

Lens power and focal length are related by \[ P=\frac{1}{f}. \] A positive power corresponds to a convex lens, while a negative power corresponds to a concave lens.

Answer 1

The Correct Option is B

Step 1: Use lens power formula.
\( P = \frac{1}{f} \) (in metres).

Step 2: Calculate focal length.
\( f = \frac{1}{P} = \frac{1}{2}\text{ m} = 50\text{ cm} \). Since P is positive, the lens is convex (converging).

\[ \boxed{f = +50\text{ cm}} \]