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: Understanding the Question:
We are given the focal length of a convex lens used in spectacles, and we need to calculate its optical power.

Step 2: Key Formula or Approach:
The power of a lens (\(P\)) in dioptres (\(D\)) is the reciprocal of its focal length (\(f\)) in meters:
\[ P = \frac{1}{f\text{ (in meters)}} \]

Step 3: Detailed Explanation:

• We are given:
Type of lens = Convex lens
Focal length (\(f\)) = \(+20\text{ cm}\) (Focal length of a convex lens is positive).

• First, convert the focal length from centimeters to meters:
\[ f = \frac{20}{100} = +0.2\text{ m} \]

• Now, use the power formula:
\[ P = \frac{1}{f} = \frac{1}{0.2} \]

• Calculate the final value:
\[ P = \frac{10}{2} = +5\text{ D} \]

• The positive sign of the power confirms that the lens is converging (convex).


Step 4: Final Answer:
The power of the lens is +5 D.