Question

A lens has focal length $ f = 20 \, \text{cm} $. What is the power of the lens?

• A. \( +5 \, \text{D} \)
• B. \( +10 \, \text{D} \)
• C. \( -5 \, \text{D} \)
• D. \( -10 \, \text{D} \)

Hint:

Remember: The power of a lens is the reciprocal of the focal length in meters.

Answer 1

The Correct Option is A

Step 1: Apply the lens power formula
The power \( P \) of a lens is calculated using the formula:\[P = \frac{1}{f}\]Here, \( f \) represents the focal length in meters.
Step 2: Convert focal length to meters
Provided:- Focal length \( f = 20 \, \text{cm} \), which is equivalent to \( 0.2 \, \text{m} \).
Step 3: Determine the lens power
Substitute the focal length into the formula:\[P = \frac{1}{0.2} = 5 \, \text{D}\]
Result:
The power of the lens is \( +5 \, \text{D} \). The correct option is (1).