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: Use the formula for the power of a lens
The power \( P \) of a lens is given by the formula: \[ P = \frac{1}{f} \] where \( f \) is the focal length in meters.
Step 2: Convert the focal length to meters
Given: - Focal length \( f = 20 \, \text{cm} = 0.2 \, \text{m} \).
Step 3: Calculate the power of the lens
Now, substitute the value of \( f \) into the formula: \[ P = \frac{1}{0.2} = 5 \, \text{D} \]
Answer:
Therefore, the power of the lens is \( +5 \, \text{D} \). So, the correct answer is option (1).