Focal length of a lens is 20 cm. Its power

Options

A. +5D
B. - 5D
C. + 5 cm
D. 0.20 m

Explanation

To find the power of a lens given its focal length, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Formula for Power**:
The power (P) of a lens is defined as the reciprocal of its focal length (f) in meters. The formula is:
\[ P = \frac{1}{f} \]

2. **Convert Focal Length to Meters**:
The given focal length is 20 cm. We need to convert this to meters because the power formula requires the focal length in meters.
\[ f = 20 \text{ cm} = \frac{20}{100} \text{ m} = 0.20 \text{ m} \]

3. **Substitute the Focal Length into the Power Formula**:
Now that we have the focal length in meters, we can substitute it into the power formula.
\[ P = \frac{1}{0.20} \]

4. **Calculate the Power**:
Performing the division gives us:
\[ P = 5 \text{ diopters} \]

5. **Determine the Sign of the Power**:
Since the focal length is positive (20 cm), the power is also positive. Therefore, the final answer is:
\[ P = +5 \text{ diopters} \]

### Final Answer:
The power of the lens is **+5 diopters**.

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