The focal length of a lens of power + 4D is :

Options

A. - 25 cm
B. + 25 m
C. - 4 cm
D. + 4 m

Answer

To find the focal length of a lens given its power, we can use the relationship between power (P) and focal length (f). The formula is:

\[ P = \frac{1}{f} \]

Where:
- P is the power of the lens in diopters (D)
- f is the focal length in meters (m)

### Step-by-Step Solution:

1. **Identify the power of the lens**:
The power of the lens is given as +4 diopters (D).

2. **Use the formula to find the focal length**:
We can rearrange the formula to find the focal length:
\[ f = \frac{1}{P} \]

3. **Substitute the value of power into the formula**:
\[ f = \frac{1}{4 \, \text{D}} \]

4. **Calculate the focal length in meters**:
\[ f = 0.25 \, \text{m} \]

5. **Convert the focal length from meters to centimeters**:
Since 1 meter = 100 centimeters,
\[ f = 0.25 \, \text{m} \times 100 \, \text{cm/m} = 25 \, \text{cm} \]

6. **Final answer**:
The focal length of the lens is 25 centimeters.

### Summary:
The focal length of a lens with a power of +4D is 25 cm.