The power of lens is `2.5 D`. What is its focal length ?

Explanation

To find the focal length of a lens when its power is given, we can use the relationship between power and focal length. The steps to solve the problem are as follows:

### Step-by-Step Solution:

1. **Understand the Formula**: The power (P) of a lens is related to its focal length (f) by the formula:
\[ P = \frac{1}{f} \]
where P is in diopters (D) and f is in meters (m).

2. **Rearrange the Formula**: To find the focal length, we can rearrange the formula:
\[ f = \frac{1}{P} \]

3. **Substitute the Given Power**: We know the power of the lens is \( P = 2.5 \, D \). Now substitute this value into the rearranged formula:
\[ f = \frac{1}{2.5} \]

4. **Calculate the Focal Length in Meters**: Perform the calculation:
\[ f = \frac{1}{2.5} = 0.4 \, m \]

5. **Convert Focal Length to Centimeters**: Since the problem may require the answer in centimeters, we convert meters to centimeters:
\[ f = 0.4 \, m \times 100 = 40 \, cm \]

6. **Final Answer**: Therefore, the focal length of the lens is:
\[ f = 40 \, cm \]