The focal length can be expressed in

Options

A. metre
B. dioptre
C. watt
D. horse power

Answer

To solve the question regarding the units in which focal length can be expressed, we will analyze each option provided.

### Step-by-Step Solution:

1. **Understanding Focal Length**:
- Focal length is a measure of the distance from the lens to the focal point where light rays converge. It is a physical length.

2. **Evaluating the Options**:
- **Option 1: Meter**:
- Focal length is a length, and the standard unit of length in the International System of Units (SI) is the meter. Therefore, focal length can indeed be expressed in meters.

- **Option 2: Diopter**:
- Diopter is a unit of measurement for the optical power of a lens. The power (P) in diopters is defined as the reciprocal of the focal length (f) in meters:
\[ P = \frac{1}{f} \text{ (in meters)} \]
- Thus, while diopter is related to focal length, it is not a direct measure of focal length itself. It is a derived unit.

- **Option 3: Watt**:
- Watt is a unit of power, defined as one joule per second. It is not related to the measurement of length or focal length. Therefore, focal length cannot be expressed in watts.

- **Option 4: Horse Power**:
- Horsepower is another unit of power, equivalent to 746 watts. Similar to watts, it is not a measurement of length and cannot be used to express focal length.

3. **Conclusion**:
- From the analysis above, the only correct option for expressing focal length is in meters.

### Final Answer:
The focal length can be expressed in meters.

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