1 D is equal to

Options

A. 1 m
B. 1 cm
C. `1m^(-1)`
D. `1cm^(-1)`

Explanation

To solve the question "1 D is equal to", we need to understand the definition of a diopter and how it relates to the focal length of a lens.

### Step-by-Step Solution:

1. **Understanding Diopter**: A diopter (D) is a unit of measurement for the optical power of a lens. It is defined as the reciprocal of the focal length (f) of the lens measured in meters.

2. **Formula for Diopter**: The formula to calculate the power (P) of a lens in diopters is given by:
\[ P = \frac{1}{f} \]
where \( P \) is the power in diopters and \( f \) is the focal length in meters.

3. **Finding the Equivalent of 1 Diopter**: If we set the power \( P \) to 1 diopter, we can express this as:
\[ 1 D = \frac{1}{f} \]
This implies:
\[ f = 1 \text{ meter} \]

4. **Conclusion**: Since the power of a lens in diopters is the reciprocal of the focal length in meters, we can conclude that:
\[ 1 \text{ D} = 1 \text{ meter}^{-1} \]

### Final Answer:
1 D is equal to \( 1 \text{ meter}^{-1} \).

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