The power of a lens is -5 D.
Find its focal length.

Explanation

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)

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