The formula is given by:
\[ \alpha = \frac{1}{D} \times 10 \log \left( \frac{P_{in}}{P_{out}} \right) \]
where:
- $\alpha$ is the attenuation,
- $D$ is the distance the light has traveled,
- $P_{in}$ is the input power, and
- $P_{out}$ is the output power.
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