Light loss on a fiber optic cable is measured in what unit?

• A. Meters
• B. Watts
• C. Segments
• D. Decibels

Answer: (D)

Light loss measures how much the signal is weakened as light travels through a fiber. It’s a ratio of input power to output power, not a distance or an absolute power value. The decibel is the natural unit for this because it expresses a power ratio on a convenient logarithmic scale, turning multiplication of losses along different sections into simple addition. That makes it easy to total up the loss from each part of the link. In contrast, meters describe distance, watts describe absolute power, and a term like “segments” isn’t a measurement. You’ll often see loss quoted in dB or as dB per kilometer (dB/km), which lets you compute the overall loss over the entire fiber length. For example, if input power is 1 mW and output is 0.1 mW, the loss is 10 dB.