Worked example: A 100 km single-mode fibre link operates at 1550 nm with 0.2 dB/km loss. A laser launches 1 mW (0 dBm). What power reaches the far end, and how many 3 dB-spaced amplifiers would a 500 km link need? Step 1: Total loss = 0.2 dB/km × 100 km = 20 dB. Step 2: Received power = 0 dBm − 20 dB = −20 dBm = 10 μW (each 10 dB is a factor of 10). Step 3: For 500 km, loss = 100 dB — far beyond any receiver. Place amplifiers every 100 km: 4 amplifiers for 500 km. Step 4: Each amplifier restores the signal by ~20 dB, so the chain delivers about −20 dBm again. Conclusion: tiny 0.2 dB/km loss still demands repeaters on intercity links — but only every 100 km, versus every few km for copper.