The three windows
Fiber is not equally transparent at every color of light. Practical systems use three infrared windows: 850 nm (multimode, cheap VCSELs, ~3 dB/km), 1310 nm (single-mode, zero dispersion, ~0.35 dB/km) and 1550 nm (single-mode, lowest loss ~0.20 dB/km, and where optical amplifiers work). The historical water peak near 1383 nm is suppressed in modern G.652.D fiber, opening the full spectrum.
What causes attenuation
Two intrinsic mechanisms: Rayleigh scattering from microscopic density variations (falls rapidly as wavelength grows — why 1550 beats 850) and absorption by residual impurities. Extrinsic causes are your responsibility in the field: macrobends (tight loops leaking light, worst at long wavelengths), microbends (pinching by clamps and ties), dirty or damaged connectors, and poor splices.
Dispersion — the pulse spreader
Modal dispersion: in multimode fiber different ray paths arrive at different times, smearing pulses and limiting distance × bandwidth. Graded-index profiles (OM3/OM4) minimize it. Chromatic dispersion: different wavelengths travel at slightly different speeds; it is zero near 1310 nm and grows at 1550 nm, which matters only for long high-rate links.
Field takeaway
Loss testing is always done at the wavelengths the system will use — 850/1300 nm for multimode, 1310/1550 nm for single-mode — because a link that passes at 1310 can fail at 1550 if a macrobend hides in the route.