The loss of a multimode graded-index fiber is greatest at which wavelength?

• A. 1300 nm
• B. 1310 nm
• C. 850 nm
• D. 1550 nm

Answer: (C)

Wavelength-dependent attenuation is the key idea here. Silica fibers lose more light at shorter wavelengths because of stronger Rayleigh scattering (which grows rapidly as wavelength gets shorter) and because absorption is higher nearer the visible edge. In a multimode graded-index fiber, this affects overall loss just as it does in other fiber types—the grading mostly changes how the modes travel (dispersion), not how much signal is lost.

Among the given wavelengths, the shortest one sits in a region where attenuation is much higher, whereas the others are in the telecom windows with much lower loss. So the loss is greatest at the shorter wavelength around 850 nm.