Fiber Optics – Light Brigade Practice Test 2026 - Free Fiber Optics Practice Questions and Study Guide

In an optical fiber, different wavelengths travel at different speeds because the glass is dispersive; its refractive index varies with wavelength. Since the speed of light in the fiber is roughly v ≈ c/n(λ), blue light sees a different index than red light, so their speeds differ. The signal-carrying quantity is the group velocity, which also depends on wavelength, leading to chromatic dispersion where different wavelengths in a pulse travel at different speeds and spread out in time. That makes the statement true. If wavelengths traveled at the same speed, there would be no dispersion; saying it’s only sometimes or not sure doesn’t fit the standard behavior of real fiber glass.