True and false: Different wavelengths travel at different speeds through a fiber.

True and false: Different wavelengths travel at different speeds through a fiber.

• A. True
• B. False
• C. Sometimes
• D. Not sure

Answer: (A)

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.

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.