Is propagation speed adjustable?

• A. Yes
• B. No
• C. It depends on the medium
• D. Only with changes to the source

Answer: (B)

Propagation speed is set by the medium’s electromagnetic properties. The speed of a wave in a given medium is determined by that medium’s permittivity and permeability (or, for optical paths, by the refractive index). You don’t control this speed just by changing the signal you send; to change how fast the wave travels, you must change the medium or its properties (for example, use a different material, alter temperature, apply an electric field in an electro‑optic crystal, or use a metamaterial). Because the ability to adjust speed depends on the medium, the correct way to think about this is that propagation speed depends on the medium.