Question
What determines the speed of electromagnetic waves through a medium?
Answer Options
- A) Resistance and reactance
- B) Evanescence
- C) Birefringence
- D) The index of refraction
Correct Answer: D
Explanation
Electromagnetic waves (radio waves, light) travel fastest in a vacuum (the speed of light, c). When a wave travels through any physical medium (like air, water, or the dielectric of a coaxial cable), its velocity is reduced. This reduction in speed is a fundamental property of the medium interacting with the electromagnetic field.
The factor that determines the reduction in speed is the index of refraction of the medium. The index of refraction (n) is defined as the ratio of the speed of light in a vacuum (c) to the phase velocity of the electromagnetic wave in that medium (v): n = c/v. A higher index of refraction means a slower wave velocity and is a core concept in radio propagation, especially in the ionosphere and transmission lines.
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