Electromagnetic wave speed

What you will learn

Common vacuum speed; electromagnetic nature and wavelength–frequency relationships.

Specification mapping: A3.1

Prerequisites: A1.4, A1.2

Physical principles

All electromagnetic waves travel at the same speed c in vacuum, approximately 3.00 × 10⁸ m s⁻¹ for calculations at three significant figures. Radio, microwave, infrared, visible, ultraviolet, X-ray and gamma radiation differ in frequency and wavelength, not in vacuum propagation speed. Electromagnetic fields oscillate transversely.

Vacuum speed is not necessarily the speed inside matter. Refractive index describes phase speed for a specified wavelength; absorption and dispersion depend on material and frequency. Signals in cables, atmosphere and devices also acquire delays. A satellite signal therefore has both propagation time and possible processing delay.

For an unobstructed path of length d in vacuum, one-way travel time is t = d/c. A reflected or echo path may involve twice the source–reflector distance. Use the actual total path and do not confuse propagation speed with data rate, which measures bits per second.

Scientific diagram and interpretation

Equations and relationships

c ≈ 3.00 × 10⁸ m s⁻¹ (adopted working value)

Worked examples and applications

Common misconceptions

Higher frequency does not mean higher vacuum speed. Data rate is not propagation speed.

Examination technique

Check whether the stated distance is one way, round trip or a complete multi-leg route.

For an explanation, connect the physical cause to its effect. For a calculation, state the model, rearrange before substitution, retain intermediate digits, and finish with units. For a data question, refer to the actual readings and distinguish a trend from proof of a cause.

Check your understanding

Five questions. Choose an option, then Check Answer. Read the feedback and use Try Again to reset. Use Tab between controls, arrow keys within radio groups, and Enter or Space on buttons. These local practice answers are not saved.