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  • Stu WØSTU

Speed of Propagation (T3B11)

The technician question pool (2018-2022) item T3B11 asks:

What is the approximate velocity of a radio wave as it travels through free space?

A. 3000 kilometers per second

B. 300,000,000 meters per second

C. 300,000 miles per hour

D. 186,000 miles per hour

Radio waves are electromagnetic waves. Like the visible light waves that we perceive with our eyes, like the invisible ultraviolet rays that may burn and damage our skin, and like the X-rays used to image our innards, radio waves have electric field and magnetic field components that move through free space very rapidly. All electromagnetic waves travel through free space at identical velocity. Just exactly what velocity is the subject of this Technician question.

A common phrase for the velocity in question is “the speed of light,” even though it applies to all frequencies of electromagnetic waves including those categories mentioned above. The term velocity simply means speed in a particular direction, and it applies well since electromagnetic waves tend to travel in straight line directions barring the effects of the environment such as the atmosphere or ionosphere, sharp edges, metal and other objects.

Free space, in this case, means the vacuum of outer space or our earth’s atmosphere at normal pressures. Electromagnetic waves may also travel through some materials that are not free space. For instance, you can transmit and receive radio waves from inside your home since the radio waves easily transit some of the materials from which your home is constructed. Light waves obviously travel through glass, else windows would be somewhat less alluring. X-rays clearly travel through your body tissues readily to expose a sensitive film, else your doctor would be more confounded by your ailment.

However, when electromagnetic waves travel through materials other than free space the velocity is somewhat reduced. Light waves slow down a bit when going through window glass or a camera’s lens. Similarly, radio waves change speed and slow down somewhat when transiting different kinds of materials. The difference in velocity between vacuum and air is very tiny, to the point of being insignificant. Hence, free space is air or vacuum.

The velocity of EM waves is approximately 186,000 miles per second, not miles per hour as the distracter option D proposes. This equates to roughly 300,000 kilometers per second, but do not be fooled by option B’s miles per hour unit attached to that numerical value. Option A is getting closer, but 3000 kilometers per second converts to 3 million meters per second, since a kilometer is 1000 meters. That’s only 1/100 of the correct answer. Electromagnetic waves travel 300 million meters per second, or 300,000,000 meters per second. It is the only response option with the unit meters per second.

The answer to Technician question T3B11, “What is the approximate velocity of a radio wave as it travels through free space?” is B: 300,000,000 meters per second.

-- Stu WØSTU


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