Which law describes how radio signal power decreases with distance in free space?

• A. Inverse Square Law
• B. Doppler Shift
• C. Planck's Law
• D. Heisenberg Uncertainty

Answer: (A)

The inverse square law describes how power from a radio transmitter spreads over the surface of a sphere, so the power per unit area falls in proportion to 1/r^2. As distance increases, the same amount of radiated power covers a larger area, causing the signal intensity to drop with the square of the distance. In free space, the received power also follows this 1/r^2 behavior, as captured in the Friis transmission equation, which shows the received power is proportional to (λ/(4πR))^2 times transmitter power and antenna gains. So doubling the distance makes the received power drop to one quarter. The other options refer to different phenomena: Doppler shift concerns frequency changes due to motion, Planck’s law describes blackbody radiation spectra, and Heisenberg’s uncertainty principle relates to the limits of measuring certain pairs of physical properties.