A small radio antenna (oscillating dipole) is 300 m away from a receiver. (a) What is the retardation time — how "old" is the signal the receiver detects? (b) If the antenna current oscillates at 1 MHz, how many oscillation cycles fit in the travel time? (c) Why can we ignore retardation for a 50 Hz power line 1 m from an appliance? Step 1: t_r delay = distance/c = 300/3.00×10⁸ = 1.0×10⁻⁶ s = 1.0 μs. The receiver "hears" the antenna as it was one microsecond ago. Step 2: Period T = 1/f = 1 μs. Travel time = exactly 1 period — one full oscillation fits in the journey; the received wave is one cycle behind the antenna's present state. Step 3: For 50 Hz at 1 m: delay = 1/3×10⁸ ≈ 3.3 ns; period = 20 ms. Delay/period ≈ 1.7×10⁻⁷ — the source barely changes during the light-travel time, so t_r ≈ t and static formulas work fine. Retardation matters when ωL/c ≳ 1 (antenna-size comparable to wavelength).