Plot capacitor reactance Xc and inductor reactance XL vs frequency on a log scale
Reactance is the frequency-dependent opposition to current flow in capacitors and inductors. Unlike resistance, reactance does not dissipate energy — it stores and returns it each cycle.
Xc = 1/(2πfC) — falls with frequency. A capacitor is an open circuit at DC and approaches a short at very high frequencies.
XL = 2πfL — rises with frequency. An inductor is a short at DC and an open at very high frequencies.
In a series LC circuit XL and Xc cancel at f₀ = 1/(2π√(LC)). Impedance is minimum (= R only) at f₀ — the basis of band-pass filters and tuned circuits.