Explain why the reactance provided by a capacitor to an alternating current decreases with increasing frequency.

(N/A) capacitor does not allow the flow of direct current $(DC)$ through it because the resistance across the gap is infinite.
When an alternating current $(AC)$ is applied across the capacitor plates,the plates are alternately charged and discharged.
The current through the capacitor is a result of this changing voltage (or charge).
Thus,a capacitor will pass more current if the voltage changes at a faster rate,which means if the frequency of the supply is higher.
This implies that the reactance offered by a capacitor is less with increasing frequency.
Mathematically,the capacitive reactance is given by $X_{C} = \frac{1}{2 \pi \nu C}$,where $\nu$ is the frequency and $C$ is the capacitance. As $\nu$ increases,$X_{C}$ decreases.