Fabry–Perot resonant frequency is the frequency of a Fabry-Perot resonator. At these frequencies, light exhibits constructive interference after one round trip. And is denoted by fo. Fabry–Perot Resonant Frequency is usually measured using the Hertz for Frequency. Note that the value of Fabry–Perot Resonant Frequency is always negative. Typically, the value of Fabry–Perot Resonant Frequency is greater than 0.