English: Electric fields in a Fabry-Pérot resonator. ^[1] The electric-field mirror reflectivities are ${\displaystyle r_{1}}$ and ${\displaystyle r_{2}}$. Indicated are the characteristic electric fields produced by an electric field ${\displaystyle E_{inc}}$ incident upon mirror 1: ${\displaystyle E_{refl,1}}$ initially reflected at mirror 1, ${\displaystyle E_{laun}}$ launched through mirror 1, ${\displaystyle E_{circ}}$ and ${\displaystyle E_{b-circ}}$ circulating inside the resonator in forward and backward propagation direction, respectively, ${\displaystyle E_{RT}}$ propagating inside the resonator after one round trip, ${\displaystyle E_{trans}}$ transmitted through mirror 2, ${\displaystyle E_{back}}$ transmitted through mirror 1, and the total field ${\displaystyle E_{refl}}$ propagating backward. Interference occurs at the left- and right-hand sides of mirror 1 between ${\displaystyle E_{refl,1}}$ and ${\displaystyle E_{back}}$, resulting in ${\displaystyle E_{refl}}$, and between ${\displaystyle E_{laun}}$ and ${\displaystyle E_{RT}}$, resulting in ${\displaystyle E_{circ}}$, respectively.