Free Spectral Range
The y-axis is the transmission of a Fabry-Perot interferometer with F=100; the x-axis is the separation distance d, going from 0.9999 cm to 1.0001 cm. One plot (the blue curve) is for lambda = 500 nm. The other plot (tan curve) is for lambda = 500 nm + (a small delta lambda), and the plot is animated based on the value of delta lambda. The changing delta lambda is given in the plot label. You can see what happens as delta lambda is varied from 0 to 2x the FSR of 0.0125 nm.