In an interferometer, the free spectral range (FSR) refers to the spacing between adjacent or consecutive fringes in the interference pattern. 

An interferometer consists of two partially reflective mirrors placed parallel to each other. When monochromatic light is incident on the mirrors, it reflects back and forth between them, creating interference fringes. The interference occurs because the light waves that travel different paths between the mirrors either constructively or destructively interfere.

The free spectral range is given by the formula:

where:

FSR is the free spectral range (in units of frequency or wavelength),

c is the speed of light in the medium (usually air) between the mirrors,

L is the distance between the two mirrors.

The free spectral range is inversely proportional to the distance between the mirrors. This means that increasing the distance between the mirrors will decrease the free spectral range, and vice versa.