For the laser to give out an intense light beam, the collection of photons have to be stimulated in a region or cavity containing the material or medium that is lashing. Usually this cavity is a region between two mirrors which reflect the photons from one side to the other. For accumulating photons in the cavity,
the wavelength
of the photon must be related to the cavity length (L) by
n * (/2) = L
(31. )
Where n is an integer. For wavelengths not following eq. (31.1), there is destructive interference. The “standing waves” in the cavity are called cavity modes and the modes (photons of given frequency) that contribute to laser action are called resonant modes.
of the photon must be related to the cavity length (L) by 
/2) = L 
