1. The problem statement, all variables and given/known data A helium-neon (He-Ne) laser has a gain bandwidth (denoted here as the frequency interval over which the laser gain equals or exceeds the minimum threshold gain) given by ΔG= 1.0 GHz, centred on the λ = 632.8 nm emission wavelength. Show that in order to tune the frequency of this He-Ne laser over its entire gain bandwidth, the length of the resonator should be changed by ΔL = -1/2λ. 2. Relevant equations L=mλ/2 3. The attempt at a solution The length and frequency changes are both small compared to the length and fre- quency, respectively so differentiation of the condition for standing waves in a resonator must be applied. The condition for standing waves is :L=mλ/2 but dL/dλ= m/2 , how can i get ΔL = -1/2λ ?