where a {\displaystyle a} is the period of the diffraction grating and λ {\displaystyle \lambda } is the wavelength of the light incident on the grating. However, if wavelength λ {\displaystyle \lambda } is comparable to grating period a {\displaystyle a} , this expression may lead to errors in z T {\displaystyle z_{\text{T}}} up to 100%.[4] In this case the exact expression derived by Lord Rayleigh should be used: