Malus' law in X-ray Formula

Equation for calculate malus' law in x-ray is,

$$I=I_{\mathrm{<mn>0</mn>}}·\frac{f}{f_{\mathrm{<mn>0</mn>}}}·(1+\frac{\lambda ·(f_{\mathrm{<mn>0</mn>}}-f)}{2·c})·{\cos \left({\theta }_i\right)}^2$$

where,
I₀ - Intensity of the polarized wave before passing through the filter
f - frequency of the radiation passes through polarizer,
f₀ - frequency of the polarized radiation falling on the polarizer,
λ - Compton wavelength of electron,
c - speed of light in vacuum (299792458 m/s),
θ_i - Angle between the light's initial polarization direction and the axis of the filter,
I - Intensity of the transmitted wave