In many cases, the attenuation coefficient does not vary with z , in which case one does not have to perform an integral and can express the law as: I ( z ) = I 0 e − − μ μ z where the attenuation is usually an addition of absorption coefficient α α (creation of electron-hole pairs) or scattering (for example Rayleigh scattering if the scattering centers are much smaller than the incident wavelength). Also note that for some systems we can put 1 / λ λ (1 over inelastic mean free path) in place of μ μ .