Lambert and Bouguer law of absorption of light The intensity of light travelling through a medium falls of exponentially with distance. For a plane wave,

I(x) = I₀e^{- a x}

where I(x) is the intensity when the wave has travelled a distance x through the medium, and I₀ the initial intensity. The constant a is called the absorption coefficient. The decrease of light intensity occurs due to:(i) Absorption of light energy in the medium which produces heating of the medium. (ii) Scattering of light.