If a beam of particles enters a thin layer of material of thickness , the flux  of the beam will decrease by  according to
where  is the total cross section of all events, including scattering, absorption, or transformation to another species. The volumetric number density of scattering centers is designated by . Solving this equation exhibits the exponential attenuation of the beam intensity:
where  is the initial flux, and  is the total thickness of the material. For light, this is called the Beer–Lambert law.