Most biological samples do not consist simply of pigments but also particles (e.g. cells, fibres, mitochondria) which scatter the incident light. The effect of scattering is an apparent increase in absorbance due to an increase in pathlength and the loss of light scattered in directions other than that of the detector. Two types of scattering are encountered. For incident light of wavelength λ, Rayleigh scattering is due to particles of diameter < λ (molecules, sub-cellular particles). The intensity of scatter light is proportional to λ4 and is predominantly backward scattering. Mie scattering is caused by particles of diameter of the order of or greater than λ (tissue cells). The intensity of scatter light is proportional to 1/λ and is predominantly forward scattering.