 The acousto-optic effect is a specific case of photoelasticity, where there is a change of a material's permittivity, formula_1, due to a mechanical strain formula_2.Photoelasticity is the variation of the optical indicatrix coefficients formula_3 caused by the strain formula_4 given by,

where formula_6 is the photoelastic tensor with components, formula_7,formula_8 = 1,2...,6.Specifically in the acousto-optic effect, the strains formula_4 are a result of the acoustic wave which has been excited within a transparent medium.