A complete theoretical treatment of the ‘‘mirage’’ (optical beam deflection) technique has been performed for thermal diffusivity measurements in solids. Numerical calculations of the transverse deflection show there is a linear relation between the first noncentral zeros of the real part of this function and the inverse root frequency. The slope of this straight line is related to the thermal diffusivity through a factor (γ) which depends on the thermal and optical properties of the sample. We found γ=1 for transparent materials as well as for opaque thermally thin materials, whereas γ=1.44 for opaque thermally thick samples. We study further the experimental limits which make a sample to be considered as an opaque or transparent and as thermally thick or thin, and present a complete classification of solids according to their optical and thermal properties. The different behaviors observed may be related to the different geometry of thermal waves propagating through the media.