Internal stresses will arise in an amorphous plastic body if it is cooled rapidly down through the glass-transition zone. This phenomenon is similar to the thermoelastic effect exhibited by conventional, i.e. inorganic, glasses and is fairly commom during the processing of plastic articles. The phenomenology of the thermoelastic effect in amorphous plastics is examined with special regard to the optical properties of the solidified material. It is found that thermal stresses may induce distinct birefringence patterns in the solid plastic body which depend on the imposed cooling rate and the rheo-optical characteristics of the material. The interpretation of these patterns should be made with caution, however, because of the questionable validity of the stress-optical law in the vicinity of the glass-transition zone. It is also found that the thermoelastic effect may give rise to density and refractive index gradients across the solid plastic body which are likely to influence its optical performance.