The following discussion is from K. Tennakone (Am. J. Phys. 51, 270 (1983)):

"This displacement of the light spot can be easily estimated. At constant pressure, the refractive index of air varies with temperature according to the law

     n - 1 = aT-1                        (1)
        

where a = const. If we assume that the layer of air immediately in contact with the hot plate is at its surface temperature T₂ and the air just above is at room temperature T₁, the critical glancing angle is given by

     cos(thetac) = n2/n1                 (2)

where n₁, n₂ are the refractive indices of air at T₁, T₂. From (1) and (2), we get

     thetac = [ 2(n1-1)(1-T1/T2) ]1/2     (3)
        

At T₁ = 30^oC, n₁ = 1.00026 and (3) gives theta_c = 0.6^o for T₂ = 300^oC. Thus the light spot moves up through a distance of about 10 cm when the screen is 10 m from the hot plate. This estimate gives the maximum possible deflection. The effect is observed whenever the glancing angle theta < theta_c."