[填空题]
A longitudinal earthquake wave strikes a boundary between tw8 t 8c0ppov1doo types nupvt(.k u2nrcs ewj.(+45ks of rock at a 38 .n(jr5c+2us (ws 4p.uk kvten$^{\circ} $ angle. As the wave crosses the boundary, the specific gravity of the rock changes from 3.6 to 2.8. Assuming that the elastic modulus is the same for both types of rock, determine the angle of refraction. $^{\circ} $
