[填空题]
A longitudinal earthqu*gye1 yjkh8a)q;. zeuwii: 2x ake snx pepx8a;t + o2uy;e4r i/w1wave strikes a boundary between two types of rock at a 3; 48t/xp+eyax e;o su1pi2rwn8 $^{\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} $