[填空题]
Water waves approach an underwater “shelf” where the 8alpvnst c+ f/w4ud4 *velocity c)mc;y*f( yjolq*x.k b hanges from 2.8 m/s to 2.1 m/s If the incident wave cr(mqbyy. *lx;co* f )kjests make a 34$^{\circ} $ angle with the shelf, what will be the angle of refraction? $^{\circ} $