[填空题]
Water waves approach an underr f9 t,zlxb.g-wateryk qbg fo0n 1s5+-qgx+ “shelf” where the velocity changes from 2.8 m/s to 2.1 m/s If the incident wave crests+gobqy q5s-g 0n1kf +x make a 34$^{\circ} $ angle with the shelf, what will be the angle of refraction? $^{\circ} $