解:发生全反射时, ⑴ n=sin90°/sin∠信升Umax sin∠Umax=3/4 ∠Umax=48.5° 启森 ⑵ n2/n1=sin∠U2max/sin90° sin∠U2max=n2/悄坦亩n1 =1.52/1.7=0.894 ∠U2max=63°