完形填空 通读下面的短文,掌握其大意,然后从每小题所给的A,B,C三个选项中选出一个最佳答案。 I had a very unusual experience(经历)on Sunday. At ten in the morning, I was 1 down the street when a UFO landed right 2 of me. You can imagine(想象) 3 strange it was! An alien 4 and walked down Center Street. I followed it to see 5 it was going, and I was very surprised when it went 6 a souvenir shop. While it was 7 at the souvenirs(纪念品), the shop assistant called the 8 . Before the p(1)如图所示,为一列在均匀介质中沿x轴正方向传播的简谐横波在某时刻的波形图,波速为2m/s,则 A.质点P此时刻的振动方向沿y轴负方向 B.P点的振幅比Q点的小 C.经过△t=4s,质点P将向右移动8m D.经过△t=4s,质点Q通过的路程是0.4m (2)如图所示,直角玻璃三棱镜置于空气中,已知∠A=60°,∠C=90°,一束极细的光于AC的中点D垂直AC入射,AD的长度为a,棱镜的折射率n=,求: (Ⅰ)光从棱镜第一次射入空气时的折射角; (Ⅱ)光从进入棱镜到它第一次射入空气所经历的时间(设光在真空中的传播速度为c)