题目

—You went home very late last night, right? —Yes, the party ended at midnight. A . was on B . was over C . was off 答案：B19．如图所示，△ABC为一直角三棱镜的截面，其顶角∠BAC=30°，AB边的长度为L，P为垂直于直线BCD的光屏，P屏到C的距离为L．一宽度也为L的平行单色光束垂直射向AB面，在屏上形成一条宽度等于$\frac{2}{3}$AB的光带，已知光速为c，求：Ⅰ．棱镜的折射率；Ⅱ．沿BC边入射的光线从照射到玻璃砖到射到屏P上所用的时间．