题目

阅读理解ThanksgivingDay was near. A first grade teacher gave her class a fun task--to draw apicture of something for which they were thankful. Mostof the students didn't come from rich families, but many would still celebratethe holiday with turkey and other traditional customs (习俗). These, the teacher thought, would be the subjects of most of thestudents' art. And they were, for the most part.Butone student named Douglas drew a different kind of picture. Douglas was adifferent kind of boy. He was often sad. While other children would play witheach other during break time, he would sta主截面是边长为d的正方形的棱柱，折射率为n，将其弯成半圆形（如图20－26所示）．要使A端垂直入射的光线全部从B端射出，求所弯半圆形的最小内半径R的值．