题目

如图所示是一透明的圆柱体的横截面，其半径为R，折射率是，AB是一条直径.今有一束平行光沿AB方向射向圆柱体，试求离AB多远的入射光线经折射后经过   B点？ 答案：R 解析:本题考查光的折射.取一条经折射后恰通过B点的光线进行分析.设一光线经折射后经过B点，光路如图所示.                                                                                 (2分) 由折射定律知n==                                                                           (2分) 由几何关系可得θ1=2θ2                                                                                                                                                                   (2分) 由以上两式可得θ1=60°  θ2=30°                                                                (2分) 所以CD=Rsinθ1=R.                                                                                  (2分)

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