题目

如图，玻璃柱的横截面为半径R=20.0 cm的半圆，O点为圆心。光屏CD紧靠在玻璃柱的右侧，且与截面底边MN垂直。一光束沿中半径方向射向O点，光束和MN的夹角为 ，在光屏CD上出现两个光斑。已知玻璃的折射率为n= 。 （i）若 =60°，求两个光斑间的距离； （ii）屏上两个光斑间的距离会随 大小的变化而改变，求两光斑间的最短距离。阅读理解(从每题所给的四个选项中选出一个最佳答案) B I was awoken at 4 a. m. the following morning by a telephone message from the Foreign Office to the effect that Germany had attacked Russia. The Prime Minister had always said that he was never to be woken up for anything but invasion (侵略) of England . I therefore put off telling him till 8 a. m. His only comment was, “Tell the BBC. I will broadcast tonight. ” He began to prepare the speech at 11 a. m. and except for lunch time, at which Sir Stafford Cripps, Lord Cranborne, and Beaverbrook were present, he devoted the whole day to it …. The speech was only ready at twenty minutes to nine. 1．The writer was probably ________. [　　] A．a secretary of the Prime Minister B．a worker in the Foreign Office C．an officer in the War Office D．a BBC. reporter 2．Which of the following is ture? [　　] A．Germany attacked Russia and four hours later the Prime Minister of England knew it. B．The writer knew invasion of England four hours earlier than the Prime Minister. C．The writer was told that Germany had attacked Russia; so he told it to the Prime Minister immediately. D．Although the writer knew invasion of Russia in the early morning, he didn't tell it to the Prime Minister right away because the Prime Minister was sleeping. 3．When the Prime Minister knew invasion of Russia, he first wanted ________. [　　] A．to tell the BBC about it B．to tell his people about it C．to write a speech D．to meet Sir Stafford Cripps, Lord Cranborne and Beaverbrook 4．The Prime Minister ________with Sir Stafford Cripps, Lord Cranborne and Beaver brook . [　　] A．had a meeting to discuss the serious situation B．broadcast at the BBC C．heard the writer' s report D．had lunch 5．The Prime Minister probably broadcast at the BBC  _________. [　　] A．at nine o' clock that night B．at nine o' clock that morning C．at twenty minutes to nine D．at lunch time