以下关于波与相对论的说法，正确的是
A．无论什么波，只要振幅足够大就可以产生明显的衍射现象
B．根据麦克斯韦的电磁场理论，变化的电场周围一定可以产生电磁波
C．波源与观察者互相靠近或者互相远离时，观察者接收到的频率都会发生变化
D．火车以接近光速行驶时，我们在地面上测得车厢前后距离变小了，而车厢的高度没有变化
CD
题目分析：发生明显衍射现象的条件是波长比障碍物的尺寸大或差不多时，与波的振幅无关；根据麦克斯韦的电磁场理论，只有周期性变化的电场周围一定可以产生周期性变化的磁场，从而形成电磁波；波源与观察者互相靠近或者互相远离时，观察者接收到的频率都会增大或者减小，这种现象是多普勒效应；火车以接近光速行驶时，我们在地面上测得车厢前后距离变小了，因为火车在竖直方向速度为零所以车厢的高度没有变化，。选项CD正确。
| 完型填空。 | ||||
| From Monday to Friday most people are busy working or studying, but in the evenings and weekends they are free and 1 themselves. Some watch television or go to the moives, other 2 sports. This is decided by their own 3 . There are many different ways to spend our free time. Almost everyone has some kinds 4 , it may be something from collecting stamps, to 5 model planes. Some hobbies are very expensive, but other don't cost anything at all. I know a man who has coin collection worth several 6 dollers. A short time ago he bought a rare fifty- cents piece which cost him $250! He was very happy about his collection and thought the price was all right. On the other hand, my youngest brother collects match boxes, he was almost 600 of them, but I wonder 7 they are worth any money. However, 8 my brother, they are quite valuable. Nothing makes him happier than to find a new match box for his collection. That's what a hobby means, I think. It is something we like to do in our 9 time just for of it. The value in dollars is not important, but the pleasure it 10 us is. | ||||
|