问题 选择题

如图所示,在一个凸透镜的主光轴上距光心2倍焦距的地方放一点光源S,在透镜的另一侧距光心3倍焦距的地方垂直于主光轴放置的屏上得到一个亮圆斑,若将透镜的上半部遮住,则(  )

A.屏上亮圆斑上半部消失

B.屏上亮圆斑下半部消失

C.屏上亮圆斑仍完整,但亮度减弱

D.屏上亮圆斑直径减小

答案

遮住凸透镜的上半部,物体上任一点射向凸透镜的下半部,经凸透镜折射后,照样能会聚成像,会聚成像的光线进行向外传播,在3倍焦距处形成光斑,光斑的大小不发生变化,折射光线减少,光斑会变暗.

故选C.

多项选择题 X型题
阅读理解

阅读理解。

      An old friend from California called from the airport (机场) to tell me that he had arrived. I was not able

to leave the office, but I had made plans for his arrival. After explaining where my new house was, I told him

that I had left the key under the doormat (门垫). Since I knew it would be pretty late before I could get home,

I suggested that he make himself at home and help himself to anything that was in the refrigerator.

      Two hours later my friend phoned me from the house. At the moment, he said, he was listening to some

of my records after having had a delicious meal. Now, he said, he was drinking a glass of orange juice. When

I asked him if he had had any difficulty finding the house, he answered that the only problem was that he had

not been able to find the key under the doormat (门垫), but luckily, the living room window by the apple tree

had been left open and he had climbed in through the window. I listened to all this in great surprise. There is

no apple tree outside my window, but there is one by the living room window of my next door neighbor's

house!

1. An old friend of mine called _____. [ ]

A. to tell me to meet him at the airport

B. to tell me about his arrival

C. to ask me to leave the office

D. to ask me to make plans for his arrival.

2. My friend climbed into the room because _____. [ ]

A. there was an apple tree outside

B. the living room window was left open

C. he had difficulty opening the door

D. he hadn't found the key

3. My friend telephoned me two hours later _____. [ ]

A. from my home

B. at the airport

C. in his office

D. from my neighbor's house

4. The purpose (目的) of writing this passage (文章的段) is to _____. [ ]

A. introduce a friend

B. tell an interesting story

C. explain something

D. show what friendship is