问题 填空题

做测定凸透镜焦距的实验时,把蜡烛和光屏放在透镜的主光轴上,与主光轴垂直.若这时在它们之间无论怎样移动透镜,光屏上都得不到清晰的蜡烛像,则应采取的措施是______.为了求得凸透镜的焦距,测出蜡烛到光屏的距离L和蜡烛在光屏上两次成像时透镜的两个位置之间的距离d,则该透镜的焦距f=______.

答案

(1)无论怎样移动透镜,光屏上都得不到清晰的蜡烛像是因为蜡烛和光屏之间的距离小于焦距,成的是虚像,或物体位于焦点上,不成像,所以要在光屏上都得到清晰的蜡烛像应加大蜡烛和光屏之间的距离.

(2)由题,u+v=L,根据透镜成像公式

1
f
=
1
u
+
1
v
,得

  

1
f
=
1
u
+
1
L-u

解得:u1=

L-
L2-4fL
2
,u2=
L+
L2-4fL
2

又由题意,有u2-u1=d

联立解得:f=

L2-d2
4L

故答案为:加大蜡烛和光屏之间的距离,

L2-d2
4L

多项选择题
阅读理解

阅读理解。

      Robots are smart. With their computer brains, they help people work in dangerous places or do difficult

jobs. Some robots do regular jobs. Bobby, the mail carrier, brings mail to a large office building in Washington,

D.C. He is one of 250 mail carriers in the United States.

      Mr. Leachim, who weighs two hundred pounds and is six feet tall, has some advantages as a teacher. One

is that he does not forget details. He knows each child's name, the parents' names and what each child knows

and needs to know. In addition, he knows each child's pets and hobbies. Mr. Leachim does not make mistakes.

Each child goes and tells him his or her name, then dials an identification (身份证明) number. His computer

brain puts the child's voice and number together. He identifies the child with no mistakes. Then he starts the

lesson.

      Another advantage is that Mr. Leachim is flexible. If the children need more time to do their lessons they

can move switches. In this way they can repeat Mr. Leachim's lesson over and over again. When the children

do a good job he tells them something interesting about their hobbies. At the end of the lesson the children

switch Mr. Leachim off.

1. The first paragraph of the passage tells us ______.

A. human beings are not as smart as robots

B. robots will take the place of man to rule the earth

C. we can only use robots to do some regular jobs

D. robots can help people in many different ways

2. Mr. Leachim's ______ makes him a good teacher.

A. knowledge

B. appearance

C. advantage

D. energy

3. What is the most important thing Mr. Leachim can do in his lessons?

A. To meet the needs of each student.

B. To talk to the students in different languages.

C. To keep everyone's interest in his lesson.

D. To introduce more hobbies to the children.

4. The word "flexible" in the last paragraph means ______.

A. not strict

B. not hard

C. suitable

D. changeable

5. Which of the following statements may be true according to the passage?

A. There are 250 robot teachers in the United States.

B. Mr. Leachim is run and controlled by electricity.

C. Bobby works in a large office building in Washington D. C.

D. The lessons taught by Mr. Leachim are given on a TV set.