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用功率P0=1W的光源，照射离光源r=3m处的某块金属的薄片．已知光源发出的是波长λ=589nm的单色光，试计算

（1）1s内打到金属板1m2面积上的光子数；

（2）若取该金属原子半径r1=0.5×10-10m，则金属表面上每个原子平均需隔多少时间才能接收到一个光子？

答案

解　（1）离光源3m处的金属板每1s内单位面积上接受的光能为

E=

P0t
4πr2
=
1×1
4π×32
J/m2•s=8.9×10-3J/m2•s=5.56×1016eV/m2•s

因为每个光子的能量为

E1=hν=

hc
λ
=
6.63×10-34×3×108
589×10-9
J=3.377×10-19J=2.11eV

所以单位时间内打到金属板上单位面积的光子数为

n=

E
E1
=
5.56×1016
2.11
=2.64×1016个

（2）每个原子的截面积为

S1=πr12=π×（0.5×10-10）2m2=7.85×10-21m2．

把金属板看成由原子密集排列组成的，则每个原子截面积上每秒内接收到的光子数为

n1=nS1=2.64×1016×7.85×10-21s-1=2.07×10-4s-1．

每两个光子落在原子上的时间间隔为

△t=

1
n1
=
1
2.07×10-4
s=4830.9s

答：（1）1s内打到金属板1m2面积上的光子数为2.64×1016个；

（2）若取该金属原子半径r1=0.5×10-10m，则金属表面上每个原子平均需隔4830.9s才能接收到一个光子．

判断题

政府机构与其他国家机构的关系是协作配合、相互制约的关系。 ( )
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单项选择题

A new study finds that blacks on death row (1) of killing whites are more likely to be executed than whites who kill minorities. It also concludes that blacks who kill (2) minorities are (3) likely to be executed than blacks who kill whites. For example, there is more than a twofold greater risk that an African-American who killed a white will be executed than a white person who kills a (4) victim. A Hispanic is at least 1.4 (5) more likely to be executed (6) such an offender kills a white.

The researchers of the study believe that there are two (7) explanations.. First, prosecutors often win (8) office if they win well-publicized cases. When a black kills a white, such killings gets more (9) and this idea can be (10) by many famous cases. (11) , the court judges at the state level are often (12) to elections, called retention elections. Retention election or judicial retention within the United States court system, is a periodic process, in which the voter (13) approval or disapproval for the judges presently (14) their position, and a judge can be removed from the position if the (15) of the citizens vote him or her out. Just as the researchers (16) out, death penalty is (17) political.

The findings of the study, in short, show that American justice systems clearly (18) white lives more than those of blacks or Hispanics. The researchers also say their findings (19) serious doubts about (20) that the U.S. criminal justice system is colorblind.

15（）

A.majority

B.minority

C.priority

D.superiority
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