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为了探究光的反射和折射规律,小名设计并进行了如图所示的实验:把一束激光从某种玻璃中射向空气,保持入射点不动,改变入射角(每次增加10°),观察反射光线和折射光线的变化,并记录了入射角α、反射角β、折射角γ.实验过程中小名发现了一个奇特的现象,当把入射角增大到45°时,发现折射光线突然消失了,把入射角减小为40°时折射光线又出现了.他以为是实验仪器发生了什么问题,经检查仪器一切正常.又试了几次结果还是一样,但又有了新的发现:当折射光线消失后反射光线的亮度比有折射光线时明显增强.这是怎么一回事呢?小名请教了物理老师,老师告诉他:光从玻璃、水等介质射向空气,入射角增大时,折射光线离法线越来越远,而且越来越弱;当入射角增大到某一角度,使折射角达到90°时,折射光线完全消失,发生这种现象时的入射角叫做临界角λ.听了老师的话,小名产生了浓厚的兴趣,他还想知道这种玻璃的临界角是多少,玻璃和水的临界角相等吗,于是他重新进行了实验(从40°开始,入射角每次增加0.2°).

下面是小名的实验数据:

表1  光从玻璃射向空气时入射角、反射角和折射角的关系(单位:度)

α/度

0

10

20

30

40

41

41.2

41.4

41.6

41.8

42

β/度

0

10

20

30

40

41

41.2

41.4

41.6

41.8

42

γ/度

0

15.2

30.9

48.6

74.6

79.8

81.1

82.7

84.8

90

表2  光从水射向空气时入射角、反射角和折射角的关系(单位:度)

α/度

0

10

20

30

40

45

48.2

48.4

48.6

50

β/度

0

10

20

30

40

45

48.4

48.4

48.6

50

γ/度

0

13.4

27.0

41.7

58.7

70.1

82.7

84.0

90

仔细考察小名的探究过程和实验数据,回答下列问题:

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    Elephants don't forget-at least, female(雌性的) elephants don't. Elephant families are matriarchal. And the social knowledge gained by the oldest females is the key to a family group's survival (生存), according to a study published in April by Karen McComb, a biologist at Sussex University in England.

    Elephants announce their presence by making a deep, long sound, a practice referred to as contact calling(联络呼叫). An unfamiliar call may mean that an elephant from outside the family group is nearby. A stranger can cause trouble. Interrupting feeding or disturbing the young. So an elephant matriarch signals the family to gather around her; then they all lift their trunks in the air to smell the unfamiliar caller. False alarms can disturb the group and take time and energy away from feeding, so survival may depend in part on getting it right.

    Working with Cynthia Moss, who founded the Amboseli Elephant Research Project in Kenya 30 years ago, McComb tested the social knowledge of 21 Amboseli elephant families with matriarchs 27 to 67 years old. She played recordings of contact calls to each family and found that the oldest matriarchs were much better at picking out unfamiliar calls. In fact, a group with a matriarch in her fifties was several thousand times more likely to form into a group upon hearing an unfamiliar contact call than when hearing a familiar call. However, families with younger matriarchs were less than twice as likely to gather together upon hearing an unfamiliar contact call as compared with a familiar call. And they gathered together a lot. Moreover, the social knowledge of older matriarchs translated into favorable results: Families with older matriarchs produced more baby elephants in each female-reproductive year.

    This finding shows how difficult it is to protect the oldest members of elephant families. As elephants age, they continue to grow larger, as do their much wanted tusks(象牙). So the older-and wiser-a matriarch is, the greater the chance she will be killed. About 800,000 elephants have been killed by people in the past 20 years.