问题 多选题

有一个干枯的井底正中央P点趴着一只青蛙,井上有一只狗,青蛙恰好看不到狗,他能看到的视野范围如图所示,天降大雨时井中全部灌满水,若青蛙和狗都保持原来位置不动,下列说法正确的是(  )

A.发生了光的折射,青蛙可以看到狗

B.发生了光的折射,青蛙的视野范围变大

C.发生了光的折射,狗不一定能看到青蛙

D.发生了光的折射,狗一定能看到青蛙.

答案

当井里没有水时,青蛙看到的视野(左图);当井里灌满水后,光线照到镜面会发生折射现象,由于光是从空气射向水,所以入射角大于折射角,因此井底之蛙看到的视野范围比没水时会看到更大;青蛙可以看到狗,由于光的折射是可逆的,所以狗一定能看到青蛙(见右图).

故选ABD.

单项选择题

The history of English is conventionally, if perhaps too neatly, divided into three periods usually called old (or Anglo-Saxon) English, Middle English, and Modern English. The earliest period begins with the migration of certain Germanic tribes from the continent to Britain in the fifth century A. D, though no records of their language survive from before the seventh century, and it continues until the end of the seventh century or a bit later. By that time, Latin, Old Norse (the language of the Viking invaders), and especially the Anglo-Norman French of the dominant class after the Norman Conquest in 1066 had begun to have a substantial impact on the vocabulary, and the well-developed inflectional system that typifies the grammar of Old English had begun to break down.

The period of Middle English extends roughly form the twelfth century through the fifteenth. The influence of French (and Latin, often by way of French) upon the vocabulary continued throughout the period, the loss of some inflections and the reduction of others accelerated, and many changes took place within the grammatical systems of the language. A typical prose passage, specially one from the later part of the period, will not have such a foreign look to us as the prose of Old English, but it will not be mistaken for contemporary writing either.

The period of Modern English extends from the sixteenth century to our own day. The early part of this period saw the completion of a revolution in vowel distribution that had begun in late Middle English and that effectively brought the language to something resembling its present pattern. Other important early developments include the stabilizing effect on spelling of the printing press and the beginning of the direct influence of Latin, and to a lesser extent, Greek on the vocabulary. Later, as English came into contact with other cultures around the world and distinctive dialects of English developed in the many areas which Britain had colonized, numerous other languages made small but interesting contributions to our word-stock.

The earliest written record of English available to us started()

A. from the seventh century

B. from the fifth century

C. from the twelfth century

D. from the ninth century

判断题