Underproperconditions,soundwaveswillbereflectedfromahillsideorothersuchobstruction(障碍)．Soundtravelsattherateofaboutone-fifthofamilepersecond．Ifthehilliselevenhundredfeetaway,ittakestwosecondsforthesoundtotraveltothehillandback．Thus,bymeasuringthetimebetweenasoundanditsreflection(theecho)youcanestimate(估算)thedistancetoanobstruction．

DuringWorldWarIItheBritishmadeapracticaluseofthisprincipletodiscoverGermanplanesontheirwaytobombLondonlongbeforetheenemywasneartheplace．Theyuseradiowavesinsteadofsoundwaves,sinceradiowavescangothroughfogandclouds．TheBritishRoyalAirForcealwaysseemedtothepuzzledGermanstobelyinginwaitattherighttimeandnevertobesurprised．ItwasradioechoesmorethananythingelsethatwonthebattleofBritain．Anditisfrom“radiodirectingandranging”thattheword“radar(雷达)”cameintobeing．

1．Soundwavereflectedfromahillcanbeusedtoestimatethe_______．

A．heightofthehillB．speedandsound

C．distancetothehillD．theintensity(强度)ofsound

2．“echo”isthe_________ofsound．

A．speedB．wave

C．intensityD．reflection

3．Theestimateisbasedonthe_______．

A．directionofthehillB．speedofsound

C．timebetweensoundanditsechoD．bothBandC

4．RadiowavehelpedtheEnglishto__________．

A．discoverGermanplanesontheirwaytoLondon

B．puzzleGermanpilotsC．knowtheplanofGermans

D．knowthedistancebetweenEnglandandGermany

5．Thewriterseemstoexplain________．

A．exactlyhowradarworks

B．whytheBritishuseradiowavesintheirbattle

C．howthewordradarcametobe．

D．howradarhelpedtheBritishwintheBattleofBritain