Lighttravelsataspeedwhichisaboutamilliontimesfasterthanthespeedof sound.Youcangetsomeideaofthisdifferencebywatchingthestartofarace.If youstandsomedistanceawayfromthestarteryoucanseesmokefromhisgun beforethesoundreachesyourears.
Thisgreatspeedoflightproducessomestrangefacts.Sunlighttakesabout8minutestoreachus.Ifyoulookatthelightofthemoontonightrememberthatthelight raysleftthemoon1.3secondsbeforetheyreachedyou.Theneareststarissofar awaythatthelightthatyoucanseefromittonightstartedtotraveltowardsyou fouryearsagoataspeedofnearly2millionkm.perminute.Insomecasesthe lightfromoneoftonight’sstarshadstartedonitsjourneytoyoubeforeyouwereborn.
Thusifwewanttobehonestwecan’tsay“Thestarsareshiningtonight.”We havetosayinstead“Thestarslookpretty.Theywereshiningfouryearsagobut theirlighthasonlyjustreachedtheearth.”
1.Lightspeedis______soundspeed
A.asfastas | B.amilliontimesslowerthan |
C.aboutmillionsoftimesfasterthan | D.aboutamilliontimesfasterthan |
A.youcanhearthesoundbeforeyouseethesmoke |
B.thesounddoesnottravelasfastaslight |
C.thesoundwillreachyoubeforethemanfires |
D.soundtravelsaboutamilliontimesfasterthanlight |
A.hastotravelagreaterdistance | B.moveslessquickly |
C.travelsmuchmorequickly | D.islesspowerful(有力的) |