資源描述：
《UNIT 20考研英語閱讀理解精讀100篇.doc》由會員上傳分享，免費在線閱讀，更多相關(guān)內(nèi)容在教育資源-天天文庫。
1、TEXTONEMostcellsaretransparent—inotherwords,theyarenotverygoodatreflectingorabsorbinglight.Tolookatthemunderamicroscopethusrequirestrickery.Manyofthesetrickskillthecells,andeventhosethatkeepthemalivelookonlyatslicesthrougheachcell,ratherthanseeingthewholethinginthreedimensi
2、ons.MichaelFeld,oftheMassachusettsInstituteofTechnology,andhiscolleagues,thinktheycanchangethat.Theyhaveinventedawaytolookatcellsthatarestillalive.Moreover,theycandosointhreedimensions.Theirmethodiscalledtomographicphasemicroscopy,anditisreportedinthisweek'sNatureMethods.In
3、steadofrelyingonabsorbedorreflectedlight,DrFeld'stechniquecelebratestransparencybylookingatlightthatgetsthroughunaltered.Itdoessobymeasuringapropertycalledtherefractiveindex.Thisindexmeasuresthespeedoflightinamaterial.(Lightzipsalongattheactual“speedoflight”,fasterthanwhich
4、nothingcango,onlywhenitistravellingthroughavacuum.)Thedifferentcomponentsofacell,thoughtransparent,havedifferentrefractiveindices.DrFeldandhisteamthereforesetouttomapwhatthesedifferencesare,withaviewtousingthemtodistinguishbetweencellularcomponents.Tomeasuretherefractiveind
5、icesofdifferentpartsofacelltheyuseatechniquecalledinterferometry,whichinvolvessplittingabeamoflightintwo.Onehalf,knownastheobjectbeam,passesthroughthecell;theotherisdirectedalongadifferentpathandactsasareference.Thelengthofthereferencepathissuchthatifnosampleispresent,thetw
6、odaughterbeamswillbeasperfectlyinphasewhentheymeetastheywerewhentheywereseparated.Thecrestsandthetroughsoftheirwaveswillreinforceeachother,andtheresultwillbebrightness.Themorethatthelightpassingthroughthesampleissloweddown,however,themorethetwobeamswillbeoutofphase.Crestwil
7、lfallontrough,andtheresultwillbedarkness.ItisthisphaseshiftthatgivesDrFeld'snewformofmicroscopyitsname.Asinglepairofbeamsdoesnot,however,produceausefulimage.Todothatrequiresscanningtheobjectbeamthroughthetargetaboutahundreddifferentways.Fromtherefractiveindexofeachpathitisp
8、ossible—withtheapplicationofsomesuitablycrunchycomputingpower—toproduceathree-dime
