p2 week 10 - diffraction & spectra
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Singleslitdiffraction
Youcans)llgetfringesevenifyouuseasingleslit Theslitbehavesasthoughithasalargenumberofpoint
sourcesspreadevenlyacrossthewidthoftheslit
Thewavefrontsfromthedifferentendsoftheslitwillbearrivingatdifferentpointsontheendscreenatdifferent)mes
Yous)llgetconstruc)veanddestruc)veinterference Thenarrowertheslit,thegreaterthediffrac)on Thelongerthewavelength,thegreaterthediffrac)on
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Singleslitdiffraction
AssumewehaveadarkfringeatP Pathdifferencecorrespondingtodarkfringesis(m+1/2 MCQandMOParesimilartrianglesso:
OP/MP=MQ/MC=sin( MC=d/2(halftheslitwidth sin(=(/2/(d/2dsin(=[forfirstdarkfringe] 2
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M Q
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Singleslitdiffraction
Darkfringesoccuratdsin(=m(wherem=1,2,3 Sameequa0onasfordoubleslitsbuthereitcorrespondstotheposi0onsofdarkfringes!
mherecannottakethevalueof03
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d
D
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M Q
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Example
Lightofwavelength632.8nmfromaHe-Nelaserilluminatesasingleslitofwidth0.1mm.Whatisthewidthofthecentralmaximumformedonascreenplaced2.0mbeyondtheslit?
[Lengthofanarcinradians:L=r,whereristheradius]
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Darkfringesat:dsin(=m
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Intensitydistribution
Awidesingleslitproducesmorefringes
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0slitwidthd
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Intensitydistribution
Singleslitintensitycomparedtodoubleslitintensity
Highintensitypeaksbecomesharperiftheslitnumberisincreased6
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Diffractionbycircularaperture
Similarprincipletothediffrac)onbyaslit
Angularposi)onoffirstminimum dsin(=1.22 7
Airydisc
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Diffractionbycircularaperture
Refrac)ngtelescope d=objec)velensdiameter Thesizeoftheairydiscdeterminesthe
resolvingpower
smallerairydisc=moreresolvingpower
Diffrac0onlimit:theminimumangularsepara)onbetweentwosourcesthatcanbedis)nguishedbyatelescope
8=1.22/d(inradians
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Example
TheHubblespacetelescopehasadiameterof2.4m.AccordingtoRayleighscriterion,whatisthetheore)calangularresolu)onthatthistelescopecanachievewithvisiblelightofwavelength5500(1=0.1nm?Howmuchbeeristhisthantheangular
resolu)onof0.5arcsecondsachievedbytelescopesonthesurfaceoftheEarth?
[1degree=60arcminutes,1arcminute=60arcseconds]
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=1.22/d
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Example
Twopointsourcesoflightareplaced12mmapartandemitlightofwavelength0.6m..Whatisthemaximumdistanceatwhichthetwosourcescanjustbedis)nguishedbyanobserverwithaneyepupildiameterof4.0mm?
[L=r,L=distancebetweenthetwopoints,r=theirdistanceaway]
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=1.22/d
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Diffractiongrating
Ahighnumberofparallelslitsarrangedcloselytogetheriscalledadiffrac0ongra0ng
Insuchanarrangementthepeaksappearassharpthinlines Theloca)onofeachline(principalmaximaisgivenby
disthespacingbetweentwoadjacentslits Diffrac)ongra)ngsareusedtomeasurewavelengthswith
highaccuracy
Transmissiondiffrac)ongra)ng:blackparallellinesontransparentplas)corglass
Reflec)ondiffrac)ongra)ng:usesareflec)vesurfacebetweentheblacklinestoreflectincidentlight
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dsin(=m
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Example
Adiffrac)ongra)nghas5000linespercen)meter.Whataretheangularposi)onsofthefirstthreeprincipalmaximaproducedbythisgra)ngwhenilluminatedwithmonochroma)clightofwavelength6500(1=0.1nm?
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Numberofdiffractedbeams
Ifdandareknown:wecangetthenumberofdiffractedbeams
sin(1m/d1 whichmeansthatmd/
d/willgiveusthenumberofdiffractedbeams Thehighestordernumberisthevalueofd/roundeddown
tothenearestinteger
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Example
Aparallelbeamofmonochroma)clightofwavelength500nmisdirectednormallyatadiffrac)ongra)ngwhichhas600linespermm.Determinethenumberofdiffractedbeamsandtheangleofdiffrac)onofthehighestorder.
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Thevisiblespectrum
Thevisiblespectrumistheregionoftheelectromagne)cspectrumconsis)ngofphotonswithwavelengthsfromapproximately400to700nm
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TypesofSpectra
Spectroscopy:isthestudyoftheinterac)onbetweenmaerandradiatedenergy
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TypesofSpectra
Spectroscopy:isthestudyoftheinterac)onbetweenmaerandradiatedenergy
Threetypesofspectra: acon0nuousspectrumisasmoothdistribu)onofwavelengths
andhasnogaps
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TypesofSpectra
Spectroscopy:isthestudyoftheinterac)onbetweenmaerandradiatedenergy
Threetypesofspectra: acon0nuousspectrumisasmoothdistribu)onofwavelengths
andhasnogaps anemissionlinespectrumisproducedbyexcitedatoms
returningtotheirgroundstate
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TypesofSpectra
Band(emission)spectraareproducedbymoleculesratherthanatoms
molecularbandspectraaremuchmorecomplexthanthespectraofsingleatoms
Theytypicallyshowbroadbandsinsteadofnarrowlines Thelinesappearclosertogetherononesideoftheband
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TypesofSpectra
Spectroscopy:isthestudyoftheinterac)onbetweenmaerandradiatedenergy
Threetypesofspectra: acon0nuousspectrumisasmoothdistribu)onofwavelengths
andhasnogaps anemissionlinespectrumisproducedbyexcitedatoms
returningtotheirgroundstate
anabsorp0onlinespectrumisproducedwhenphotonsfromasourcewithacon)nuousspectrumpassthroughadiffusegas
cloud
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TypesofSpectra
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TheSolarspectrum
Thehotdensecentreofthesunemitsacon)nuousspectrum Thecoolerouterlayerofthesun,theHeliosphere,absorbs
someofthelightatspecificenergies
Lookingattheabsorp)onspectrumwecanfindoutwhattheSunismadeof
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TheSolarspectrum
TheSunemitsprimarilyinthevisiblespectrumbutalsoatotherwavelengths
MuchoftheUVradia)onfromtheSunisabsorbedbytheatmosphere
TheSunatX-raywavelengths
TheSunatUVwavelengths
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ToDo
Readsec)on18fromthebook[from18.3un)lendofchapterp.351-p.363PhysicalOp)cs]
HomeworkAssignmentwk10:ques)ons18.1,18.3,18.4,18.11,18.14and18.16
Handitinnolaterthan4:pmnextWednesday-LATEWORKWILLNOTBEACCEPTED
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