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Chapter1
BasicElectronOptics
ResolutionofopticallensResolutionofhumaneyes:0.1-0.2mm1.1Resolution
Twopointobjectsarejustresolvedwhenthefirstdarkfringeinthediffractionpatternofonefallsdirectlyonthecentralbrightfringeinthediffractionpatternoftheother.Rayleighcriterion
Theoreticalresolutionofopticalmicroscopel:wavelengthofthelightn:refractiveindexofthemediumbetweentheobjectandtheobjectivelensa:semi-angleNA=nsina:numericalaperture??=d?;NA?=d?
Forglasslens:?~90o,sin?~0.95.Whenthemediumisair,n=1,theresolutionisWavelengthofvisiblelight:4000—8000?Resolutionofopticalmicroscope~2000?Toimprovetheresolution,useshortwavelength!X-ray:wavelengthis~?,butcannotbefocused.Electron:wavelengthis~10-2?,canbefocused!
Wavelengthofelectronh:Plank’sconstant(6.626?10-34J.s)m:massofmovingelectronv:velocityofmovingelectron
InTEM,weimpartmomentumtotheelectronbyacceleratingitthroughaacceleratingvoltageV,givingitapotentialenergyeV.Thispotentialenergymustequalthekineticenergy.
WhenacceleratingvoltageV500Volts,thevelocityofelectronismuchsmallerthanthespeedoflight,m?m0(massofstaticelectron,9.109?10-31kg).so(lisinunitof?,VisinunitofVolt)
FormostTEM,Visabout100-200kV,thevelocityoftheelectronsbecomesgreaterthanhalfthespeedoflight!Relativisticeffectsshouldbetakenintoaccount.(lisinunitof?,VisinunitofVolt)
V(kV) ?(?)-------------------------100 0.0370 200 0.0251 300 0.0197 1000 0.0087 Comparewiththewavelengthoflight,electronwavelengthis105timessmaller!FromTheoreticresolutionofelectronmicroscopeshouldbe~0.02?Inpractice,resolutionofelectronmicroscopeis~1-2?.
ThereasonswhytheresolutionofelectronmicroscopeisnotasgoodasexpectedLensinelectronmicroscope(magneticlens)isnotasperfectastheopticallens.Ithasaberra
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