▁Bro g lie ▁equation , ▁which ▁says ▁that ▁the ▁w avel ength ▁is ▁in vers ely ▁proportional ▁to ▁the ▁momentum . ▁T aking ▁into ▁account ▁relativ istic ▁effects ▁( as ▁in ▁a ▁T EM ▁an ▁electron ' s ▁velocity ▁is ▁a ▁substantial ▁fraction ▁of ▁the ▁speed ▁of ▁light ,   c ) ▁the ▁w avel ength ▁is ▁ ▁where , ▁h ▁is ▁Plan ck ' s ▁constant , ▁m 0 ▁is ▁the ▁rest ▁mass ▁of ▁an ▁electron ▁and ▁E ▁is ▁the ▁kin etic ▁energy ▁of ▁the ▁acceler ated ▁electron . ▁Elect rons ▁are ▁usually ▁generated ▁in ▁an ▁electron ▁mic ros cope ▁by ▁a ▁process ▁known ▁as ▁therm ionic ▁emission ▁from ▁a ▁fil ament , ▁usually ▁t ung sten , ▁in ▁the ▁same ▁manner ▁as ▁a ▁light ▁bul b , ▁or ▁altern atively ▁by ▁field ▁electron ▁emission . ▁The ▁electrons ▁are ▁then ▁acceler ated ▁by ▁an ▁electric ▁potential ▁( me as ured ▁in ▁vol ts ) ▁and ▁focused ▁by ▁electro static ▁and ▁elect romagnet ic ▁l enses ▁onto ▁the ▁sample . ▁The ▁transm itted ▁beam ▁contains ▁information ▁about ▁electron ▁density , ▁phase ▁and ▁period icity ; ▁this ▁beam ▁is ▁used ▁to ▁form ▁an ▁image . ▁ ▁Electron ▁source ▁ ▁From ▁the ▁top ▁down , ▁the ▁T EM ▁consists ▁of ▁an ▁emission ▁source ▁or ▁c ath ode , ▁which ▁may ▁be ▁a ▁t ung sten ▁fil ament ▁or ▁need le , ▁or ▁a ▁l anth an um ▁hex abor ide ▁( La B 6 ) ▁single ▁cry stal ▁source . ▁The ▁gun ▁is ▁connected ▁to