FP ) ▁is ▁an ▁index ▁of ▁how ▁far ▁an ▁electron ▁on ▁average ▁travel s ▁through ▁a ▁solid ▁before ▁losing ▁energy . ▁ ▁If ▁a ▁mon och rom atic ▁primary ▁beam ▁of ▁electrons ▁is ▁incident ▁on ▁a ▁solid ▁surface , ▁the ▁majority ▁of ▁incident ▁electrons ▁lose ▁their ▁energy ▁because ▁they ▁interact ▁strongly ▁with ▁matter , ▁leading ▁to ▁pl as mon ▁exc itation , ▁electron - hole ▁pair ▁formation , ▁and ▁v ibr ational ▁exc itation . ▁The ▁intensity ▁of ▁the ▁primary ▁electrons , ▁, ▁is ▁d amp ed ▁as ▁a ▁function ▁of ▁the ▁distance , ▁ ▁d , ▁into ▁the ▁solid . ▁The ▁intensity ▁decay ▁can ▁be ▁expressed ▁as ▁follows : ▁▁▁▁ ▁where ▁ ▁is ▁the ▁intensity ▁after ▁the ▁primary ▁electron ▁beam ▁has ▁tra ve led ▁through ▁the ▁solid ▁to ▁a ▁distance ▁. ▁The ▁parameter ▁, ▁term ed ▁the ▁in el astic ▁mean ▁free ▁path ▁( IM FP ), ▁is ▁defined ▁as ▁the ▁distance ▁an ▁electron ▁beam ▁can ▁travel ▁before ▁its ▁intensity ▁dec ays ▁to ▁ ▁of ▁its ▁initial ▁value . ▁( Note ▁that ▁this ▁is ▁equation ▁is ▁closely ▁related ▁to ▁the ▁Be er - L am bert ▁law .) ▁ ▁The ▁in el astic ▁mean ▁free ▁path ▁of ▁electrons ▁can ▁roughly ▁be ▁described ▁by ▁a ▁universal ▁curve ▁that ▁is ▁the ▁same ▁for ▁all ▁materials . ▁ ▁See ▁also ▁ ▁Sc atter ing ▁theory ▁ ▁Be er - L am bert ▁law ▁ ▁References ▁ ▁Category : At omic , ▁mole cular , ▁and ▁optical ▁physics <0x0A> </s> ▁Z era ▁a ▁fict ional ▁character