Practical Electron Microscopy and Database

The plane wave of electron beam propagating in z-direction (optical axis) can be given by,

          ---------------------- [4203]

where,
         a -- Amplitude
         k -- Wavenumber (=1/λ)
         λ -- Wavelength
         ω -- Frequency

By weakening the strength of the condenser lens (e.g. in TEM), the electron beam incident on the specimen is enlarged so that a wide area on the specimen is covered. That is, the electron beam can be approximately considered as a plane wave incident on the specimen.