We derive an equation for the differential cross-section dσ/dΩ of reflected electrons by using Electron Mirror method. The mathematical derivation of differential cross-section equation is based on the Rutherford scattering model as well as the equation of electric potential difference. We describe the interaction of focused electron beams with a polyethylene terephthalate sample using focused ion beam-scanning electron microscope electron mirror image. The electron differential cross-section dσ/dΩ for different working distances h=33, 20, and 10 mm, with a scattering angle θ ranged from 10 deg to 55 deg, incident electrons energies η=500, 750, and 1000 eV, and scanning potential Δφ ranged from 1 to 2 kV has been calculated. According to our findings, the differential cross-section dσ/dΩ of reflected electrons decreases with increasing scattering angle θ and the working distance h and is directly proportional to the difference potential Δφ and the energy of incident electrons η. Distort the electron mirror images with increased Δφ.