3.2 The resolution of a TEM is limited by diffraction (Rd) as discussed in and by the spherical aberration which is expressed  as: Rsph = Csα3, where Cs is the spherical aberration coefficient and α is the half-angle of the cone of light entering the objective lens. The TEM resolution is the quadratic sum of the diffraction resolution and spherical aberration resolution, given by the expression: (R = R2 d + R2 sph). The minimum R is obtained when Rd ∼= Rsph. Estimate the optimum value of α and resolution limit of TEM at 100–200 kV. Cs ∼= 1 mm for an electromagnetic objective lens.