The effects of Coulomb collisions on photoelectrons inside the magnetosphere are studied in detail. As was suggested by Sanatani and Hanson (1970), it is shown that angular diffusion of photoelectrons on electrons and ions must be considered, as well as energy loss of photoelectrons on electrons. Diffusion causes trapping of photoelectrons in the middle of the field line and thus decreases the transparency of the magnetosphere for photoelectrons streaming between conjugate ionospheres. It is shown that the transparency of the field line depends mainly on C=E0 2/I (N), where En is the mean energy of the escaping photoelectron flux and I (N) is proporational to the integral of the electron density along the field line, and that the other parameters (L value, electron temperature at the boundary, and anisotropy of the escaping flux) affect only the repartition of the photoelectron flux along the field line. The transparency of the field tube is approximately divided by 2 when angular diffusion is considered.