A 100 GeV photon initiates an electromagnetic shower in matter. The photon converts into an electron-positron pair when interacting with a nucleus. The electron and positron shares the 100 GeV energy. When getting close to a nucleus, the electron (or the positron) emits a photon, a so called bremsstrahlung photon. This photon can then make a new electron-positron pair and the process continues until the photon energy is below the threshold for producing an electron-positron pair, about 1 MeV. A shower is created with photons, electrons and positrons. The characteristic length for producing either a pair or a bremsstrahlung photon is called radiation length, about 6 mm in lead and 300 m in air at sea level. The 100 GeV photon initiated shower contains about 100 000 photons, electrons and positrons.