Not all of the photons will reach the focus of the telescope because of reflection and absorption losses from the components in the optical train . 由于光学系统部件所引起的反射和吸收的损失，不是所有的光子都能到达望远镜的焦点。
An atom on an excited upper level can execute a downward transition not just spontaneously but also when photons of the radiation field surrounding the atomic interact with it . 处在较高的激发态上的原子不只可以自发地而且可以在它与原子系统周围辐射场的光子相互作用时发生向下的跃迁。
