AbstractDuring the exposure of a silver-halide grain to light within the absorption band of the halide, the net rate of formation of either surface or internal silver is considered to be the difference between the rate of electron trapping and the rate of hole (or halogen) reaction at the appropriate location. Since photoelectrons and photoholes are formed in equal numbers by the light absorption, the net rate of total (i.e. surface plus internal) silver formation is equal to the difference between the rate of loss of holes (or halogen) from the grain (e.g. by reaction with permanent hole traps in either the grain itself or its environment) and the rate of loss of electrons from the grain.On this basis a simple expression can be derived for the quantum efficiency of surface silver formation. The variables involved in the expression are the rate of electron trapping at the grain surface relative to that in the interior, and the rate of hole (or halogen) attack at surface sites. An analogous expression can ...