Black and white photography
The emulsion used to coat photographic film contains silver bromide and small
amounts of silver iodide. Photographic printing papers are coated with silver
chloride. Each of these emulsions contains microscopic crystals or “grains” of the
silver halide. The grains are typically about 500 nm in diameter and are held in
gelatin that has been coated on flat film or paper.
The reaction that records the image is a photochemical reaction. When the
emulsion is exposed t light, an electron is driven out of the halide ion wherever
the light falls
Br-
Br +e
The liberated electron attaches to a nearby silver cation
Ag+ + e
Ag
The overall photochemical process is
Ag   Br  light

 Ag  Br
The reaction occurs only where the light falls. . Slowly, as this process continues,
tiny specks of elemental silver start to appear in the crystal lattice, darkening its
surface.
.A photo fixative solution containing thiosulfate ions removes undeveloped grains
of silver bromide from the negative and creates a soluble silver thiosulfate
complex, leaving a negative that retains the silver image after bathing.
Referred to as negatives, the darkest areas (most elemental silver) of the
developed film correspond to portions exposed to the most light. A positive print
is made by exposing print paper also coated with a silver halide emulsion to light,
which passes through the superimposed negative. The exposed print paper is
then developed and fixed.