How is glass made so that one can see through it from one side but from the other side it is a mirror.

A half silvered mirror is used when there is a requirement
that the glass separation allows light only from one side to pass through, so it appears
as plain glass from one side but as a mirror from the other
side.

To make this glass it is very lightly coated with
atoms of silver. The coating is such that only half the light can pass through the glass
and the other half is reflected back. When the glass is being used, the side from which
the glass is intended to appear as a mirror is brightly lit while the room on the other
side is very dimly lit.

This allows light from one side to
pass through as it is brightly lit but from the other side no light passes
through.