Hi Muhannad,
Mirrors are some of the simplest devices to make and also some of
the most useful. To make a mirror, all you need is a polished piece of
shiny material. Most metals are shiny when polished, and there is an
important reason for this. Metals are very good conductors of
electricity. For this reason, the electric field inside a metal has to
be zero. If the electric field were not zero inside a metal, then
electrons will move very quickly to cancel any such field. If a field
is applied from outside, then electrons will move around to cancel it
inside.
Light consists of traveling electric and magnetic waves. When a
light wave strikes a shiny surface, that is, a flat one which conducts
electricity well, then the electric field must vanish everywhere on the
surface. The only way to satisfy this condition and to have a light
wave coming in is if there is also a light wave coming out, with
exactly the opposite electric field as the incoming wave in all places
on the surface of the mirror at all times. This process is very similar
to the reflection of a mechanical wave on a rope or chain that is
attached to a post at one end. If you give the free end of the rope a
good shake (just once), you can watch the wave travel to the fixed end
at the post, and then an upside-down wave will come back at you. Here
are some
nice animations
of this process and others. Reflection of an electromagnetic wave from
a mirror corresponds to the "hard boundary" where the endpoint is fixed
at zero for the electric field.
If the light wave hits a mirror not quite head on but at an angle,
then the reflected wave comes off at the same angle w.r.t. to a line
drawn perpendicular to the plane of the mirror. This process (plus
another animation!) is described on our
answer on the law of reflection.
Mirrors can be curved to focus light as in telescopes, or curved
the other way to allow a wider range of view, as in the right-hand-side
rear-view mirrors in many cars these days.
Tom
(published on 10/22/2007)
