Yes, shiny materials are good reflectors of visible light, one of many
kinds of radiation. They may not be such good reflectors of other kinds
of radiation, like x-rays, for example, which are just like light
except they have higher frequencies.
Many different kinds of metal are shiny. Gold is a metal which
stays shiny for a long time because it does not react much chemically
with the air. Silver is shinier, but tarnishes easily. Many other
metals, like iron or steel, aluminum, and copper are also shiny.
What makes them that way is that some of the electrons in these
metals can move around very very easily. Electrons feel pushes and
pulls when an electric field comes along. Light waves are made up of
electric and magnetic fields. When a light wave hits a metallic
surface, the electrons on the surface are pushed and pulled by the
field of the incoming wave. How far do they go? They slosh around until
the field that they create (after all, electrons are charged and they
make their own field) cancels out the incoming field exactly. They stop
when the net force is zero on them.
This means that the electric field inside a conducting metal is
zero. If an incoming wave hits the material, and the electric field is
zero on a plane surface at all times, you can express this as a sum of
two waves -- one coming in, and an equal and opposite one coming out.
The sloshing electrons in the metal radiate a wave going out that
exactly matches the one coming in. If there is some resistivity to the
metal, or some corrosion, the metal becomes less shiny.
There's another way a material can be shiny. If a material is
transparent, but has a different index of refraction than the air, then
light rays will bend when they strike the surface of the material.
Some, but not all, will also be reflected from the surface at the same
angle of reflection as if the material had been made out of metal. If
the light starts out in the dense material (like water or glass) and
hits the surface with air, if the angle is steep enough so that the law
of refraction cannot be satisfied, then all of the light will bounce
back into the water or glass, in a process called "total internal
reflection", making that surface look shiny too.
(republished on 07/29/06)