|Why do some things reflect your image (like
metall or dark window glass) and other things do
not (like wood, concrete, paper)?
|Question Date: 2005-10-05|
Everything that is not "luminous" itself, that is,
does not produce its own light, does reflect light
- or you would not see it! So, if everything
reflects light, then why don't you see an
image?Because only very, very smooth surfaces
reflect light rays parallel to each other, so that
you get an image. This is called
"specular"reflection, which means "mirror-like"
(from the Latin word speculum, meaning "mirror").
Every surface that is not smooth like metal or
glass reflects light in all different directions,
so that the light gets scattered. We call that
"diffuse" reflection. Then you just see the object
itself, because it reflects light, but the
scattered rays that go off in all different
directions cannot produce an image.
Smooth surfaces reflect light, and rough surfaces
do not. Because of the nature of the substances,
it is impossible to make wood, concrete, paper,
and the like smooth enough to reflect coherent
images, but it is possible to make metals,
crystals, liquids, etc. sufficiently smooth.
This is actually not a simple question to answer.
Things reflect because of a combination of things,
including their refractive index, how smooth the
surface is, and finally, in the case of metals,
and mirrors, because there are electrons in the
material that are free. Consider that a shallow
pool of water will reflect, but the ocean does not
usually reflect, particularly if there are waves
on it. One reason that things reflect is that the
speed at which light travels changes depending on
whether it is traveling in air, water etc. and
when it goes from air to water, some of the light
is bounces off, which causes reflections. The
scientific term for this is refraction, and water
has a higher refractive index than air. Wood,
concrete and paper don't reflect, partly because
their surfaces are not very smooth, and he light
that hits the surface of these materials is
scattered all over. A smooth sheet of paper (a
glossy magazine cover) will reflect.
I'm a geologist so I like your question! Let's
start by talking about what a mineral is. A
mineral is an inorganic (was never alive)
substance with a specific chemical composition and
The atoms of some minerals
are arranged in such a way that the mineral has a
shiny or metallic luster. The word luster is used
to describe the way a mineral's surface reflects
light. Be careful, though! Some surfaces are
shiny because we polish them and not because they
look that way naturally.
Glass is naturally
shiny because it is composed of the mineral quartz
(very common in beach sand) which has a shiny
luster. Concrete is made up of pebbles and a
matrix of clayey material.
Although some of
the individual pebbles might be shiny, the clay
minerals all around them and covering them are
dull. Wood and paper are in a completely
different category because they were once alive
and are considered organic.
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