Mirrors and Reflection
Name: Nicholas S.
What makes a mirror such a better reflector than anything
else from the point of view of physics? Is it only because it is polished
to be very flat? Isn't a mirror, glass with a silver coat painted on the
back? A black surface that is polished to be very flat, would not work..
so what are the prerequisites for a mirror? Also how does this apply when
water reflects light & images? Ultimately I would like to know if surface
molecules can be oriented, by electrical or magnetic forces, to acquire
mirroring reflective capacities?
The color of an object is the first factor of whether something will
reflect. Some molecules can absorb visible light and hold it long enough to
convert it to heat. This is what makes something look black. Some
molecules release visible light almost as soon as it is absorbed. These
materials look white. Most materials absorb some colors better than others.
These have various colors.
The next concern is smoothness. A rough surface reflects in all directions.
A piece of concrete is a good example. A very smooth surface reflects in a
very ordered fashion, sending most light out in a single direction. This is
a mirror. A good mirror is silvered because of color and smoothness.
Metals are very flexible, very easy to smooth with just a small amount of
heat. Metallic paint produces a very smooth surface. Of all metals, silver
is probably the most "white". It reflects almost every color. Gold and
copper would not reflect blue very well.
Water surfaces reflect a little bit when shining a light straight down on
it. Most of the light continues on into the water (refraction). Some
reflects back. Because the surface is very smooth (most liquids are),
reflected light forms a clear image. When light is shined at a shallow
angle, something different happens. Light coming in almost parallel to the
water surface cannot pass through. What little that does get into the water
gets bent right back out. The process is called Total Internal Reflection.
Light passing from one material to another has its direction changed a
little. A prism is a good example of this. At too shallow an angle, this
new angle is too big. The light gets bent back out.
Dr. Ken Mellendorf
Illinois Central College
The electromagnetic theory of metallic reflection is too detailed for a
forum such as this. The best (most understandable and accurate) description
I've found is in Richard Feynman's "Lectures on Physics" -- I forget which
of the three volumes. It's funny how this answer applies to so many
questions about physics. Feynman was a master teacher.
Hi, Nicholas !!!
As you know, when the light reaches a surface,
the beam will be reflected and part of it absorbed.
A metallic surface does it better, due to the
interaction between free electrons and the photons.
A non-metallic element which atoms are bound
by another type of chemical bond dont reflect
the light or only with some difficulty can reflect it.
Anyway, reflection of light is
the result of interaction between electromagnetic
fields. To avoid scattering of the beams the sur-
face must be flat, so that the reflected beams
will travel parallel and you can get a better image.
The common mirror is indeed a glass surface
(very flat) with silver deposited on a face and
protected with resin to avoid its destruction.
When you say that "a black surface would not work",
this is due to the light absorption ( that is why it
is black...- a black hole ?? - ) and cannot be used
as a mirror. About your last question : " if surface
molecules can be oriented to acquire mirroring capacities".
I only can say that to be oriented by electrical / magnetic
field it is possible. But about acquiring reflective properties
it depends on a number of conditions.
Click here to return to the Physics Archives
Update: June 2012