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Fundamentals of Reflections
Name: Neville P.
Status: other
Grade: other
Location: CA
Country: N/A
Date: 7/28/2005
Question:
Hello. Can you explain to me (and my students) what is it
that makes something reflective. How does a mirror work? I already
understand about incident and reflected light but am looking for a more
detailed answer about the physics at the atomic level.
Replies:
Neville,
Every atom has certain "natural frequencies", frequencies of light it can
easily absorb. Even molecules have natural frequencies. Crystal structures
also have natural frequencies. For all such objects, there are other
frequencies that pass right through. Then there are the frequencies that
"almost" absorb. They can be absorbed, but not held for any significant
length of time. These frequencies are the basis of reflection.
If a reflective frequency enters strikes a material, it will be absorbed and
then almost immediately released. The direction of release is random. If
releases INTO the material, it is re-absorbed and re-emitted. Most of the
light ends up leaving the material very quickly.
Materials with a rough surface can emit the light in a wide variety of
directions. This is the reflection that lets us see things. This is how
light from anywhere in the room lets us see where we are walking.
Materials with a very smooth surface, through properties of symmetry, end up
emitting most of the light in the same direction. This is the reflection
that makes a mirror work. It also produces glare. Because of the smooth
pattern on the surface, all light emitted in the direction of the reflection
formula (angle in equals angle out) interferes constructively. Light in
this one direction is very bright. Light emitted in other directions
interferes destructively, some directions worse than others. Very little
light gets out in these other directions.
Kenneth E. Mellendorf
Physics Instructor
Illinois Central College
Advancing beyond the angle of incidence/reflection and decrease in the
speed of light as it moves from one transparent medium to another (for
which no explanation is usually given) to find a "more detailed answer
about the physics at the atomic level." is challenging because the
fundamental laws governing the behavior of the light are Maxwell's
equations for electromagnetic fields. The solution(s) involve vector
analysis and calculus, which unfortunately is beyond what the typical high
school student is prepared for. There are many good treatments in college
level physics texts on electricity and magnetism, but these are usually 1
or 2 semester college physics courses. I wish I could be of more help, but
I just do not know how to lower the level and do justice to the subject. A
tough question with no easy answer I know of.
Vince Calder
I found your question very interesting because I did not know of a
source or reference "off the top of my head". I searched the Internet and
my library and arrived at the very unsatisfying conclusion that there
is not a "simple" analysis of the behavior of light (and other
electromagnetic radiation) once you want to go deeper than the
phenomenological properties like the laws of incidence and reflection etc.
There is a definitely a need for such an analysis.
The next level of analysis starts with the four Maxwell equations. These
of course require vector calculus and knowledge of functions of complex
variables. Unfortunately, in our present educational system these topics are
not even addressed until the second year of college calculus and/or physics.
I do not think that it has to be that way, we have just lowered the
mathematical bar so low in most of our secondary schools that those topics
are not even found in most math and physics texts.
The books I searched were: "Lectures on Physics" by Richard Feynman
(usually the best place to start); "Physics of Waves" by William C. Elmore
and Mark A. Heald; "Light" by R. W. Ditchburn; "The Electromagnetic Field"
by Albert Shadowitz. They all assume a knowledge of integration of functions
of complex variables, matrix algebra, and vector analysis.
Despite the fact that there does not appear to be a text that starts at
an approachable level, I think there could be. Not an easy assignment, but a
do-able one in my opinion. For your immediate needs you might be able to
pick out some results from these (or other) texts, but it appears that the
higher level math is a prerequisite to a full understanding of light at the
microscopic level.
Vince Calder
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