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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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