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Name:  Ng Chieng M.
Status: student
Age: 20s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
Does anyone know how one can change the optical density of a material? I understand that light travels within a material by being absorbed by the atom/molecule and then re-emitted and then travels through space to until it collides with the next particle. However, there should be a time difference in the rate of re-emission of light. What accounts for the difference and what are the factors affecting them. Can we control the optical density? If anyone knows the answer or knows of a book that contains the answer, please let me know. Thank you!


Replies:
Ng Chieng M.

Optical density can be changed by causing the radiation to pass through increasing thicknesses of the material in question. The thicker the specimen, the greater the optical density and the less its transparency.

Regards,
ProfHoff


Ng Chieng M.,

The rates of absorption and re-emission can only be explained with quantum mechanics. At the level of individual atoms and molecules, there are no exact times. Everything is based on probability. A ray of light is actually millions of photons: tiny bundles of electromagnetic energy moving together as a wave. Each individual photon may or may not be absorbed by an atom it meets. Different frequencies (different colors for visible light) have different chances of being absorbed. If a photon is not absorbed, it may be absorbed by a later atom. After absorbed, the photon's energy raises the energy level of the atom. This extra energy may stay within the atom for quite a while, or it may be released almost immediately. Once released, the photon continues on.

In a material only a few molecules thick, every photon would take a different amount of time to get through. When a material is millions of molecules thick (like almost every object in the world), this randomness averages out. What we usually see are average values. The more molecules in the path of the photon, the more reasonable it is to assume the average values are precise.

What affects these quantities have to do with the natural states of the atoms. Certain energy levels work well for atoms. Other energy levels do not. A material that is red, for example, cannot hold red light. The energy of a red photon can be absorbed, but it emitted back out almost immediately. That same material can hold the energy level of a blue photon quite easily. In fact, very little blue light is ever re-emitted. It is held long enough to become heat energy rather than light energy. All this depends on the structure of the individual atoms, as well as how the atoms combine to form the individual molecules. To significantly affect such things would require changing the atomic and molecular structure of the material: changing it to a new material. Heat can affect things a little. To make it matter a great deal, in most cases, you would have to heat the material until it glows.

Dr. Ken Mellendorf
Illinois Central College



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