MadSci Network: Physics

Re: Light waves

Area: Physics
Posted By: Adrian Popa, Staff Optical/Microwave Physics
Date: Mon Apr 29 15:21:29 1996

Hello: I wish you great success in your work on K-12  science 

curriculum, It is a great challenge and a most important effort. 

Please pardon my long answer to your brief question.  

However, I would like to make several important points (to me at least) 

about the wave - particle issue. 

To answer your question let me start by saying that

the speed of light in the vacuum of space is one of the

fundamental constants of the universe as we currently

understand it. As Einstein predicted early in this century, 

even time must be changed to explain the constant speed of

light in space-time coordinates. Today we have navigation satellites

that must set the clocks in orbit ahead in time so that their  time 

signals received on earth are corrected for retavistic time shift.

When light encounters matter 

it rattles through the atoms and molecules delaying the passage 

through the material. The light does not slow down in the matter, 

it is detoured in a zig zag path so that it takes longer to pass 

through the matter. We call the delay the index of refraction of 

the material. If the light takes twice as long

to pass through the matter than it would through an equal 

(straight line) distance in a vacuum, the refractive index is 2. 

Three times longer gives an index of 3 etc. The index is related 

to how dense the matter is along with other factors in the chemical

structure of the material.

It was also demonstrated about the turn of the century

by Maxwell, Hertz and others that all electromagnetic

energy including radio, infrared, visible light,

ultraviolet x-rays and cosmic rays also travel at the speed 

of light in a vacuum. Consequently all electromagnetic

energy can be described in terms of particles (photons) or

as waves (electromagnetic waves). As formulated by Max

Planck, the energy of a photon is equal to Planck's

constant times the frequency of the oscillation. Thus there are 

photons in radio signals, television signals , radar, light etc. 

A recent example of research with radio frequency photons is

the work of Professor Daniel Kleppner and his associates at 

the Massachusetts Institute of Technology using  masers operating 

in the microwave spectrum where today's cellular telephones operate

(Kleppner et al,Physics Today, 42, 24, 1989). This work, which has not 

been reported in the popular press, has also uncovered many

strange effects at photon levels of energy which raise interesting 

questions  similar to  those published in the popular press about optical

photons.  However, an important point to make is that explaining signals,

such as used in cellular telephony, using a 

few photon levels of energy, at any frequency in the electromagnetic 

spectrum, would be unfathomable to anyone except experts in quantum

electrodynamics. However; for completeness, if we did

discuss optical photons in our science texts (as your

question suggests) should we not also discuss Kleppner's

work with microwave photons? 

I believe that emphasis on photons (particles)

would confuse the student, the teacher and for that matter many scientists 

including myself. The problem is that we are discussing esoteric 

phenomena at the leading edge of scientific thought which may or may 

not be explainable in a systematic way for decades, just as the search

still continues for the gravity  waves predicted by Einstein in his famous

papers on  general relativity. The same argument holds true for the 

strange behavior of photons in optical region of the spectrum. 

In the 21st century the new photonic devices now emerging from the

laboratory to replace and /or augment electronic devices will generate

,transmit, store, manipulate and display information at incredible rates. 

Today's  fiber optic links  can

transmit the contents of a 30 volume encyclopedia in 1/10

of a second. However, these systems operate by using

many millions of photons per second. Signals at these levels,

, measured in microwatts and milliwatts, completely

overwhelm the few photon effects being studied at the

leading edge of physics. In Industry today we are having to 

retrain optical physicists in the complex wave theory used in 

microwave research to enable them to  understand the design and

operation of the new generation of photonic devices. 

In my opinion, for completeness, the particle/wave duality issue 

should be mentioned in science curriculum (just as

relativistic time shift effects and gravity waves are). 

However, any attempt to present the behavior of small numbers of photons

while explaining the 

revolutionary new applications of photonics in information technology 

would best be delayed until students thoroughly understand the wave 

like nature of electromagnetic energy. In the U.S. today this generally

occurs in university level education. This understanding is well beyond the 

scope of a K-12 science curriculum.

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