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Wednesday, December 30, 2015


Light is a wave phenomenon (though this statement is not the whole story). Our best evidence for this proposition is interference, as in double slit interference & Refraction is very good evidence, too.

Out of what, is a light wave formed?

The straight answer is from "Electric & Magnetic Fields". A light wave is always formed from both electric & magnetic fields. The fields always point perpendicular to the propagation direction, that is perpendicular to direction in which light is traveling.

When light interacts with electrons, the exchange of energy occurs in a grainy fashion. The amount of energy exchanged is always hv, where 'h' denotes Planck's Constant and 'f' is the frequency of the light wave. The graininess introduces the photon.

In which way should we think of a Photon as a particle? Indeed as a "particle of light"?

a) A photon has energy, and it has momentum. these are nice particle like properties.

b) We cannot ascribe to a Photon, a definite continuous trajectory.

c) It is tempting to think of a light beam as merely a hail of Photons, but it is also incorrect. Don't do it. A wave traveling through space is better picture.

d) The graininess of light manifests itself primarily when light interacts with charged particles. Thus the Photon notion is needed primarily at the start and finish of a Light beam's travel through space.

Indeed, the Title of Einstein's " Photon" paper was this:

"On a heuristic point of view concerning the production and transformation of Light." The word "heuristic" means "stimulating discovery or further investigation" and indicates a tentativeness. More significant for us  is Einstein's specification of "Production & Transformation of Light". We should understand production to mean the emission of light. By "Transformation", Einstein meant the combination of absorption followed by emission.

And in the body of paper, Einstein devoted extensive space to the absorption of light by metals and molecules. Einstein had clearly in mind the interaction of light and electric charges when he introduced his particle theory of Light.

It is better to represent Photon as

as it reminds us the "Particle" aspect of light - The Dot; The wiggly line - "Wave" aspect of the light.

Physics is not able to provide a valid, intuitive picture of Photon, and that is why none appears in this book.

Photons and Waves are inextricably linked.

Major connections are following three

i) Energy of Photon=hv

ii) Momentum of Photon=hv/c

iii) Probability of appearance of Photon ~ square of 'E' of EM wave

Note:- The Photon idea appears on Left and Wave idea on Right.

When to use Wave concept & Photon concept?

use wave picture for describing interference, refraction, reflection and phenomena with Polaroid sheets; use the Photon idea for describing the interaction of light and electric charge, that is, for describing absorption and emission of light.

In a letter dtd. 12 Dec 1951, when Einstein was 72 years old, Einstein wrote these lines to Besso:

"The entire 50 years of deliberate pondering have not brought me closer to an answer to question "what are light quanta(i.e. Photons)? Today, every Tom, Dick and Hary believes that he knows, but he deceives himself."

Does Physicists understand light?

When it comes to equations, Physicists understand light very well indeed. For instance, theoretical calculations in 1950's laid the foundation for the first Lasers(which were constructed in early 1960's). In no way were LASERs accidental discoveries.

Providing pictures to capture the essentials of what the equations have to say is quite a different task. Sometimes a wave picture will do the job; at other times, it will not. And the same is true for Photon description. But where one description fails, the other succeeds thus there in lies the "complimentary" and for us there is a lesson in humility.

Light is a more subtle phenomenon than any single one of our everyday concepts can describe.