Photon theory of light

The Photoelectric Effect and the Photon Theory of Light Despite the idea of quantization, physicists still pictured energy as traveling in waves. But, the wave model could not explain the second confusing observation, the flow of current when light strikes a metal. 

Some qualities of light are best explained if we describe it as consisting of moving particles, often called photons or quanta (called the particle theory of light). Other qualities are best explained if... 

The Bohr model of the atom took shape in 1913. Niels Bohr (1885-1962), a Danish physicist, started with the classic Rutherford model and applied a new theory of quantum mechanics to develop a new model that is still in use, but with many enhancements. His assumptions are based on several aspects of quantum theory. One assumption is that light is emitted in tiny bunches (packets) of energy call photons (quanta of light energy). 

The wave aspects of the photon are completely described by charge-frcc Maxwell equations. Therefore, it is natural to try to reconcile Planck s hypothesis with the wave theory of light. 

The phenomenon of optical interference is commonly describable in completely classical terms, in which optical fields are represented by classical waves. Classical and quantum theories of optical interference readily explain the presence of an interference pattern, but there are interference effects that distinguish the quantum (photon) nature of light from the wave nature. In this section, we present elementary concepts and definitions of both the classical and quantum theories of optical interference and illustrate the role of optical coherence. 

We can also try to deduce the radiation formula, not as above from the pure wave standpoint by quantisation of the cavity radiation, but from the standpoint of the theory of light quanta, that is to say, of a corpuscular theory. For this we must therefore develop the statistics of the light-quantum gas, and the obvious suggestion is to apply the methods of the classical Boltzmann statistics, as in the kinetic theory of gases the quantum hypothesis, introduced by Planck in his treatment of cavity radiation by the wave method, is of course taken care of from the first in the present case, in virtue of the fact that we are dealing with light quanta, that is, with particles (photons) with energy hv and momentum Av/c. It turns out, however, that the attempt to deduce Planck s radiation law on these lines also fails, as we proceed to explain. 

The photoelectric effect conld not be explained by the wave theory of light. Einstein, however, made an extraordinary assumption. He snggested that a beam of light is really a stream of particles. These particles of light are now called photons. Using Planck s quantum theory of radiation as a starting point, Einstein deduced that... 

Experiments by MilUken in 1908 soon confirmed Einstein s predictions. In 1921, A.H. Compton succeeded in determining the motion of a photon and an electron both before and after a collision between them. He found that both behaved like material bodies in that both kinetic energy and momentum were conserved in the collision. The photoelectric effect and the Compton effect, then, seemed to demand a return to the corpuscular theory of light. The reconciliation of these apparently contradictory experiments has been accomplished only since about 1930 with the development of quantum electrodynamics, a comprehensive theory that Includes both wave and particle properties of photons. Thus, the theory of light propagation is best described by an electromeignetic wave theory while the Interaction of a photon with matter is better described as a corpuscular phenomenon. 

How does one go from the classical wave theory of light as an electromagnetic field, to the quantum mechanical theory of Einstein for absorption (and of course emission) that considers the corpuscular interaction between a photon and an electron. 

But the idea of photons does not dispense with the need for the wave theory of light, which is categorically demanded by the phenomenon of interference. Therefore, a great abnegation of naive realism... 

Powerful as the theory of photons has proved itself, and deeply as the statistical method permits us to penetrate into the nature of radiation, there are aspects of the subject which can only be understood in terms of the electrical theory of matter and of the electromagnetic theory of light. The introduction of the electrical theme cannot be much longer deferred. 

Electromagnetic radiation travels in straight lines in a uniform medium, has a velocity of 299,792,500 m/sec in a vacuum, and possesses properties of a wave motion and also of a particle (photon). The wave theory of light can satisfactorily account for properties such as wavelength, interference, refraction, polarization, reflection, and diffraction. 

The scattering cross section depends on the matrix element (3.9) of the polarizability tensor and furthermore contains the (o frequency dependence derived from the classical theory of light scattering. One obtains [309] analogously to the two-photon cross section (Sect. 2.5)... 

Although Einstein s theory of light as a stream of photons rather than a wave explains the photoelectric effect and a great many other observations, it also poses a dilemma. Is light a wave, or does it consist of particles The only way to resolve this dilemma is to adopt what might seem to be a bizarre position We must consider that light possesses both wave-like and particle-like characteristics and, depending on the situation, will behave more like waves or more like particles. We will soon see that this dual wave-particle nature is also a characteristic trait of matter. 

Sason True, the findings of Rydberg and Balmer formed a basis for the development of the quantum theory of light by Planck and Einstein, who viewed light as a stream of energy bundles called photons (see Retouches section 9.R.5). 

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