Microscopic World -1- (Mysteries of Atomic World)
Epilogue : Opening to Quantum Mechanics
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Throughout all the pages of the present Seminar, Microscopic World -1-, we have learned that matter and light in the microscopic world possess both the particle nature and the wave nature.
In addition, we could understand that the Bohr model of atomic structure is constructed by considering both the natures. By using this model, it has become possible to explain the stability of atoms and the atomic spectra which was impossible to be explained with the classical theory (Newtonian mechanics and Maxwellian electromagnetism).
It seems that the double nature (the duality) of matter and light, i.e. the wave and particle natures, is a very fundamental property of "things" or "beings" existing in our world. In other word, the duality seems to be a fundamental philosophy for the "existence" of things. This looks quite strange and mysterious for us, because we are very much used to the thinking way in the classical theory.

[Young's Experiment]
Let us look back at Young's experiment by which the wave nature of light was confirmed for the first time. For the sake of convenience, the contents of Young's experiment presented in the page 3-8: Summary of Part 3 are shown here again.

Schematic drawing of Young's experiment
The monochromatic light from the source passes through the two slits, S1 and S2, and produce an interference pattern (fringes) on the screen.

An example of the picture of the pattern is represented in the following picture (A) and (B). In these pictures, (A) shows the result when one of the slits is closed, and (B) the result when both of the slits are opened.

We will discuss on some details of these pictures below. Before that, let us study a little about the relation between the basic mechanism of photography and the particle nature of light.

[The Mechanism of Photography and the Light Particle]
In Young's experiment, the reason that the photographic plate (or film) can response to light is due to the particle nature of light. Let us explain this below.
Photography is based on the chemical reactions brought about by the action of light (photochemical reactions). Namely, the molecules of halogenated silver are dissociated by the illumination of light and silver atoms are educed. For example, silver bromide (AgBr) that is one of the halogenated silvers is dissociated by light as
AgBr --> Ag + Br
to be silver and bromine atoms. After being treated with special chemicals, only the silver atoms remain to be black coloured and to make a photographic negative.
To dissociate a molecule of halogenated silver, roughly speaking, more energy than about 2 eV is necessary. Namely, one molecule of silver bromide (AgBr) or silver iodide (AgI) can be dissociated only by the accumulation of more energy than about 2 eV This amount of energy is almost the same as the value of the work function which is the necessary minimum energy to bring about the photoelectric effect. (Refer to the page 3-6: The Hypothesis of Light Quanta and the Photoelectric Effect.) Hence, the discussion on that page about the time to make the photoelectric effect take place is valid for the present photochemical reaction as well; i.e., if we consider the time necessary for the dissociation of the halogenated silver within the classical theory, it should take a time longer than the shutter speed of an ordinary camera that is usually shorter than 1/100 s or 1/1000 s. Consequently, we cannot help considering that light can dissociate the halogenated silver molecules being absorbed instantly as a particle of light (photon).

[The Mysterious Particle-Wave Duality]
The image of the interference fringes recorded on the photographic plate (film) is consists of huge number of silver atoms educed by the photochemical dissociation of halogenated silver molecules. These silver atoms collect together to make the interference pattern on the film. We recognize this collection of the silver atoms as a picture of the interference pattern. Namely, we are looking at the collection of marks or traces of photons. Each mark denotes a point where a particle of light (photon) has struck. The collection of these huge number of marks constitutes a interference fringes which characterize the wave nature of light. How can we understand this mechanism?
If light were simply particles, one light particle could not pass through two slits simultaneously, so that it could never produce interference fringes. If light were simple particles, then the picture on the screen should be what is obtained by superposing such a picture in the case of a single slit opened as shown in the above picture (A) with another picture obtained by displacing it by the interslit distance d. However, we have an interference pattern as shown in the picture (B) in practice. Hence, we must consider that light is not a simple particle, but possesses some kind of wave nature as well.
If light were pure waves, we could not have a picture of the interference fringes. Because light has the duality of wave nature and particle nature simultaneously, we can have the picture of the interference pattern. We therefore cannot deny the dual nature of light. Then, what part of light is a wave? And what part of light is a particle?
You might suspect that two different photons interfere with each other after they pass through two slits separately and consequently they produce the interference pattern on the screen. However this idea is not valid by the following reason: If the intensity of the incident light to the double-slit experiment is so weak that only one photon runs at every moment of time, two different photons are impossible to pass through two slits at one time and no interference occurs among different photons. In spite of such extreme weakness of the intensity of the incident light, the same interference pattern is still obtained on the photographic plate by exposing it for an extremely long time. Such an experiment of three-month exposure time was carried out in 1909 by a British student, and he got a clear interference pattern that is exactly the same as those in usual Young's experiment. This tells us that the interference pattern in Young's experiment is not caused by the interference between different photons. Then, why does the interference pattern come into existence?

[Opening to Quantum Mechanics]
As mentioned above, we have encountered a serious discrepancy between the particle nature and the wave nature of light. This discrepancy is not only for light but also for electrons.
In order to perfectly overcome this discrepancy, people have had to construct a completely new theory getting over the difficulties in the classical theory. This is nothing else than Quantum Mechanics proposed by W. K. Heisenberg (Germany, 1901 - 76) and E. Schroedinger (Austria, 1887 - 1961).
By this quantum mechanics, the mystery of the particle and wave natures of light and electrons were solved and the atomic structure was clarified. You are strongly expected to challenge to study Quantum Mechanics proceeding to the Second Part of the present seminar,
Microscopic World -2- (Introduction to Quantum Mechanics).
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