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Quantum Physics

Quantum physics concerns the behavior of the smallest things we know. These smallest things are very small indeed. Hold up two fingers together in front of you. The diameter of an atom is approximately the same fraction of the width of two fingers is of the diameter of the earth. Our expectations about how things behave are shaped by experiences with objects large enough to see and handle: no wonder these expectations are sometimes wrong when applied to objects as small as an atom. In the same way the classical laws of physics, particularly Newtonian mechanics, devised to describe the behavior of objects of visible size, must be modified and in some respects completely replaced-in order to account for the behavior of atoms and subatomic particles. Although the world of the very small is remote from our senses, it shapes every day experience. Almost everything we touch and see owes its character to the subtle architecture of atoms and molecules, an architecture, whose building code is quantum mechanics. And when we come to large scale phenomena that depend in a direct way on the details of atomic processes-for example lasers, superconductors and solid state electronics-then the explicit use of quantum physics is essential.

Energy of a Photon

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The energy of a photon can be expressed as
E = h$\nu$
where h is Plank's constant and nu is the frequency of the mode of the harmonic oscillator to which the photon belongs. As h = 6.63×10-34 Js and is a constant, energy can be added or subtracted in steps only; that is, energy is quantized. The wavelike nature of a photon is revealed in the frequency $\nu$, which can be related to the wavelength by the equation $\nu \lambda$ = c, the velocity of light in vacuum as in classical theory. The unit of energy is joules. Its practical unit is electron-volts (eV). But a convenient unit is reciprocal wavelength (cm-1). Thus, wavelength $\lambda$ = 1 $\mu$ m has energy 1.24 eV; its reciprocal wavelength is 10000 cm-1.

Photoelectric Emission of Electrons

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Photoelectric absorption occurs in the vicinity of an atom where the photon's energy is wholly converted into releasing an electron from its site in the material, which is usually one of the inner atomic shells. This so called photoelectron emerges with kinetic energy given by

T = $E_{\gamma }$ - $B_{e}$
Where $B_{e}$ is the binding energy of the electron.

The atom which has lost the electron may be excite by releasing other, less tightly bound electrons. Electrons emitted by this process are called Auger electrons. Alternatively, an electron from a higher shell may fill the vacancy in the inner shell with the emission of a characteristic X ray photon. This is known as X ray fluorescence. This X ray in turn may interact and be absorbed by the medium.

Wave Particle Duality

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In the classical point of view, the waves and particles are different phenomena, which had to be modified in early 20th century. During the period of time, scientists discovered that, under certain circumstances, particles act as waves and waves act as particles. This dual behaviour is known as wave particle duality of matter.  The phenomenon of electron diffraction by atomic lattices implies that electrons sometimes possess wave like properties. The photoelectric effect shows that electromagnetic waves sometimes act like swarms of massless particles called photons. Wave particle duality usually only manifests itself on atomic and subatomic length scales. The classical picture remains valid on significantly longer length scales.

Energy Levels in Atoms

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The emission line spectrum of an element tell us that atoms of that element emit photons with only certain specific frequencies f and hence certain specific energies E = hf. During the emission of a photon, the internal energy of the atom changes by an amount equal to the energy of the photon. Therefore, each atom must be able to exist with only certain specific values of internal energy. Each atom has a set of possible energy levels. An atom can have an amount of internal energy equal to any one of these levels, but it cannot have an energy intermediate between two levels. All isolated atoms of a given element have the same set of energy levels, but atoms of different elements have different sets.

Line Spectra

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The spectrum of white light is continuous. It shows up each wavelength component of the white light as an unbroken band of colors from red to violet. The spectra produced by substance in the gaseous state are discontinuous. They consist only a limited number of colored lines with dark spaces between them. Such discontinuous spectra are called line spectra. A line spectrum is one in which radiations of only specific wavelengths appear as lines. Line spectra are also called atomic spectra because they are caused by energy changes taking place within the atoms. The light from a mercury vapor lamp or from sodium chloride heated in a Bunsen flame produces a line spectrum.

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