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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

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

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

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.