29-11-2012, 05:58 PM
Engineering Physics PH 104
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Classical Physics treats particles and waves as separate components.
Mechanics of particles and optics of waves are independent disciplines.
In the microscopic world (atoms, molecules, electrons, nuclei and several
other elementary particles etc. ), there are neither particles nor waves in
classical sense.
We may regard the electron in the picture tube, as a particle since it has
charge, mass and behave as a mechanical particle (following Newton’s
Laws)
It is equally correct to consider the moving electrons as wave.
We consider electromagnetic waves as waves under certain suitable
conditions they exhibit diffraction, interference, polarization etc.
Under other suitable conditions, EM-waves behave as they consist of a
stream of particles.
This wave particle duality along with the special theory of relativity is the
central theme of modern physics.
Electromagnetic Waves
Coupled electric and magnetic oscillations that move with the
speed of light and exhibit typical wave behaviour.
Maxwell (1864) made a remarkable suggestion:
accelerated electric charges generate linked electric and
magnetic disturbances that can travel indefinitely through space.
If the charges oscillate periodically, the disturbances are waves
whose electric and magnetic components are perpendicular to
each other and to the direction of motion.
Black Body Radiation
Only the quantum theory can explain its origin
What is the origin of the radiation emitted by bodies of
matter?
Glow of a hot piece of metal – gives off visible light
whose color varies with the temperature of the metal from
red to yellow to white as it becomes hotter and hotter.
Other frequencies are also present to which our eyes do
not respond.
All objects continuously radiate such energy whatever their
temperatures – At room temperatures most of the
radiations are in the infrared part of the spectrum and
hence is invisible.
Planck’s Quantum Theory
In order to explain the black body radiation phenomenon, Max
Planck (1900) introduced the quantum theory of radiation. He
assumed that
- the atoms in the walls of a black body behave like simple
harmonic oscillator
- a simple harmonic oscillator can not have any arbitrary
values of energy but only those values of energy given by
, where n = 0, 1, 2, …..
n is called quantum number and h=6.626 X 10-34 JS .
- As long as the oscillator is in one of the allowed stated, it
can not emit or absorb energy (stationary state). The
emission and absorption occurs only when the oscillator
jumps from one state to another state.
According to Planck this quantum theory is only applicable to the
radiant energy. Later on Einstein (1905) extended the theory.