Dr.Eman Zakaria Hegazy Quantum Mechanics and Statistical Thermodynamics Lecture 1
Quantum mechanics
Historical Background of Quantum Mechanics
- The development of quantum mechanics began in 1900 in
connection with the study of light emitted by heating
solids, so we begin by discussing the nature of light.
- In 1801Thomas Young gave convincing experimental
evidence for the wave nature of light by showing that
light exhibited diffraction and interference when passed
through two adjacent pinholes
- In 1860 Maxwell developed Maxwell’s equations
predicted that an accelerated electric charge would
radiated energy in the form of electromagnetic waves
would radiated energy in the form of electromagnetic
waves
=c
All
electromagnetic
waves
travel
at
speed
c= 3.00×1010 cm/sec in vacuum where  : frequency,
 : wavelength .
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Dr.Eman Zakaria Hegazy Quantum Mechanics and Statistical Thermodynamics Lecture 1
II- Black body radiation:
The radiation given off by material bodies when they heated,
when the burner of an electric stove is heated , it first turns a
dull red then white and blue. In terms of frequency, the
radiation goes from a lower frequency to a higher frequency as
the temperature increases ,because red in a lower frequency
region of the spectrum than is blue.
Black body = an ideal body which absorbs and emits all
frequencies and the radiation emitted by a black body is
called blackbody radiation.
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Dr.Eman Zakaria Hegazy Quantum Mechanics and Statistical Thermodynamics Lecture 1
*A plot of the intensity of black body radiation versus
wavelengths for several temperatures is given in figure (1)
*Many theoretical physicists tried to derive expression but
they were
all unsuccessful, the expression
that is derived
according to the laws of nineteenth century is by Rayleigh and
Jeans Law
dv(T)d (8πBT)c3. deq(1-1)
where
v(T)d is the radiant energy density between
frequencies  and +d and has a unit s J/m3 ,T is the absolute
temperature , and c is the speed of light. The quantity kB is
called Boltzmann constant, KB = R/n J/K
R: Molar gas constant
n: Avogadro’s constant = 6.022× 1023 mol-1
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Dr.Eman Zakaria Hegazy Quantum Mechanics and Statistical Thermodynamics Lecture 1
according to this hypothesis the energy of a wave is related to
its amplitude and the amplitude varies continuously from
zero on up; hence physicists expected the energy of an atom
to vary continuously , so they show
correlated to physicists cannot
this
equation was
explain the blackbody
radiation.
III-Black body radiation and energy quantization:
Planck assumed that the radiation emitted by the body was
due to the oscillations of the electrons in the constituent
particles of the material body.
Planck able to derive the equation:
d(T)d 8πh c3 d(eh/KT-1) eq (1-2)
h:planck’s constant = 6.63×10-34 J.Sec
This equation is called planck’s distribution Law for Black
body radiation.
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