The document outlines the emergence of quantum mechanics, highlighting its development in response to classical physics' inability to explain certain microphysical phenomena, such as blackbody radiation. It discusses significant contributions from physicists like Gustav Kirchhoff, Max Planck, and Albert Einstein, particularly Planck's introduction of energy quantization and its implications for blackbody radiation and the photoelectric effect. Key concepts include the spectral distribution of radiation, the ultraviolet catastrophe, and the successful application of quantum ideas in atomic theory.

1. Dept. of Physics, ZU
Q. Physics: 4318
Lecture 1

2. THE EMERGENCE OF Q. PHYSICS
The introduction of quantum mechanics was
prompted by the failure of classical physics in
explaining a number of microphysical phenomena
that were observed at the end of the nineteenth
and early twentieth centuries.

3. The study of radiation emitted by a heated body in
equilibrium, initiated by the German Physicist Gustav
Kirchhoff in 1859.
Blackbody radiation

4. Blackbody radiation
Fig.1. Cavity approximating
an ideal black body
One of the most puzzling phenomena studied
near the end of the nineteenth century was the
spectral distribution of blackbody radiation.
 A black body is an ideal
system that absorbs all the
radiation incident on it
It can be approximated by
a cavity with a very small
opening, as shown in the
figure

5.  The characteristics of the radiation in such a
cavity depend only on the temperature of the wall.
 At ordinary temperature , the thermal radiation
emitted by a blackbody is not visible because the
energy is concentrated in the infrared region of
the electromagnetic spectrum
 As the body is heated, the amount of energy
radiated increases according to the Stefan–
Boltzmann law, and the concentration of energy
moves to shorter wavelength.
, (: Power Radiated/ Area),

6. T : Absolute Temperature
 (for a perfect blackbody surface), How to prove it?
,
Stefan’s constant.
 Between about and , there is enough energy in
the visible spectrum from the body to glow a dull
red.
 At higher temperatures, it becomes bright red or
even “white hot”.
 The following figure shows the power radiated by a
blackbody as f(λ) for different temperatures.

7. Fig.2. Spectral distribution of radiation from
blackbody for different temperatures

8.  The power radiated by a blackbody as a
function of wavelength for a different
temperatures can be written as and is called
the spectral distribution function.
 This function has a maximum at a wavelength
that varies inversely with temperature according
to Wien’s displacement law:
 can be calculated from classical
thermodynamics in a straightforward way, and
the result can be compared with the
experimentally obtained curves of Fig.2.

9.  The result of this classical calculation, known as
the Rayleigh–Jeans law, is
or,
where k is Boltzmann’s constant.
 This result agrees with experimental results in
the region of long wavelengths, but it disagrees
violently at short wavelengths.
 As λ approaches zero, the experimentally
determined also approaches zero, but the

10.  the calculated function approaches ∞. (why?)
 Thus, according to the classical calculation,
blackbodies radiate an infinite amount of energy
concentrated in the very short wavelengths.
 This result was known as the,
Ultraviolet catastrophe

12. German Theoretical
Physicist Max Planck who
originated quantum
theory, which won him
the Nobel Prize in Physics
in 1918.
Planck’s Theory

13.  In 1900 Max Planck announced that by making
a strange modification in the classical
calculation he could derive a function that
agreed with the experimental data at all
wavelengths.
 Max Planck postulated that the energy emitted
and absorbed by the blackbody was not
continuous but was instead emitted or absorbed
in discrete packets or quanta.
 Planck found that the size of an energy
quantum is proportional to the frequency of the
radiation: E = hf

14. 
 Planck was unable to fit the
constant h into the framework
of classical physics, and the
fundamental importance of his
assumption of energy
quantization, implied by his eq.
was not generally appreciated.
 where h: Planck’s constant. The value of h was
determined by fitting Planck’s function to the
experimentally obtained data.

15.  In general, A turning point for the theory came
with Planck’s explanation of blackbody radiation
and Einstein’s description of the photoelectric
effect (1905). Both of these “quantum theories”
postulated a discreteness of energy.
 For a blackbody the radiating oscillators were
allowed to have only certain discrete energies,
and in the photoelectric effect the radiation was
assumed to consist of energy quanta or photons.
 A major success of the quantum idea was Bohr’s
theory of one-electron atoms (1911).

16. A major step in the direction of
quantum mechanics was de
Broglie’s association of wave
properties with matter (1923).