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- Lecture Physics A2: Quantum Nature of Light (Quantum Optics) - PhD. Pham Tan Thi
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- Quantum Nature of Light
(Quantum Optics)
Pham Tan Thi, Ph.D.
Department of Biomedical Engineering
Faculty of Applied Sciences
Ho Chi Minh University of Technology
Courtesy of N. Brunner and J. Simmonds
- Light behave like a wave or a particle
(This is called the duality of the behavior of light)
- Wave Characteristics
- Electrically Charged Particles and Electromagnetic Waves
Electrons have (-) charge
Protons have (+) charge
Both have electric fields
+- attract
++ and -- repel
• The changing position of a charged
particle creates “waves” called
electromagnetic waves
• The electromagnetic waves travel
through empty space eventually
interacting with a distant charged
particle.
• Visible light is an electromagnetic
wave.
- Magnetism
(Effect on electric charges)
Moving electric charges also
produce magnetic fields.
Example: electric current passing
through a coil.
Another interesting example: the
Earth’ magnetic field is produced by
the spinning of charges in the liquid
metal core of the Earth.
Conversely, magnetic field force
charged particles to move…
- Accelerated Charges (electrons, protons) Produce
(Ripples in the Electromagnetic Field)
An electromagnetic wave is
composed of two oscillating fields,
and electric field and a magnetic
field perpendicular to each other
Antenna receives electromagnetic signal
from TV station
- Wavelength of Electromagnetism means COLOR
- The Temperature Scale
(Conversion from wavelength/energy to temperature)
The scale mostly used in sciences, physics and astronomy is Kelvin.
The unit is Kelvin (K).
- Thermal Radiation
• The fundamental sources of all
electromagnetic radiation (EMR) are
electric charges in accelerated motion.
• All bodies emit electromagnetic radiation
as a result of thermal motion of their
molecules. This radiation, called thermal
Reference
radiation, #1 of different
is a mixture
wavelengths.
Reference #2
• Thermal radiation is emitted by all objects An electric heating element emits
primarily infrared radiation. But if its
above absolute zero (-273.15°); but some temperature is high enough, it also
of objects is in visible. emits a discernible amount of
visible light.
- What is a Blackbody
A blackbody is an idealized object that
absorbs all EMR that falls on it - no
EMR passes through it and none is
reflected (i.e. perfect emitter and
perfect absorber).
Because no light (visible EMR) is
reflected or transmitted, the object
appears black
Leave appears green because green
color is reflected to human eye
- Blackbody in Lab Experiment
• An object of controlled temperature T contains a cavity, joined to the
outside by a small hole.
• If the hole is small enough, the radiation in the cavity comes to
equilibrium in the walls.
• The hole allows a small fraction of the radiation to pass to a
spectrometer — the radiation coming out has the same spectrum as
what is inside.
• The radiancy is the power emitted per unit area per increment of
wavelength and so has unit of Wcm-3
- Blackbody in Lab Experiment
• The spectral radiance from the hole is independent of the material
used and only depends on the temperature.
- Rayleigh - Jeans Approximation for Blackbody
- Kirchhoff’s Law of Thermal Radiation in Thermal Equilibrium
- Ultraviolet Catastrophe
- Quantum Theory
1. Stefan-Boltzmann’s Law
2. Wien’s Law
3. Planck’s Theory
4. Planck’s Formula
- Stefan - Boltzmann’s Law
The total emitted radiation (Mλ) from a blackbody is proportional to the
fourth power of its absolute temperature.
M = T4
where σ is the Stefan-Boltzmann
constant, 5.6697 x 10-8 Wm-2K-4
—> the amount of energy
emitted by an object such as the
Sun or the Earth is a function of
its temperature
—> This can be derived by
integrating the spectral radiance
over entire spectrum
Z 1
2⇡ 2 k 4 4
L= L d = 2 3T OR M = ⇡L = T 4
0 5c h
- Wien’s Displacement Law
In addition to computing the total amount of energy existing a
theoretical blackbody such as the Sun, we can determine its dominant
wavelength (λmax) based on Wien’s displacement law:
k
max =
T
where k is the is the Wien’s
displacement constant = 2.898
x 10-3 Km, and T is the absolute
temperature in K
i.e. there is an inverse
relationship between the
wavelength of the peak of the
emission of a blackbody and its
temperature.
Therefore, as the Sun approximates a 6000 K blackbody, its dominant
wavelength (λmax) is 0.48 µm
- Planck’s Quantum Theory
Max Planck found a correct law for the black body radiation by
assuming that each oscillator can only exchange energy in discrete
portions or quanta
The energy exchanges between radiation and matter must be discrete and
energy of radiation E = nhν
Average energy per standing wave
h⌫
"=
eh⌫/kT 1
Planck’s modifications
8⇡h ⌫ 3 d⌫
u(⌫)d⌫ = 2 h⌫/kT
c e 1
h = 6.626 x 10-34 J.s is Planck’s constant
- Einstein’s Theory of Photoelectric Effect
(Provides a direct confirmation for the energy quantization of light)
Heinrich Hertz in 1887
later on, it was explained by Albert Einstein in 1905
It was during this study of electromagnetic waves that he saw the if
one sends ultraviolet light onto metals, you’ll get sparks coming off.
The phenomenon of ejection of electron from the surface of a metal when
light of a suitable frequency strikes it is called photoelectric effect. The
emitted electrons are called photo-electrons.
Experiment:
The time between the incidence and
emission of a photoelectron is very
small, < 10-9 s.
Classically: raise intensity => more
electrons
independent of frequency
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