Lecturer: Ivan Kassamakov, Docent
Assistant: Kalle Hanhijärvi, Doctoral student
Course webpage:
http://electronics.physics.helsinki.fi/teaching/optics-2014
Nature of light
What am I?
Waves?
Particles?
In the17th century, two scientists had
different views about the nature of light…
Light is
particles
No! Light
is waves
Isaac Newton
1643 - 1727
Christian Huygens
1629 - 1695
In the 17th century, some properties of
the light were well known already.
For example:
 Light has different colours.
 Light can travel through a vacuum.
 Light can be reflected and refracted, these
processes are described by the Laws of Reflection
and Laws of Refraction.
Newton proposed his “particle theory of light”
(or “corpuscular theory of light”) to explain the
characteristics of light.
(source: “Opticks”, published by Isaac Newton in 1704)
I think light is a stream of tiny
particles, called Corpuscles …
Newton's particle theory
Why does light have different colours?
 The particles of different colours have
different properties, such as mass, size
and speed.
Why can light travel through a vacuum?
• Light, being particles, can naturally pass through
vacuum. (At Newton’s time, no known wave could
travel through a vacuum.)
Newton's particle theory
Why does light travel in straight lines?
 A ball thrown into space follows a curved path
because of gravity.
 Yet if the ball is thrown with greater and greater speed,
its path curves less and less.
 Thus, billions of tiny light particles of extremely low
mass travelling at enormous speeds will have paths
which are essentially straight lines.
Newton's particle theory
Why does a prism separates a beam of
white light into the colours of the rainbow?
Why does red light refract
least and violet light
refract most?
Newton’s assumptions:
1. The light particles of different colours have mass.
Red light particles have more mass than violet particles.
2. All light particles experience the same refracting force
when crossing an interface.
Thus, red light particles with more inertia will be refracted
less by the same force than violet light particles by the same force .
Newton's particle theory
How does the particle theory explain
the Laws of Reflection?
Steel Ball
Rebound
 The rebounding of a steel ball from
a smooth plate is similar to the
reflection of light from the
surface of a mirror.
Many light
particles in a light ray
Light
Reflection
Mirror
Newton's particle theory
Laws of Refraction
 A cannon ball hits the surface of water, it is acted upon by a
“refracting” force which is perpendicular to the water surface.
It therefore slows down and bends away from the normal.
Light does the opposite. Newton explained this observation by
assuming that light travels faster in water, so it bends
towards the normal.
 The problem:
Does light really travel
faster in water?
In fact nobody could measure
the speed of light at the time of
Newton and Huygens
Air
Cannon ball
 Another problem:
How to explain the
textures of the shadows
Water
Light
The concept of the luminiferous ether.
Sound waves
Mechanical waves
Through what do light waves
travel?
Luminiferous ether
Light waves
The concept of the luminiferous ether.
Through what do light
waves travel?
Light waves
1704 - Isaac Newton publishes Opticks, in which he proposes a
particle theory of light. This had trouble explaining refraction, so
he adds a "fudge factor", claiming that an "Aethereal Medium"
is responsible for this effect, and going further to suggest it
might be responsible for other physical effects such as heat.
The concept of the luminiferous ether.
Strange characteristics
predicted of the ether:
• Since light travels through
space, the ether would have to
permeate all space.
• Since light travels through
matter, the ether would have
to permeate all matter.
• It does not affect motions in
the sky. It did not affect the
earth’s motion.
• It was stationary.
Query 19:
“Doth not the Refraction of Light
proceed from the different density of
this Ætherial Medium in different
places, the Light receding always from
the denser parts of the Medium? And
is not the density thereof greater in
free and open Spaces void of Air and
other grosser Bodies, than within the
Pores of Water, Glass, Crystal, Gems,
and other compact Bodies?”
Let’s see how Huygens used his “wave
theory” to explain the characteristics of
light …
(source: Treatise on light, published by Huygens in 1690)
I think light is emitted as a series
of waves in a medium he called
“aether”
(“aether” commonly also called “ether”)
Light as a wave motion
Dutch physicist and mathematician Christiaan
Huygens (1629 – 1695) invented the first pendulum
clock and discovered one of Saturn’s satellites, Titan.
• In 1690 he published a theory that
describes light as a wave motion.
•Huygens’ Principle says that every point
on a wavefront can be considered as a
source of secondary wavelets, which
spread out with the speed of the original
wave.
•His theory is able to describe reflection,
refraction, diffraction and interference
Huygens' pendulum clock had an error of less than 1 minute
a day, the first time such accuracy had been achieved.
Light as a wave motion
Trait de la Lumiere
(Treatise on Light)
Christiaan Huygens, 1690
Huygens’ own illustration of his
principle of secondary wavelets to
explain the propagation of light
Light as a wave motion
How do waves propagate?
P
A wave starts at P and a “wavefront” W
moves outwards in all directions.
After a time, t, it has a radius r, so that
r = ct if c is the speed of the wave.
Each point on the wavefront starts
a secondary wavelet. These secondary
wavelets
to form a new wavefront
W’ at time t’.
Wave theory of light
• But Huygens’ theory left a problem: if light was
a wave, through what medium did it travel?
• The only waves known were mechanical.
Something needed to oscillate for a mechanical
wave to propagate.
In 1678 and 1690 Huygens proposed a wave theory of light in which
waves propagated longitudinally through a stationary aether. The speed
of propagation was finite. This aether was continuous throughout space
and consisted of hard elastic particles which transmitted impulses
without being displaced themselves.
This theory describes: Propagation, reflection, refraction,
diffraction, interference and attenuation
Wave theory does not describe the absorption of light by a
photosensitive materials
Particle theory of light
 People tend to accept “authority” when there is not
enough evidence to make judgement. Newton’s
particle theory could only explain refraction by
incorrectly assuming that light travels faster in a
denser medium. No one could prove he was wrong
at that time.
 The uncertainty about the speed of light in water remained
unresolved for over one hundred years after Newton's
death.
Wave theory of light
However, the wave theory of light was
re-examined 100 years after Newton’s particle theory
of light had been accepted……
Light is not
particles!
Thomas Young
1773 - 1829
Wave theory of light
• Thomas Young
successfully
demonstrated the
interference of light
(which Huygens failed to
show), by his famous
double-slit experiments.
• Since then the wave
theory of light has been
firmly established.
Problems in
th
19 -century
physics
In a speech to the Royal Institution in 1900, Lord
Kelvin himself described two “dark clouds on the
horizon” of physics:
The question of the
existence of an electromagnetic medium—
referred to as “ether” or
“aether.”
The failure of classical
physics to explain
blackbody radiation.
"There is nothing new to be discovered in physics now. All that
remains is more and more precise measurement."
The Aether
Space is permeated by an invisible lumineferous aether
(light-bearing medium)
Medium through which light can propagate
The Earth must be moving relative to the aether
So light will travel faster or slower, depending on the
orientation
Differences can be determined by experiment
Test for the existence of the aether
The Michelson-Morley Experiment
Michelson-Morley experiment
Resultant Vector 4 km/hr east
Resultant Vector 2 km/hr west
Michelson-Morley experiment
Michelson and Morley
realized that the earth
could not always be
stationary with respect to
the aether.
And light would have a
different path length and
phase shift depending on
whether it propagated
parallel and anti-parallel
or perpendicular to the
aether.
Parallel and
anti-parallel
propagation
Mirror
Perpendicular
propagation
Beamsplitter
Mirror
Supposed velocity of
earth through the aether
Michelson’s and Morley’s set up
They folded the path to increase the total path of each arm.
The Michelson interferometer
should’ve revealed a fringe shift as
it was rotated with respect to the
aether velocity. MM expected 0.4
Their apparatus
periods of shift and could resolve 0.005
periods. They saw none!
Interference fringes showed no
change as the interferometer
was rotated.
Michelson and Morley's results
from A. A. Michelson, Studies in
Optics
Michelson’s Conclusion
 In several repeats
and refinements
with assistance
from Edward
Morley, he always
saw a null result.
 He concluded that
the hypothesis of
the stationary
aether must be
incorrect.
 Thus, aether
seems not to
exist!
Albert Michelson
(1852-1931)
Edward Morley
(1838-1923)
The Ultraviolet Catastrophe
 Lord Rayleigh used the classical theories of
electromagnetism and thermodynamics to
show that the blackbody spectrum should be:
Rayleigh-Jeans Formula
This worked at longer wavelengths but deviates badly at short ones.
This problem became known as the ultraviolet catastrophe and
was one of the many effects classical physics couldn’t explain.
Black-body
What is a black-body?
An object that absorbs all incident radiation, i.e. no reflection
A small hole cut into a cavity is the most
popular and realistic example.
⇒None of the incident radiation
escapes
What happens to this radiation?
•The radiation is absorbed in the walls of the cavity
•This causes a heating of the cavity walls
•Atoms in the walls of the cavity will vibrate at frequencies
characteristic of the temperature of the walls
•These atoms then re-radiate the energy at this new characteristic
frequency
The emitted "thermal" radiation characterizes the
equilibrium temperature of the black-body
Photons
Max Planck (1858-1947)
Max Planck in 1900
stated that the light
emitted by a hot
object (black body
radiation) is given off
in discrete units or
quanta. The higher
the frequency of the
light, the greater the
energy per quantum.
Photons
All the quanta associated with a particular
frequency of light have the same energy. The
equation is E = hν where E = energy, h = Planck's
constant (6.63 x 10-34 J • s), and ν = frequency.
Electrons can
have only certain
discrete energies,
not energies in
between.
The wave theory of light was widely
accepted until 1905……
Wave theory of
light? “No way!”
Albert Einstein
1879 - 1955
Photoelectric Effect (1905)
 Light as particles
 Einstein’s (1879-1955) explanation
 light as particles = photons
Light of frequency ν
Kinetic energy = hν - Ф
Electrons
Material with work function Ф
Photons
Einstein expanded Planck's
hypothesis by proposing that light
could travel through space as
quanta of energy called photons.
Einstein's equation for the
photoelectric effect is h ν = KE + Ф.
Although photons have no mass
and travel with the speed of light,
they have most of the other
properties of particles.
The higher the frequency (or shorter
the wavelength) the higher the
energy.
Albert Einstein
(1879-1955)
Einstein’s explanation
 Electrons are knocked free from the metal by incoming
photons, with each photon carrying an amount of energy E
that is related to the frequency (ν) of the light by
E=hν
Where h is Planck’s constant (6.62 x 10-34 J seconds).
 Only photons of high enough energy (above a threshold value)
can knock an electron free. e.g. blue light, but not red light,
has sufficient energy to free an electron from the metal.)
What is light?
Light exhibits either wave characteristics or particle
(photon) characteristics, but never both at the same
time. The wave theory of light and the quantum theory
of light are both needed to explain the nature of light
and therefore complement each other.
Albert Einstein provided a piece of convincing
evidence for the particle nature of light ……
Has the story ended yet?
Is light particles or waves?
Light is not particles,
not waves, but BOTH!
Louis de Broglie
1892 - 1987
 Louis de Broglie in 1924 proposed that particles also
have wave-like properties, this was confirmed
experimentally three years later.
 Most scientists did not understand de Broglie’s Ph.D.
dissertation at that time. One scientist passed it on to
Einstein for his interpretation. Einstein replied that de
Broglie did not just deserve a doctorate but a Nobel
Prize!
 De Broglie was awarded the
Nobel Prize in 1929.
What is light?
Aristotle - Light was emitted from our eyes
Christian Huygens - Wave theory of light
Sir Isaac Newton - Particle theory of light
Thomas Young - Wave theory of light
Albert Einstein - Particle theory of light
de Broglie - Wave-particle duality of all matter
What is light?
• Light is a form of electromagnetic energy –
detected through its effects, e.g. heating of
illuminated objects, conversion of light to current,
mechanical pressure (“Maxwell force”) etc.
• Light energy is conveyed through particles:
“photons”
– ballistic behavior, e.g. shadows
• Light energy is conveyed through waves
– wave behavior, e.g. interference, diffraction
• Quantum mechanics reconciles the two points of
view, through the “wave/particle duality”
assertion
Properties of Light
 The wave-particle duality.
Light as a wave.
A principle of quantum
mechanics which implies that
light (and, indeed, all other
subatomic particles)
sometimes act like a wave,
and sometimes act like a
particle, depending on the
experiment you are
performing.
E = E0 cos 2πνt
c = λν
E = amplitude of electric field (J)
ν = frequency (Hz)
λ = wavelength (m)
c = speed of light (2.998 x 108 m/s in vacuum)
Light is also viewed as particles or packets of energy - photons.
Energy of a photon: E
= hν
E = energy (J)
h = Planck' s constant ( 6.626 ×10 −34 J ⋅ s )
Also written as:
E=
hc
λ
= hcν~
ν~ =
1
= wavenumber
λ
One photon of visible light contains about 10-19 Joules
Plane Electromagnetic Waves
Photons
 Photons are stable, chargeless, massless
elementary particles that exist only at the speed c.
 Unlike ordinary objects, photons cannot be seen
directly; what is known of them comes from
observing the results of their being either created or
annihilated
 Photons begin and end on charged particles; most
often they are emitted from and absorbed by
electrons.
Light is Packets of Energy - Photons
Packet with
energy h ν
"No amount of experimentation can ever prove me right; a single
experiment can prove me wrong“ – Albert Einstein
Why can’t we see a light beam?
Unless the light beam is propagating right into your eye or is scattered
into it, you won’t see it. This is true for laser light and flashlights.
This is due to the facts that:
air is very sparse (N is relatively small),
air is also not a strong scatterer,
and the scattering is incoherent.
This eye sees almost no light.
This eye is blinded
(don’t try this at home…)
To photograph light beams in laser labs, you need to blow
some smoke into the beam…
Photons
"What is known of [photons] comes from observing the
results of their being created or annihilated."
Eugene Hecht
What is known of nearly everything comes
from observing the results of photons being
created or annihilated.
Prof. Rick Trebino Georgia Institute of Technology School of Physics Atlanta
Light is not particles,
not waves, but BOTH!
Louis de Broglie
1892 - 1987
“Light is, in short, the most refined
form of matter.”
Sources of light
When a charge moves nonuniformly, it radiates
Linearly accelerating charge
Synchrotron radiation -
light emitted by charged
particles deflected by a
magnetic field
Bremsstrahlung (Braking radiation) light emitted when charged particles
collide with other charged particles

B
Тhe majority of light in the universe
comes from molecular vibrations
emitting light.
Electrons vibrate in their motion around nuclei
High frequency: ~1014 - 1017 cycles per second.
Nuclei in molecules vibrate
with respect to each other
Intermediate frequency:
~1011 - 1013 cycles per second.
Nuclei in molecules rotate
Low frequency: ~109 - 1010 cycles per second.
The Emission of Light from Atoms
 Surely the most significant mechanism responsible for the
natural emission and absorption of radiant energyespecially of light - is the bound charge, electrons
confined within atoms.
 Excitation of the ground state
 De-excitation with emission of a photon
 Ground state ~ 10-8 seconds later
Atomic and molecular vibrations
correspond to excited energy levels
in quantum mechanics.
Energy levels are everything in quantum mechanics.
⇔
Energy
Excited level
∆E = hν
Ground level
The atom is vibrating
at frequency, ν.
The atom is at least partially in
an excited state.
Excited atoms emit photons
spontaneously.
When an atom in an excited state falls to a lower energy level, it
emits a photon of light.
Energy
Excited level
Ground level
Molecules typically remain excited for no longer than a few
nanoseconds. This is often also called fluorescence or, when it
takes longer, phosphorescence.
Photon absorption
Excited level
Energy
This is, of
course,
absorption.
Ground level
Absorption lines in an
otherwise continuous
light spectrum due to
a cold atomic gas in
front of a hot source.
Einstein showed that stimulated
emission can also occur.
Before
Spontaneous
emission
Absorption
Stimulated
emission
After
Bohr's Model of the Hydrogen Atom
Spontaneous and Stimulated Transitions