THE VISION OF LIGHT -Wave particle-duality of radiation and
matter�INTRODUCTION Since the beginning
of the 19th century, the industrial revolution had
produced great technological innovation and engineering progress.
Thanks to new equipment, more accurate experiments were performed
and new branches of physics were born: spectroscopy, diffraction,
interference, and so on. All this remarkable scientific development
paved the way for the observation of many unknown phenomena which
physics was unable to describe and thus, gradually, classic physics
entered into crisis. In fact, physics is an experimental science: a single
experiment may be sufficient to show the inadequacy of a model,
hence, every theory has ever to be consistent with observations.
Since the second half of '800, both e.m. radiation and matter had little
by little revealed their so-called WAVE-PARTICLE duality:
1) Radiation, which up to that time had been deemed to have a wave�like behavior only, started
to reveal a particle-like behavior as well. 2) Symmetrically, matter, always considered
particle-like, started to show also a wave-like behavior.
The overcoming of the crisis led to the birth of quantum mechanics,
which replaced classic physics in the insight of the world, in model
formulation, in the definition of physical quantities.
But let's start our journey from the beginning.
Since Newton's time, many optic experiments had been carrying out,
trying to investigate the nature of light. At that time, there were two
opposite factions: the proponents of a corpuscular theory of light, like
Newton, Gassendi, and the proponents of a wave theory of light, as
Huygens, Hooke, and Fresnel. It is said that spectroscopy
was born in 1672, by the famous Newton's prism experiment: he sent a beam of white
light through a glass prism and observed the dispersion
of light into rainbow colors. Newton concluded that light was
particle-like, made up of tiny colored corpuscles which blend to compose white
light. He also introduces the term "spectrum of colors" and
divided it into 7 colors: red, orange, yellow, green, blue, indigo, violet.
In 1678, the Dutch mathematician Christiaan Huygens, refused
Newton's corpuscular theory, mathematically demonstrating the laws of
reflection and refraction with a wave-like model of light.
On the one hand, the corpuscular theory could justify reflection and
refraction (even if only assuming that light accelerated upon entering a
denser medium because of the greater gravitational pull), but on the
other hand, explained the diffraction phenomenon (which had been first
observed by Francesco Grimaldi) by allowing that a light particle could
create a localized wave in the "aether", a hypothetical medium having
zero density , filling the void. Instead wave-like theory necessarily
assumed the presence of aether, that oscillating, would make the
propagation of the wave possible. In 1845, English physicist
Michael Faraday discovered the so-called magneto-optic Faraday effect, a particular
interaction between light and a magnetic field in a medium.
This was significant because represented the first experimental evidence that light
and electromagnetism are related.
5 years later, French physicist Léon Foucault showed that light travels
slower through water than through air in contrast with Newton's
assumption, hence, Newton's corpuscular light theory was abandoned.
Moreover, Faraday's work inspired James Clerk Maxwell to study
radiation and light. The father of electromagnetism, unified electric and
magnetic phenomena in a single field deriving 4 equations for the e.m.
field. He went beyond: trying to solve them he found an
electromagnetic wave equation: a self-propagating solution of the
electromagnetic field that would have been travelled through space at a
constant speed consistent with the previously measured speed of light,
performed by Foucault in 1862. Maxwell thus concluded that light itself
was a form of e.m. wave. Between 1886 and 1889,
the German physicist Heinrich Hertz conducted a series of experiments
which confirmed Maxwell's theory by generating and detecting radio waves
in the laboratory, and demonstrating that these
waves behaved exactly like visible light, exhibiting properties such as reflection,
refraction, diffraction, and interference.
So, the issue on the nature of light, seemed to be solved: as a matter of
fact, wave-like theory was satisfactory for the major part of optic
phenomena. However, some unsolved questions
were still remaining: unexplainable phenomena had been observed and
there was no theory able to justify them: thermodynamic couldn't explain
the specific heat in solids at low temperature, while spectroscopy
couldn't justify the spectral lines in phenomena of emission and absorption of light.
All these facts originally were merely detected and recorded, since a
theory able to describe them haven't been developed yet, but it was the
sign of somewhat unclear in the comprehension of the nature.
Spectroscopy deals with emittion and absorption Spectra. Spectra are
patterns that sum up how the radiation emitted or absorbed by a body is
distributed in terms of wavelength. Dark or bright line on a uniform and
continuous spectrum were observed and little by little was it realized
that every substance had its own characteristic spectral lines that were,
someway, its fingerprint. In 1860, Kirchhoff
and Bunsen suggested a first chemical classification of
some substances based on their spectral analysis: this was the birth of modern
spectroscopy which contributed to the development of quantum physics.
In particular, in order to study the radiation emitted and absorbed by
objects at a certain temperature the so-called "black body" spectrum
model was widely used. A black body is an ideal object which can
absorb and reissue all incident electromagnetic radiation. Up to 1900,
all attempts to modeling black body radiation had failed and led to
absurd predictions. It happened, for example, that every object, even at
very low temperature, could emit an infinite amount of e.m. energy. This
prediction, clearly false, as experience suggests, was called "ultraviolet
catastrophe" by physicist Paul Ehrenfest. In 1900, in order to make the math work,
the physicist Max Planck suggested a new calculation
method and derived an equation that perfectly reproduces black body spectrum data.
In order to justify it, he introduced Planck's hypothesis: e.m. radiation is
absorbed and emitted in a discontinuous way, through discrete amounts.
These tiny packages have corpuscular behavior and a few years later
will be named "QUANTA" by Albert Einstein. This is an important moment:
the concept of quantized energy is introduced for the first time. This
is the dawn of quantum physics. EINSTEIN AND THE PHOTOELECTRIC EFFECT
5 years later, Einstein took up Planck's hypothesis in order to explain the
photoelectric effect. In general, this indicates
the emission of electrons by a conductor or a metal, triggered by an incident e.m. radiation.
This effect has been first detected by Heinrich Hertz
in the same framework of the discovery of the e.m waves and then has been studied
by many scientists since the end of '800. Afterward, german
physicist Lenard demonstrated that the radiation incident on the metal
surface triggered the emission of electrons, whose energy not depended
on the intensity of the radiation, but on its frequency only.
Einstein, not only provided a satisfactory explanation of the phenomena,
he went beyond: he not only suggested that energy is exchanged from
radiation to matter through discrete quanta, but also that is e.m.
radiation itself to be made up of quanta, discrete amounts of energy,
afterward named "PHOTONS" by the quantum field theory. In other
words, it's just e.m. radiation to be quantized: energy is not evenly
distributed on the whole amplitude of the wave, but it's located in tiny
packages of "energetic density". Curiously Einstein was awarded the
Nobel Prize for photoelectric effect and not for relativity theory!
COMPTON'S EFFECT The particle-like behavior of
radiation was definitely confirmed in 1923, by Arthur Compton's experimental results.
The American physicist sent an X-RAY beam against a
graphite target and observed an X-ray scattering with different angles.
The wider was the scattering angle, the longer was the wavelength of the x-rays detected.
This was understandable only by
describing X-rays as a photon beam with a corpuscular behavior. Therefore Compton,
following Einstein's suggestion, described the
phenomenon as a photon beam, deflected because of the elastic scattering with
the electronic lattice of the graphite. His results confirmed
unquestionably Einstein's hypothesis and interaction between photons and
electrons occurs one by one. In 1927, Arthur Compton was
awarded the Nobel Prize. DE BROGLIE HYPOTHESIS
In 1924, french physicist Louis De Broglie, in front of the evidence of
particle-like behavior of radiation, suggested symmetrical behavior for
the matter. The reasoning was about the following: since we can relate e.m. waves to particles having
a specific amount of energy, why can't we
associate "material waves" with a specific frequency and wavelength to massive particles?
He was right because the evidence of wave-particle duality of the matter
was shown in 1927 by Davisson and Germer's experiment. The 2
physicists, in fact, observed a diffraction pattern by sending an electron
beam against a surface of a crystal of nickel metal. A diffraction pattern
is typical of wave-like phenomena, hence, De Broglie's insight was
confirmed. In conclusion, both in the
case of radiation and in the case of a massive particle, some phenomena can be justified with
the notion of the wave only, and some others
with the notion of the particle only. The interesting aspect is that you can't
reveal both behaviors at the same time, in the same experiment:
as soon as one aspect is detected, the other is no longer observable at the same time.
This concept is called complementary principle and was first formulated
by Danish physicist Niels Bohr: particle-like behavior and wave-like
behavior are complementary and the features of radiation and of the
matter need both aspects to be explained in a complete way.
The experimental evidence of this fact is shown in the so-called double�slit experiment.
In the basic version of this experiment, a coherent light source, such as a
laser beam, illuminates a plate pierced by two parallel slits, and the
radiation passing through the slits is observed on a photographic film on
a screen behind the plate. We reduce the intensity of the
radiation so that in the corpuscular vision we are sure to send photons one by one
against the slits. On the screen behind the plate, we observe an
interference pattern as if every single photon behaves wave-like, crosses both slits,
interfering with itself. The same experiment performed
replacing photons with electrons (thus using matter instead of radiation) gave
the same typical interference pattern.
At this point, physicists decided to watch the path of the photons.
They put a measuring device in front of the 2 slits in order to detect from
which of the two slits the photon actually passed. As if weren't strange
enough, each detected photons passed effectively through only one slit
(as would a classical particle), and not through both slits(as would a
wave) but the interference pattern disappeared. It's like the photon
decided to act differently as if it was aware of being watched.
The experiment has been performed in many variants, using both
particles and radiation, electrons and photons,
and has given always the same results.
These results demonstrate the principle of wave-particle duality.
In conclusion, what is the real essence of nature?
what is radiation? Is it a particle or is it a wave?
And matter? Quantum mechanics world is
the world of infinitely small objects: we cannot perceive them, we can only deduce
their properties depending on which experiment we do.
What we must keep in mind is that whenever we perform a quantum
experiment, meaning that we can't neglect or ignore quantum effects,
we are dealing neither with particles nor with waves. They are what they
are: quantum objects! Whether they are matter or radiation, we cannot
deduce their features a-priori: we must do the experiment and see what
happens! Wondering "what is reality
made of", or " what is the real essence of nature" are not physical issues. These
are matters related to philosophy and in particular with ontology.
There is a phrase often attributed to Richard Feynman, but coined by the
lesser-known American physicist David Mermin, which gives a good
idea of the pragmatic point of view of many physicists:
"Shut up and calculate!".