WAVE PARTICLE DUALITY EXPLAINED: WHAT’S THE NATURE OF LIGHT?
Nothing is black and white! How many times have you heard this sentence? Well,
that sentence is completely true and not only when talking about people’s relationships and emotions.
Even in science we have understood that sometimes reality can’t be reduced to just
one single model, but it’s necessary to deal with phenomena by using different approaches
for different cases. Although this might seem really strange and not intuitive,
that’s the method physicists use to deal with the wave-particle duality of light! What is that?
Around the end of nineteenth century and the beginning of the following one,
scientists discovered some weird behaviors of light, which was considered as a wave.
In some cases, the result of experiments suggested that radiation was responding to the tests like
a particle, and this was really unexpected. We’re talking about the wave particle duality of light.
How was it possible to understand this double nature of light?
What were the most important experiments that led to this discovery? These and other
questions are the main topic of this subject, so, if you are interested, stick with me and
prepare to an extraordinary journey in the wave particle duality of electromagnetic radiation.
We’re dealing with one of the most important revolutions in science,
that has brought to the development of Quantum Mechanics, actually the most precise model that we
have to describe the world, particularly at very small sizes like those of atoms and particles.
To understand this concept better we’ll proceed step by step considering how different
researchers have expressed themselves about the nature of light in the course of history.
Even if some theories were elaborated by great Greek thinkers like Democritus or Euclid,
the major contributions were made in 17th century. Firstly, in 1630, the famous René
Descartes popularized and accredited the wave description in his treatise on light, The World,
showing that the behavior of light could be re-created by modeling wave-like disturbances
in a universal medium called luminiferous aether. Some years later, a brilliant British physicist
developed and documented an opposite theory that described light as a particle.
Want to know its name? Isaac Newton. Starting in 1630, Sir Isaac studied
for three decades the behavior of this incredible thing called light which allows us
to see the entire world. The British scientist defined rigorously the effects of reflection
and refraction. He understood that the angle of incidence is the same of the angle of reflection,
which is practically the same behavior of a particle that hits a rigid wall.
By observing these phenomena, Newton came to the conclusion that light was
definitely considerable like a particle. He developed and championed his corpuscular
theory, arguing that the perfectly straight lines of reflection demonstrated light's
particle nature, only particles could travel in such straight lines. He explained refraction by
positing that particles of light accelerated laterally upon entering a denser medium.
Meanwhile other famous scientists like Christiaan Huygens, Augustin
Jean Fresnel and Robert Hooke elaborated the concept of wave by using mathematical tools
and they discovered that both reflection and refraction could be explained through this model:
it was the born of the wave theories of light. The different speed in different materials
could be easily derived as a consequence of the medium-dependent propagation of waves.
Around one century later, an experiment carried by Thomas Young in 1801 apparently ended the debate
regarding the wave particle duality of light: the double slit experiment. What was it about?
Let’s start with a more simplified version, called the “single slit experiment”. It’s based on firing
light through a slit that allows it to reach a screen. If light consisted strictly of ordinary
or classical particles, we would expect to see a pattern corresponding to the size and shape of the
slit. However, when this "single-slit experiment" is actually performed, the pattern on the screen
is a diffraction pattern in which the light is spread out. The smaller the slit, the greater the
angle of spread. What is diffraction? When a bunch of direct rays of light passes through an obstacle
its wave front becomes more and more curved as the size of the slit gets smaller. To be more precise,
the rays that pass into the slit interferes with each other generating a spherical wave.
It’s a very interesting phenomenon that we can even see even on sea waves.
So, this version of the experiment confirms the wavy nature of light.
What does it happen if we use two different slits of the same size? The pattern on the screen gets
more pronounced with a series of dark and light bands. How can we explain this result by using
the properties of waves? Well, we have two curved wavefronts similar to the one of the single slit,
but this time they interact with each other. We have a case of interference. In some regions this
interaction will be constructive amplifying the intensity of light, whereas in other it will be
destructive, the waves cancel with each other. As I was saying before this experiment made
scientists refuse Newton’s corpuscular ideas and ended the debate regarding
the wave particle duality of light. However, in the 19th century something
really incredible for Physics totally changed the perspective on this topic…
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Physicists observed that electric wires interacted with each other and slight changes in current
made magnetic compasses turn around. Briefly speaking they understood that electricity and
magnetism were two different aspects of one unique interaction: electromagnetism. Although the major
experiments and laws were elaborated by Faraday and Henry, the man who gave the major contribute
to the development of electromagnetic theory was the Scottish James Clerk Maxwell. He reunited
all the precedent discoveries into four equations named after him the “Maxwell’s equations”.
These laws showed that the two different phenomena can be united in one single theory. Moreover,
these equations can be reassembled to produce what is known as a wave equation both for the Electric
and Magnetic field. The result of Maxwell’s theory was the discovery of electromagnetic
radiation that travels at the speed of light in different medium and even in empty space
(differently from sound waves that need a medium to propagate). This fact was phenomenal as all
optics could be derived from that model, and it was another confirmation for the debate
regarding the wave particle duality of light. At this point of history physicists thought
that our entire world could be described just by using gravity and electromagnetism that
reproduced perfectly the experimental results, some scientists even discussed if humanity had
reached the end of physics research. But some important discrepancies in the description of
effects of metals totally revolutionized both the way of thinking about matter
and the opinions regarding the wave particle duality of electromagnetic radiation.
What are we talking about? Well, to go on with our journey we need to talk about stars. Yes,
it might be a bit surprising and confusing, but you’ll understand everything later.
Usually, astronomers analyze the emission of celestial bodies by studying the spectrum,
which is a density of the power radiated by the star in dependance to different frequencies.
More simply, the spectrum is a catalogue of the colors emitted by a star.
The Sun for example will have higher values of spectrum in the region of yellow and orange.
The spectrum changes together with temperature: the most predominant frequency of emission is
higher when the star is hotter. At the end of 19th century the main theoretical model used
to classically describe this phenomenon was named after Rayleigh, but it had a tremendous problem:
it predicted that for very high frequencies the emission was infinite. This topic was also known
as the “ultraviolet catastrophe” and it generated major concerns even in the public community. The
only way to reproduce the experimental results, that didn’t have these spike before the violet
region, was developed by the Deutsch Max Planck. He proposed that the energy of a particle in the
star was directly dependent to its frequency through a constant called “Planck constant”, as
opposed to Rayleigh’s model that was based on the dependence to temperature (energy was proportional
to temperature through the Boltzmann constant K). This mathematical adjustment was really strange
and not rigorous since it didn’t have any theoretical foundation and physical meaning.
However, these assumptions became crucial some years later.
To better describe a particular phenomenon called photoelectric effect in 1904
Einstein stated that light is quantized in small packets of energy called photons,
each one with an energy equal to the product of the Planck constant and the frequency (E=hf)
What is this effect about? Photoelectricity is a proper of some metals that emit electrons
if stimulated by electromagnetic radiations. The classical predictions regarding this phenomenon
were based on intensity: the more radiation on the metal, the more electrons could be emitted.
However, experiments showed a different pattern: the emission just depended on frequency;
under a specific value f 0 no electrons could be seen and above that value the relation
between frequency and number of electrons was linear (if you double the frequency the number
of the electrons emitted by the metal doubles). Einstein concluded that light could be treated as
a group of small packets with energy E=hf, called photons. They interact with electrons by following
the same laws of mechanical bodies. The energy of the photons is used in the collision partly
to free one electron (and this is E0=hf0) and partly to increase the velocity of the electron.
If a radiation is more intense it is composed by more photons, whereas if it has a higher frequency
this means that each photon has a big energy. Thanks to this explanation of the photoelectric
effect Einstein received the Nobel Prize for Physics in 1921.
And it doesn’t end here, with this discovery Einstein made evident that energy emitted through
electromagnetic radiation is quantized: it’s the beginning of quantum mechanics.
This exciting model made physicists go crazy as it revolutionized what they considered as a truth:
the wavy nature of light. Einstein’s discovery underlined that light can be treated also as a
particle. Further studies on the so-called Compton effect confirmed this hypothesis.
But at the end what is the solution to the wave particle duality of light?
As I was saying at the beginning, it’s not possible to state that light is
only a particle or a wave, it can be considered as both at the same time.
And there’s more: this is true not only for electromagnetic radiation.
Physicists have discovered that the same behaviour of light
is proper of all matter: the wave particle duality of matter.
In 1924 the famous physicist De Broglie tried to reverse the relation discovered by Einstein.
He proposed that all particles could be treated also as waves with a wavelength
equal to the Planck constant divided their momentum which is velocity per mass-- λ=h/p
This hypothesis was then proved by Davisson and Germer who realized a very important experiment.
They fired a beam of electrons against a nickel target and they observed that
the particles were scattered with the same diffraction pattern of light. Absolutely crazy!
But why does nature behave like that? To understand this, we need quantum mechanics.
In this extraordinary theory each particle is linked to a wave function. The square of this
function represents the density of probability of finding the particle on a particular point.
So, particles are not waves, but they behave following a wavy function that is linked to
the probability of observing them in space. This is also the concept under the uncertainty
principle of Heisenberg that states that the product of uncertainties in the position
and the momentum (which is linked to wavelength) must be higher than a constant value. If one of
these quantities is very precise the other is totally uncertain. And here is the wave particle
duality explained in a few words: when we consider an object as a particle, position is relatively
well defined, the wave is pulse-like and has a very ill-defined wavelength, and thus momentum.
And conversely, when we consider an object like a wave, momentum, and thus wavelength, is relatively
well defined, the wave looks long and sinusoidal, and therefore it has a very ill-defined position.
"This video ends here! Thanks for watching everyone! Did you find this topic exciting?
Are you amazed by wave particle duality of light? Did you already know the existence of wave
particle duality of electromagnetic radiation? Do you want to know more about wave-particle duality
of matter? Have you ever heard about diffraction before? Let me know in the comments below,
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