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In the beginning, it was proposed that light is composed of small particles travelling in space. This theory tried to explain light as particles travelling through a medium that filled the universe and was named aether.


The wave-particle duality is one of the most intriguing and fundamental aspects of quantum mechanics. This duality was first highlighted through experiments with light and electrons. At the beginning of the 19th Century light was considered to be a particle with a fixed energy that is called “quantum”; light was thought to have either properties of particle and wave.



Diffraction = the spreading of waves around obstacles. For example when a beam of light falls on the edge of an object, it won’t continue in a straight line but it will be bent by the contact.

Light’s diffraction could not be explained by the corpuscular theory. During light refraction, light particles enter a small gap and should pass through as a single beam. However, particles spread in a phenomenon known as diffraction, just as ocean waves pass through a bay.

However, the corpuscular theory of light being small objects was not able to explain all the properties of light, such as how waves reduced their speed and changed their direction when entering water if they were not travelling through the water. An important argument against the corpuscular theory was this inability to explain the diffraction of light.



Louis De Broglie

Albert Einstein

Thomas Young


In 1801, Thomas Young, with the double slit experiment, demonstrated the wave nature of light by observing a pattern of light interference when light passes through two narrow slits.

Thomas young's experiment




Einstein proposed that light was made up of small particles, called photons, and that its energy depended on its frequency. He thought that a photon was a quantum of light carrying an Energy defined as E =hv where h is Planck's constant, otherwise v is the frequency of the radiation.His concepts evolved in correlation with his research on the photoelectric effect (the term "photoelectric effect" refers to the emission of electrons from a metal when it is struck by photons of a sufficiently high frequency).


Although Young's experiment was not specifically influenced by Einstein's photon theory, his work on the dualistic nature of light laid the foundation for the modern understanding of the interaction between light and matter, underlining the fact that light can manifest itself both as a wave and as a particle, depending on the experimental context.

It was expected that brighter light would cause the electrons to jump more, but this did not happen. Only when the frequency of the light increased did the electrons jump from the metal plate. Einstein, therefore, proposed that it was the energy of a particle called quantum that hit the metal plate and that this was responsible for the expulsion of the electrons from the plate.

The hypothesis of Louis de Broglie

Light behaves like an electromagnetic wave. Indeed, we can observe this behavior in experiments of refraction or interference. However, there are cases in which light behaves as if it were made up of particles, which are photons, and are characterized by an energy that depends on the frequency of the wave. In analogy with this dual behavior of light, in 1924 De Broglie introduced the same type of behavior also for matter for charged particles, for example electrons.

So the electron particle will be associated with a wave that is a standing wave for the system. A standing wave is a wave, like the one you have for example in the string of a guitar, which is constrained at the ends and when plucked vibrates remaining fixed at the ends; so it is a wave that does not propagate in space but there is only a change in amplitude over time.

It can be considered that there is a wave behavior in which the characteristic of the wave associated with the particle is to have a frequency that is given by the energy of the particle, divided by Planck's constant. While the wavelength is nothing but Planck's constant divided by the momentum of the particle.What these equations mean in the case of an electron moving in an atom, the allowed trajectories for an electron are discrete, which corresponds to discrete energy levels, and these are the configurations corresponding to standing waves.

The experimental confirmation of De Broglie's hypothesis occurred in 1927, during an experiment carried out by Davisson-Germer in which was observed the diffraction of electrons from a metal foil.Indeed, the behavior of the wave of the particles can be observed by observing the peculiar behaviors of a wave (diffraction and interference).But, to observe them, we must see an interaction between wave and target, which presents obstacles, slits, or openings that are of the order of magnitude or smaller, of the wavelength of the radiation that we are studying.

All matter can be associated with a wave. In real-life objects, which have high mass, they cannot be understood as dual in nature because their wavelength tends to zero; if instead the momentum is small, the dual nature is manifested.According to de Broglie: nature is symmetrical. Light can be a wave or it can behave as a set of particles called photons, but also charged particles like electrons, matter can behave like a wave.

In 1927, Davisson and Germer fired electrons at reduced velocity against a crystalline nickel target. The dependence of the angle of incidence of the reflected electron was measured. This experiment confirmed de Broglie's hypothesis about the wave nature of particles and completed the hypothesis of wave-particle duality.

In physics, the Davisson-Germer experiment provided an important confirmation of de Broglie's hypothesis, namely that particles, such as electrons, could exhibit a behavior like a wave.


Davisson and Germer designed and built a vacuum instrument with the purpose of measuring the energies of electrons released from a metal surface. The electrons were accelerated by an electric potential and made to impinge on a nickel surface. The result showed that there was a peak in the intensity of emitted electrons, and this peak was evidence of the wave-like behavior of electrons.
Angelica Schiffo, Giulia Rodaro, Melissa Halili, Sofia Movio



https://www.dbcf.unisi.it/sites/st13/files/allegati/15-10-2015/lezione4_la_meccanica_quantistica.pdf https://it.m.wikipedia.org/wiki/Esperimento_di_Young https://www.britannica.com/science/diffraction