I can’t seem to get it. All I know is that light particles are called photons and that they simultaneously serve as both a wave and a particle. [?]

• David says:

It was the study of the external photoelectric effect, which paved the way for a quantum theory of light! Discoveries made from experiments, beginning in 1887, showed that the external photoelectric effect had certain properties that could not be explained by classical, continuum theories in which light and all other kinds of electromagnetic radiation were considered to behave like waves. In classical theories, as the light shining on a metal becomes increasingly intense, the wave theory of light suggests that the electrons that absorb the light will be liberated from the metal with more and more energy. However, these experiments demonstrated that the maximum possible energy of the ejected electrons depends only on the frequency of the incident light, and not on its intensity.

Thus, in 1905, Einstein suggested that light could be considered to behave like particles in some instances, and that the energy of each light particle, or photon, depends only on the frequency of the light. In accounting for the external photoelectric effect, he envisioned light as a collection of particles or quanta striking the metal. Hence, a free electron in the metal that is struck by a photon absorbs the photonâs energy. If the photon has acquired enough energy from the photon, it may escape from the metal. Furthermore, Einsteinâs theory explained many features of the external photoelectric effect, such as why the maximum energy of the ejected electrons is independent of the intensity of the incident light. According to his quantum theory of light particles or photons, the maximum energy of a dislodged electron depends only on the energy of the photon that ejects the electron, which in turn depends only on the lightâs frequency. Einsteinâs photoelectric equation expresses these ideas mathematically, as: –

E = hf = ĂÂ˝mevĂÂ˛ + Ăâ

where h is Planckâs constant, and f is the frequency of the incident light, with ĂÂ˝mevĂÂ˛ as the maximum kinetic energy of the photoelectron. The quantity Ăâ  represents the work function of the metal surface (irradiated by the photons) which is the energy required to completely remove an electron from the metal.

Einstein’s photoelectric effect explanation and quantised theory of photons has become an integral part of the wave / particle duality model of light. Other optical phenomena, such as diffraction, and interference require a wave nature explanation. In 1921, Einstein won the Nobel Prize for Physics for his explanation of the photoelectric effect. Three years later, Louis Victor Prince de Broglie used Einstein’s quantum theory of photons to show that all particles may have wave-like properties and, thus, paved the way towards Erwin SchrĂÂśdinger’s wave equation, published in 1926, and a full quantum theory known as quantum mechanics.

• Terry D says:

It is called QED, quantum electrodynamics. It is the relativistic quantum field theory that explains how photons mediate the force between charged particles and their anti-particles. It is based on Yang-Mills symmetry and was developed in particle form by Richard Feynman and in field form by Julian Schwinger.

• Tuomo says:

Everything is at the same time a particle and a wave.
This was realized by de Broglie in 1924.
It means that everything has both, particle like and wave like properties.
The duality of light in this matter was discovered before theory of de Broglie.

Quantum theory of light means just that light has particle like properties, which only depend on wavelength of the light, not the intensity of the light. Photoelectric effect is one example of this. In photoelectric effect the photons are absorbed by material which then releases electrons and thus creates current. If the wavelength of the light is not small enough the effect doesn’t happen as the photons don’t have enough energy to release electrons from material.

• Kes says:

Quantum means quantity or amount. Light travels (at the speed of light) as mass-less packets of pure energy. The amount of energy of the photon depends upon its wavelength. Infrared (long wavelengths) is relatively weak while ultraviolet (shorter wavelengths) is stronger and more penetrating (sunburns through thin clouds, etc.). X-rays have even shorter wavelengths and can penetrate skin and bone while gamma rays have still shorter wavelengths and can penetrate lead to a certain extent. Photons can interact with electrons orbiting atoms (and molecules) forcing electrons to higher more energetic orbits. When the electrons return to rest orbits new photons are emitted and the wavelength of the photon is determined by the excess energy given up by the electron. Every element has its own unique set of electrons and orbits and therefore the energy involved in the interaction of photons and its electrons can be used to identify the element. Helium was discovered within the light spectrum of the sun before it was discovered on earth (and is named for the sun, Helios).

• Queenie says:

yeah, the quantum theory of light says that light exists as tiny packets, or particles, which are called photons.

Just as light is thought to have a dual nature, sometimes showing the characteristic of a wave, and sometimes that of a particle (photon), quantum theory attributes a similar dual wave-particle nature to subatomic particles.

• Mario says:

E=mc^2