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Principle of absorption and emission atomic

La nature de la lumière

 Automatic translation  Automatic translation Updated October 13, 2013

Light is the only information that scientists have at their disposal to understand the world around us. Over the centuries, observing sunlight, scientists have made "talking" light.
Already in 1670, Isaac Newton decomposes white sunlight through a prism of glass and realize that the light beam is decomposed. He thinks that light is composed of particles. In 1676, Ole Christensen Rømer (1644-1681) determines the speed of light by observing the moons of Jupiter.
In 1690, Christian Huygens (1629-1695) states that light consists of a series of waves propagated through the ether, intangible substrate which supports the vacuum to convey light.
In 1801, Thomas Young (1773-1829) gets an interference pattern (image opposite), showing that light is a wave because only waves can add and subtract to produce interference (dark areas interspersed bright areas). This experience allows us to understand the behavior and the nature of light.
In 1814, Joseph von Fraunhofer (1787-1826) notes rays into visible light of the solar spectrum. This German optician and physicist was the first to study the diffraction of light by optical networks (Fraunhofer diffraction). At this time we do not know the reason for the presence of these lines in the visible spectrum of light. The answer will come much later.
In 1850, Robert Wilhelm Bunsen (1811-1899) and Gustav Robert Kirchhoff (1824-1887) discovered that the spectral lines of light emitted by an incandescent body, constitute a signature to identify the body. By observing the spectrum of sunlight, they recognize several chemical elements present on Earth which, cesium and rubidium.
In 1864, James Clerk Maxwell (1831-1879) make a synthesis of electrical and electromagnetic waves. He determines that the light is an electromagnetic wave and that the entire electromagnetic spectrum is light. What differentiates between them electromagnetic waves, is the wavelength. The various windows of the electromagnetic spectrum characterized by a range of wavelengths, but also by a range of frequencies.
In 1900, Max Planck solved the enigma of the black body, its formula perfectly describes the light emitted by a body according to its temperature. In other words, a high temperature indicates a high energy, low temperature indicates a low energy.


In 1905, Albert Einstein (1879-1955) explained the photoelectric effect, these are photons of the incident light who grub up electrons to matter. The photons act as quantum of energy that Planck had suggested, but it is Einstein who watch it. So these photons have a certain energy, these energy can extract the electrons of the metal. When we receive on our skin, bright rays of the sun, we feel much energy they carry. The light is therefore composed of photons with wave-like behavior and to each photon, corresponds an energy. More the wavelength of the photon is shorter and more it is energetic.
In 1911, Ernest Rutherford (1871-1937) specifies the structure of the atom and gives the atomic nucleus size of about 10-14 meters.
In 1913, Niels Bohr (1885-1962) proposed the structure of the hydrogen atom, the electrons are located in orbits quantified. The electron traveling at a certain distance on one of the layers of onion skin around the nucleus. This is the principle of absorption and emission of light in an atom (see next chapter).

NB: Photon energy E = hν = hc / λ.
E is the energy expressed in joules, h is Planck's constant (6.62 x 10-34), ν is the frequency (number of electromagnetic oscillations), c is the speed of light in vacuum and λ is the wavelength. The energy of a photon is infinitely small. In other words, more the the wavelength is short, more the frequency is high and more the energy is powerful.

Image: The nature of light falls within the quantum mechanics to which it is both, a wave and a particle. The light has a wavelength, which determines color, the red emits in the wavelength of 700 nanometers, the Violet 450 nm. It is in this small window that we see the world, but the invisible light is spread over a larger electromagnetic field.
Credit image:

 Double-slit experiment or Young's experiment

Image: Double-slit experiment or Young's experiment is a physics experiment that consists in interfere two beams of light from the same source. The light is passed through two small slits drilled in an opaque panel.

electromagnetic spectrum, absorption and emission

Principle of atomic absorption and emission


In 1913, Niels Bohr (1885-1962) proposed the structure of the hydrogen atom, the electrons are located on the quantized orbits (Bohr model). The outer electron sailing at a certain distance, on one of the layers of onion skin around the nucleus. Bohr shows that this electron can make some jumps, a quantified layer to another. For the electron located on a higher layer, jumps on the inner layer, it needs an energy equal to the energy difference between the two layers quantified. If energies involved are moderate, only the outer electrons of the electron cloud are concerned. A light beam is a wave which propagates as a set of particles, so called, photons. The amount of energy carried by the photon, is called quantum of energy. When the photon reaches the atom, the atom absorbs the photon and we will see an absorption line. The electron that has changed layer is not in a stable condition and will look to return to its original layer. When the electron returns to its original layer, a photon is emitted, the atom loses energy and we'll see an emission line. The electron is returned to its original layer and regained its ground state, transmitting energy that it had been receive, equal to the difference of energy between the two layers quantified. Displacement of electrons from one layer to another, will highlight the absorption and emission spectral lines.


Light, wave, particle, energy and matter are related to this phenomenon, atomic absorption and emission. The atom is the result of the interaction between a nucleus and electrons, in other words, it is a linked system of particles having energy. When there is energy input or loss of energy, matter reacts by absorbing or emitting a wavelength of light.

NB: In classical physics the atoms are made up of a certain number of punctual negatively charged electrons, and a point-like nucleus, positively charged, but this raises a paradox. In classical physics, the matter should disappear, annihilate because an electron, which radiates around a nucleus loses energy (Maxwell's theory) and therefore should fall on the nucleus. Which means that the stability of an atom is incomprehensible in the context of the classical theory. By cons, quantum physics explains the mystery of the atom and the stability of matter. Quantum physics has appeared between 1925 and 1927, it derives from the quantum physics initiated by Max Planck in 1900 and developed by Albert Einstein, Niels Bohr, Arnold Sommerfeld, Hendrik Anthony Kramers, Werner Heisenberg, Wolfgang Pauli and Louis de Brogli.e. between 1905 and 1924. This scientific and conceptual revolution which explains the existence of matter, is the basis of our understanding of the physical world.

 Absorption and emission of photons

Image: Principle of absorption and emission of a photon. If the energy involved is moderate, electronic transitions occur only on the outer layers of atoms. It correspond to the passage of an electron from the sub-layer unfilled in an unoccupied higher energy sub-layer (absorption) or the return of an electron in the valence sub-layer (emission). If the energy employed is sufficiently high (in very high frequencies), there is electron grubbing.

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