MATTER AND RADIATION

 

1. The structure of matter

The matter that constitutes the Universe is of the same nature everywhere, despite the variety of its shapes. It is composed of various types of atoms. The atom, in its turn, is composed by a nucleus of  protons (positive electrical charge) and neutrons (particles with no electrical charge), and by the electrons, with a negative electrical charge, that rotate around the nucleus, bound by the attraction exerted on them by the protons. In normal conditions, the atom is electrically neutral, in other words it contains an equal number of electrons and protons. It is this number that determines the type of atom, that is the chemical element and its physical and chemical properties. The total number of protons and neutrons in the nucleus varies from 1 to 260 approximately, for the most complex atoms.
The nucleus is very small, but it contains 99.9% of the mass of the whole atom, while the electrons rotate around it at a large distance. Therefore, the atom is practically "empty". Protons and neutrons are composed of even smaller particles, called quarks.
According to the modern physical theories, matter has symmetry, in other words, for every particle there is a corresponding antiparticle with opposite properties (for example, there are antielectrons, particles with the same mass and properties of the electrons, but with a positive electrical charge). The existence of antimatter has been demonstrated, although apparently it is present in the Universe in much smaller quantity than ordinary matter.

Much of the matter that composes the Universe is in the gaseous state (atomic or molecular), or in form of plasma. The plasma is the so-called "fourth state" of matter, besides the solid, the liquid and the gaseous state; it is like a gas, but it is composed of ions and electrons instead of neutral atoms. On the contrary, solids and liquids are relatively rare in the Cosmos; the rocky bodies like the planets or asteroids are only a small fraction of the mass of the Universe.

The particles of a gas have a thermal motion, in other words they move randomly colliding with each other millions of times per second, and the higher the temperature of the gas, the higher the speed of the collisions. More precisely, the temperature of the gas is proportional to the average speed of its particles, that is to their kinetic energy. (The kinetic energy of a body equals 1/2 m v 2  , where m is its mass and v its speed). The thermal energy of the gas is the sum of the kinetic energy of its particles.
The absolute zero in the temperature scale, O degrees Kelvin or -273.16 °C, corresponds to a state where the average kinetic energy of the gas particles is zero.

2. The radiation

The light that we see is nothing but electromagnetic radiation: it is a "means of transportation" of the energy from one point in space to another. The radiation consists of an electromagnetic wave, that is an oscillating electromagnetic field, that travels in a straight line, transporting energy. The radiation travels in the vacuum with a very high speed, but finite, equal to 299,792 Km per second; this value is indicated as c. Its speed is slightly lower when it propagates through a material medium, like a gas.
An electromagnetic wave is characterized, besides its speed, by its frequency (f) and its wavelength (L). The wavelength is the distance between two subsequent peaks of intensity of the field at a particular instant. The frequency is the number of wavelengths that travel in one second through a given point in space.
Among these quantities there is a relation:
f L = c
meaning that if we multiply the wavelength by the frequency we obtain a constant, which is equal to the speed of light: the higher the frequency of a wave, the smaller its wavelength, and vice versa.
The energy transported by an electromagnetic wave is proportional to its frequency.
The radiation present in the Cosmos has very different wavelengths. What we are able to see is only a very small part of the spectrum of the radiation, in other words of the interval of the existing wavelengths. Such spectrum can be divided into various regions:

radio waves (wavelengths between 102 and 106 cm)
microwaves (from 10-2  to 102 cm)
infrared (from 10-4 to 10-2 cm)
visible light (from 3 10-5 to 7 10-5 cm approximately)
ultraviolet (from 10-6 to 10-5 cm)
X-rays (from 10-8 to 10-6 cm)
gamma-rays (from 10-10 to 10-8 cm)

The unit of measurement for the wavelengths of visible light, ultraviolet light and X-rays is the Angstrom, 10-8 cm; that of the infrared rays is the micron, that is 10-4 cm. The unit of measurement of the frequency is the Hertz and its multiples.
 

The radiation, as already said, is an electromagnetic wave, and therefore it is subject to phenomena such as reflection, refraction, interference, etc... However, light does not always behave like a wave: the energy it carries is not diffused uniformly along the wave, but it travels in space as if it were concentrated in corpuscles, and in this form it is emitted and absorbed by the atoms. Like the particles, radiation exerts a pressure on the bodies it meets, and when an electromagnetic wave hits a particle, it transfers to it part of its energy, just like another particle would do. So, light has a dual nature: it is a wave and a particle at the same time, and it behaves in one way or the other according to the circumstances. These corpuscles are called photons, they have no mass or electric charge, and they move with the speed c.
The energy transported by a photon and the frequency of the corresponding electromagnetic wave come into the relation:

E = hf = hc/L

where h is a constant called Planck constant.

3. Emission and absorption of radiation by matter

As already said, light travels in space like a wave, but when it interacts with matter it behaves like a particle. This means that the energy transported by the radiation is exchanged by the atoms in form of "packets", the photons.
Within an atom, the electrons move around the nucleus in a region of space called orbital. The electron has a well-defined energy, that depends on the type of orbital in which it moves. If an electron moves from one orbital to the other, its energy varies. This mechanism takes place through the absorption or the emission of a photon. In order to move to an orbital with a higher energy, the electron must receive energy, and therefore absorb a photon. To move to an orbital with a lower energy, it must release some, in other words it must emit a photon, which, in turn, will be "caught" by another atom.
All types of atoms have only a certain number of orbitals, and therefore only particular levels of energy available for their electrons. So, an electron can absorb or emit only some quantities of energy, and not others, to move from one orbital to the other. In other words, an atom can emit or absorb only photons with definite energies, and therefore fixed wavelengths.
These photons are in a way the "signature" of an atom or a molecule: if you observe a radiation with a certain wavelength rather than another, you know you are in the presence of a certain chemical element, or its ion, or its isotope.