# Wave theory of light

Wave theory of light.

For other uses of this term, see Light (disambiguation).

Ray of sunlight scattered by dust particles in the Antelope Canyon in the United States. Light (from Latin lux, lucis) is the part of electromagnetic radiation that can be perceived by the human eye. In physics, the term light is used in a broader sense and includes the entire field of radiation known as the electromagnetic spectrum, while the term visible light specifically refers to radiation in the visible spectrum. Optics is the branch of physics that studies the behavior of light, its characteristics and its manifestations. The study of light reveals a series of characteristics and effects when interacting with matter, which allow the development of some theories about its nature. Finite speed It has been theoretically and experimentally shown that lightit has a finite speed. The first successful measurement was made by the Danish astronomer Ole Roemer in 1676 and since then numerous experiments have improved the precision with which the data is known. Currently the exact accepted value for the speed of light in a vacuum is 299,792,458 m / s.1 The speed of light when propagating through matter is less than through vacuum and depends on the dielectric properties of the medium and light energy. The relationship between the speed of light in a vacuum and in a medium is called the refractive index of the medium.

Refraction

This illustration shows the decomposition of light as it passes through a prism.

Refraction example. The straw seems broken, due to the refraction of light as it passes from the liquid to the air. Refraction is the abrupt change in direction that light undergoes when changing media. This phenomenon is due to the fact that light propagates at different speeds depending on the medium through which it travels. The change in direction is greater the greater the change in speed, since the light travels more distance in its displacement by the medium in which it goes faster. Snell’s law relates the change in angle to the change in velocity by means of the refractive indices of the media. Since refraction depends on the energy of light, when white or polychromatic light is passed through a non-parallel medium, such as a prism, the separation of light into its different components (colors) according to its energy occurs, in a phenomenon called refractive dispersion. If the medium is parallel, the light is recomposed when leaving it. Very common examples of refraction are the apparent break that is seen in a pencil when you put it in water or the rainbow.

Propagation and diffraction

Shadow of a marble. One of the most obvious properties of light with the naked eye is that it propagates in a straight line. We can see it, for example, in the propagation of a ray of light through dusty environments or saturated atmospheres. Geometric optics starts from this premise to predict the position of light, at a certain moment, throughout its transmission. From the propagation of light and its encounter with objects, shadows arise. If we interpose an opaque body in the path of light and then a screen, we will obtain on it the shadow of the body. If the source of the light or focus is far from the body, so that it is relatively smaller than the body, a defined shadow will be produced. If the focus is brought closer to the body, a shadow will appear in which a lighter region called penumbra and a darker region called umbra are distinguished. However, light does not always spread in a straight line. When light passes through a pointed obstacle or a narrow opening, the beam bends slightly. This phenomenon, called diffraction, is responsible for the fact that when looking through a very small hole everything is distorted or that telescopes and microscopes have a maximum number of magnifications.

Interference

The simplest way to study the phenomenon of interference is with the so-called Young’s experiment, which consists of making monochromatic (single-color) light fall on a screen that has a very narrow slit. The diffracted light coming out of said slit is re-incident on another screen with a double slit. The light from the two slits is combined into a third screen, producing light and dark alternative bands. The phenomenon of interference can also be seen naturally in oil stains on puddles or on the face with information from compact discs; both have a surface that, when illuminated with white light, diffracts it, producing a cancellation due to interferences, depending on the angle of incidence of the light, of each of the colors it contains, allowing you to see them apart, like a rainbow. Reflection and dispersion As light strikes a body, the matter of which it is made retains its energy for a moment and then re-emits it in all directions. This phenomenon is called reflection. However, on optically smooth surfaces, due to destructive interference, most of the radiation is lost, except that which propagates at the same angle as it struck. Simple examples of this effect are mirrors, polished metals, or river water (which has a dark bottom). Light is also reflected through the phenomenon called total internal reflection, which occurs when a ray of light tries to leave a medium in which its speed is slower to another faster, with a certain angle. Refraction is produced in such a way that it is not able to cross the surface between both media, fully reflecting itself. This reflection is responsible for the sparkles in a cut diamond. In a vacuum, the speed is the same for all wavelengths in the visible spectrum, but when material substances pass through, the speed decreases and varies for each of the different wavelengths of the spectrum, this effect is called dispersion. Thanks to this phenomenon we can see the colors of the rainbow. The blue color of the sky is due to sunlight scattered through the atmosphere. The white color of the clouds or that of milk is also due to the scattering of light by water droplets or by the suspended fat particles that they contain respectively. the speed is the same for all wavelengths of the visible spectrum, but when material substances pass through, the speed decreases and varies for each of the different wavelengths of the spectrum, this effect is called dispersion. Thanks to this phenomenon we can see the colors of the rainbow. The blue color of the sky is due to sunlight scattered through the atmosphere. The white color of clouds or milk is also due to the scattering of light by water droplets or by the suspended fat particles that they contain respectively. the speed is the same for all wavelengths of the visible spectrum, but when material substances pass through, the speed decreases and varies for each of the different wavelengths of the spectrum, this effect is called dispersion. Thanks to this phenomenon we can see the colors of the rainbow. The blue color of the sky is due to sunlight scattered through the atmosphere. The white color of clouds or milk is also due to the scattering of light by water droplets or by the suspended fat particles that they contain respectively. Thanks to this phenomenon we can see the colors of the rainbow. The blue color of the sky is due to sunlight scattered through the atmosphere. The white color of clouds or milk is also due to the scattering of light by water droplets or by the suspended fat particles that they contain respectively. Thanks to this phenomenon we can see the colors of the rainbow. The blue color of the sky is due to sunlight scattered through the atmosphere. The white color of clouds or milk is also due to the scattering of light by water droplets or by the suspended fat particles that they contain respectively.

Polarization

The phenomenon of polarization is observed in certain crystals that are individually transparent. However, if two are placed in series, parallel to each other and with one rotated a certain angle with respect to the other, the light cannot pass through them. If one of the crystals is rotated, the light begins to pass through them, reaching maximum intensity when the crystal has been rotated 90 ° sexagesimal with respect to the angle of total darkness. Polarized light can also be obtained through light reflection. The reflected light is partially or totally polarized depending on the angle of incidence. The angle that causes full polarization is called the Brewster angle. Many sunglasses and camera filters include polarizing lenses to eliminate annoying reflections.

Chemical effects

Some substances when absorbing light, undergo chemical changes; They use the energy that the light transfers to them to reach the energy levels necessary to react, to obtain a more adequate structural conformation to carry out a reaction or to break any link in its structure (photolysis). Photosynthesis in plants, which generate sugars from carbon dioxide, water and light; the synthesis of vitamin D in the skin; the breakdown of dihalogens with light in radical reactions or the vision process in the eye, produced by the isomerization of retinol with light, are examples of photochemical reactions. The area of ​​chemistry in charge of studying these phenomena is photochemistry.

Historical approach

== Isaac Newton ==. In the early eighteenth century it was widely believed that light was made up of small particles. Phenomena such as reflection, refraction, and body shadows could be expected from torrenting particles. Isaac Newton demonstrated that the refraction was caused by the change of speed of the light when changing means and tried to explain it saying that the particles increased their speed with increasing the density of the medium. The scientific community, aware of Newton’s prestige, accepted his corpuscular theory. In the gutter was the theory of Christian Huygens who in 1678 proposed that light was a wave phenomenon that was transmitted through a medium called ether. This theory was forgotten until the first half of the 19th century, when Thomas Young was only able to explain the phenomenon of interference by assuming that light was actually a wave. Other studies from the same period explained phenomena such as diffraction and polarization, taking wave theory into account. The final blow to the corpuscular theory seemed to come in 1848, when it was possible to measure the speed of light in different media and it was found that it varied completely opposite to what Newton had supposed. Because of this, almost all scientists accepted that light had a wave nature. However, there were still some points to explain such as the propagation of light through vacuum, since all the known waves moved using a physical medium, and the light traveled even faster than in air or water. This medium was supposed to be the ether Huygens was talking about, but no one could find it. James Clerk Maxwell. In 1845, Michael Faraday discovered that the angle of polarization of light could be changed by applying a magnetic field (Faraday effect), proposing two years later that light was a high-frequency electromagnetic vibration. James Clerk Maxwell, inspired by Faraday’s work, mathematically studied these electromagnetic waves and realized that they always propagated at a constant speed, which coincided with the speed of light, and that they did not need a propagation medium as they self-propagated. The experimental confirmation of Maxwell’s theories eliminated the last doubts that were held about the wave nature of light. However, at the end of the 19th century, new effects were found that could not be explained assuming that the light was a wave, such as, for example, the photoelectric effect, that is, the emission of electrons from the surfaces of solids and liquids when illuminated. The works on the process of absorption and emission of energy by matter could only be explained if one assumed that light was made up of particles. Then science reached a very complicated and uncomfortable point: many effects of light were known, however, some could only be explained if light was considered to be a wave, and others could only be explained if light was a particle. . The attempt to explain this wave-particle duality drove the development of physics during the 20th century. Other sciences, like biology or chemistry,

Nature of light

Light has a complex nature: depending on how we observe it, it will manifest as a wave or as a particle. These two states are not excluded, but are complementary (see corpuscle wave duality). However, to obtain a clear and concise study of its nature, we can classify the different phenomena in which it participates according to its theoretical interpretation:

Wave theory

Description This theory, developed by Christiaan Huygens, considers that light is an electromagnetic wave, consisting of an electric field that varies in time, generating in turn a magnetic field and vice versa, since variable electric fields generate magnetic fields (law of Ampère) and variable magnetic fields generate electric fields (Faraday’s law). In this way, the wave propagates itself indefinitely through space, with magnetic and electric fields being generated continuously. These electromagnetic waves are sinusoidal, with the electric and magnetic fields perpendicular to each other and with respect to the direction of propagation. In order to describe an electromagnetic wave we can use the usual parameters of any wave: Amplitude (A): It is the maximum length with respect to the equilibrium position that the wave reaches in its displacement. Period (T): It is the time necessary for two successive maximums or minimums to pass through a fixed point in space. Frequency (ν): Number of oscillations of the field per unit of time. It is an inverse quantity to the period. Wavelength (λ): It is the linear distance between two equivalent points of successive waves. Propagation speed (V): It is the distance the wave travels in a unit of time. In the case of the speed of propagation of light in a vacuum, it is represented by the letter c. The speed, frequency, period, and wavelength are related by the following equations: It is the time necessary for the passage of two successive maximums or minimums through a fixed point in space. Frequency (ν): Number of oscillations of the field per unit of time. It is an inverse quantity to the period. Wavelength (λ): It is the linear distance between two equivalent points of successive waves. Propagation speed (V): It is the distance the wave travels in a unit of time. In the case of the speed of propagation of light in a vacuum, it is represented by the letter c. The speed, frequency, period, and wavelength are related by the following equations: It is the time necessary for the passage of two successive maximums or minimums through a fixed point in space. Frequency (ν): Number of oscillations of the field per unit of time. It is an inverse quantity to the period. Wavelength (λ): It is the linear distance between two equivalent points of successive waves. Propagation speed (V): It is the distance the wave travels in a unit of time. In the case of the speed of propagation of light in a vacuum, it is represented by the letter c. The speed, frequency, period, and wavelength are related by the following equations: It is the distance the wave travels in a unit of time. In the case of the speed of propagation of light in a vacuum, it is represented by the letter c. The speed, frequency, period, and wavelength are related by the following equations: It is the distance the wave travels in a unit of time. In the case of the speed of propagation of light in a vacuum, it is represented by the letter c. The speed, frequency, period, and wavelength are related by the following equations:

Wave phenomena

Some of the most important light phenomena can be easily understood if it is considered to have a wave behavior. The principle of wave superposition allows us to explain the phenomenon of interference: if we join together two waves with the same wavelength and amplitude, if they are in phase (the wave crests coincide) they will form a constructive interference and the intensity of the resulting wave will be maximum and equal to twice the amplitude of the waves that make it up. If they are out of phase, there will be a point where the phase shift is maximum (the wave crest exactly coincides with a valley), forming a destructive interference, canceling the wave. Young’s experiment, with its slits, allows us to obtain two light bulbs of the same wavelength and amplitude, creating an interference pattern on a screen. The waves change their direction of propagation when crossing a pointed obstacle or passing through a narrow opening. As stated in the Fresnel – Huygens principle, each point of a wavefront is an emitter of a new wavefront that propagates in all directions. The sum of all the new wave fronts causes the disturbance to continue propagating in the original direction. However, if one or a few of the new wave emitters are separated by means of a slit or a pointed obstacle, the new direction of propagation will prevail over the original one. each point of a wavefront is an emitter of a new wavefront that propagates in all directions. The sum of all the new wave fronts causes the disturbance to continue propagating in the original direction. However, if one or a few of the new wave emitters are separated by means of a slit or a pointed obstacle, the new direction of propagation will prevail over the original one. each point of a wavefront is an emitter of a new wavefront that propagates in all directions. The sum of all the new wave fronts causes the disturbance to continue propagating in the original direction. However, if one or a few of the new wave emitters are separated by means of a slit or a pointed obstacle, the new direction of propagation will prevail over the original one.

Wave propagating through a slit.

Light diffraction is easily explained if this unique wave effect is taken into account. Refraction can also be explained using this principle, taking into account that the new wave fronts generated in the new medium will not transmit with the same speed as in the previous medium, generating a distortion in the propagation direction:

Refraction of light according to the Huygens principle.

Another phenomenon of light easily identifiable with its wave nature is polarization. Non-polarized light is made up of waves that vibrate at all angles, when reaching a polarizing medium, only waves that vibrate at a certain angle get through the medium, by putting another polarizer below, if the angle that the medium coincides with the vibration angle of the wave, the light will pass completely, if not only part will pass until reaching an angle of 90º between the two polarizers, where no light will pass.

Two polarizers in series.

This effect, in addition, allows demonstrating the transverse nature of light (its waves vibrate perpendicular to the direction of propagation). The Faraday effect and the calculation of the speed of light, c, from electrical (permittivity) and magnetic (permeability) constants by Maxwell’s theory: confirm that the waves of which light is composed are electromagnetic in nature. This theory was also able to eliminate the main objection to the wave theory of light, which was to find a way for the waves to move without a material medium.

Corpuscular theory

Description The corpuscular theory studies light as if it were a torrent of particles without charge and without mass called photons, capable of transporting all forms of electromagnetic radiation. This interpretation resurfaced because light, in its interactions with matter, exchanges energy only in discrete amounts (multiples of a minimum value) of energy called quanta. This fact is difficult to combine with the idea that the energy of light is emitted in the form of waves, but it is easily visualized in terms of light corpuscles or photons.

Corpuscular phenomena

Max Planck. There are three effects that demonstrate the corpuscular nature of light. According to the historical order, the first effect that could not be explained by the wave conception of light was the radiation of the black body. A black body is a theoretically perfect radiator that absorbs all the light that falls on it and therefore, when heated, it becomes an ideal emitter of thermal radiation, which allows the energy exchange process between radiation and matter to be clearly studied. The observed frequency distribution of the radiation emitted by the box at a given cavity temperature did not correspond to the theoretical predictions of classical physics. In order to explain it, Max Planck, at the beginning of the 20th century, postulated that to be correctly described, it had to be assumed that light of frequency ν is absorbed by integer multiples of a quantum of energy equal to hν, where h is a universal physical constant called the Planck constant. In 1905, Albert Einstein used Planck’s newly developed quantum theory to explain another phenomenon not understood by classical physics: the photoelectric effect. This effect is that when a monochromatic ray of electromagnetic radiation illuminates the surface of a solid (and sometimes that of a liquid), electrons are released in a phenomenon known as photoemission or external photoelectric effect. These electrons possess kinetic energy that can be measured electronically with a negatively charged collector connected to the emitting surface. It could not be understood that the emission of the so-called “photoelectrons” was immediate and independent of the intensity of the lightning. They were even capable of being fired at extremely low intensities, which excluded the possibility that the surface would somehow accumulate enough energy to fire the electrons. Furthermore, the number of electrons was proportional to the intensity of the incident beam. Einstein demonstrated that the photoelectric effect could be explained by assuming that the incident light was made up of photons of hν energy, part of this energy hν0 was used to break the forces that united the electron with matter, the rest of the energy appeared as energy kinetics of the emitted electrons: where m is the mass of the electron, vmax the maximum observed speed, ν is the frequency of the illuminating light and ν0 is the characteristic threshold frequency of the emitting solid. The final demonstration was provided by Arthur Compton, who observed how by making X-rays fall on light elements, these dispersed with less energy and also gave off electrons (a phenomenon later named in his honor as the Compton effect). Compton, using the previous theories, gave a satisfactory explanation of the problem by treating light as particles that collide elastically with the electrons like two billiard balls. The photon, a corpuscle of light, strikes the electron: the electron shoots out with part of the photon’s energy, and the photon reflects its lowest energy in its frequency. The relative directions in which they are both fired are in agreement with calculations using conservation of energy and momentum. Another phenomenon that corpuscular theory demonstrates is light pressure. These were dispersed with less energy and electrons were also detached (a phenomenon later named after him as the Compton effect). Compton, using the previous theories, gave a satisfactory explanation of the problem, treating light as particles that collide elastically with the electrons like two billiard balls. The photon, a corpuscle of light, strikes the electron: the electron shoots out with part of the photon’s energy, and the photon reflects its lowest energy in its frequency. The relative directions in which they are both fired are in agreement with calculations using conservation of energy and momentum. Another phenomenon that corpuscular theory demonstrates is light pressure. These were dispersed with less energy and electrons were also detached (a phenomenon later named after him as the Compton effect). Compton, using the previous theories, gave a satisfactory explanation of the problem, treating light as particles that collide elastically with the electrons like two billiard balls. The photon, a corpuscle of light, strikes the electron: the electron shoots out with part of the photon’s energy, and the photon reflects its lowest energy in its frequency. The relative directions in which they are both fired are in agreement with calculations using conservation of energy and momentum. Another phenomenon that corpuscular theory demonstrates is light pressure. He gave a satisfactory explanation to the problem by treating light as particles that elastically collide with electrons like two billiard balls. The photon, a corpuscle of light, strikes the electron: the electron shoots out with part of the photon’s energy, and the photon reflects its lowest energy in its frequency. The relative directions in which they are both fired are in agreement with calculations using conservation of energy and momentum. Another phenomenon that corpuscular theory demonstrates is light pressure. He gave a satisfactory explanation to the problem by treating light as particles that elastically collide with electrons like two billiard balls. The photon, a corpuscle of light, strikes the electron: the electron shoots out with part of the photon’s energy, and the photon reflects its lowest energy in its frequency. The relative directions in which they are both fired are in agreement with calculations using conservation of energy and momentum. Another phenomenon that corpuscular theory demonstrates is light pressure. the electron shoots out with a portion of the photon’s energy and the photon reflects its lowest energy in its frequency. The relative directions in which they are both fired are in agreement with calculations using conservation of energy and momentum. Another phenomenon that corpuscular theory demonstrates is light pressure. the electron shoots out with a portion of the photon’s energy and the photon reflects its lowest energy in its frequency. The relative directions in which they are both fired are in agreement with calculations using conservation of energy and momentum. Another phenomenon that corpuscular theory demonstrates is light pressure.

Quantum theories

Feynman diagram showing the exchange of a virtual photon (symbolized by a wavy line) between a positron and an electron. The need to reconcile Maxwell’s equations of the electromagnetic field, which describe the electromagnetic wave nature of light, with the corpuscular nature of photons, has led to the emergence of several theories that are still far from giving a satisfactory unified treatment. These theories incorporate, on the one hand, the theory of quantum electrodynamics, developed from the articles by Dirac, Jordan, Heisenberg and Pauli, and on the other hand, the quantum mechanics of de Broglie, Heisenberg and Schrödinger. Paul Dirac took the first step with his wave equation that provided a synthesis of the wave and corpuscular theories, since being an equation of electromagnetic waves, its solution required quantized waves, that is, particles. His equation consisted of rewriting Maxwell’s equations in such a way that they resembled the Hamiltonian equations of classical mechanics. Then, using the same formalism that, through the introduction of the quantum of action hν, transforms the equations of classical mechanics into equations of wave mechanics, Dirac obtained a new equation for the electromagnetic field. Solutions to this equation required quantized waves, subject to the Heisenberg uncertainty principle, the superposition of which represented the electromagnetic field. Thanks to this equation we can know a description of the probability of a given interaction or observation occurring in a given region.

Relativistic effects

However, there were still some situations in which light did not behave as expected by previous theories.

Moving light

The first of these inexplicable situations occurred when light was emitted, transmitted, or received by moving bodies or media. It was to be expected, according to classical physics, that the speed in these cases was the result of adding to the speed of light, the speed of the body or the medium. However, several cases were found in which it was not: Augustin Fresnel. In 1818 Augustin Fresnel proposed an experiment to measure the speed at which light passed through a moving liquid. To do this, a column of liquid would flow through the light, flowing at a speed v relative to the observer. Knowing the speed v ‘at which light is transmitted through that medium (through the refractive index), it was calculated that the total speed of light in that fluid would be: However, when in 1851, the French physicist Hippolyte Fizeau carried out the experiment, found that the speed at which light passed through the liquid in motion was not calculated but: that is, that the speed of the fluid counted less in the final speed if the speed with the that the light passed through that fluid was greater. In 1725, James Bradley discovered that the observed position of the stars in the sky varied annually from the actual position in an interval of 41 arcseconds. The theory he put forward to explain it was that this variation was due to the combination of the speed of the earth rotating around the sun with the finite speed of light. Thanks to this theory he was able to calculate the speed of light in an acceptable way. Based on this effect, English astronomer George Airy compared the angle of aberration in a telescope before and after filling it with water, and found that, contrary to his expectations, there was no difference in his measurements (the light did not vary in speed despite the fluid was moving at the speed of the earth). Taking this experiment into account, two astronomers, the German Albert Michelson and the American Edward Morley proposed an experiment (see Michelson and Morley’s Experiment) to measure the speed at which the ether flowed with respect to the earth. They assumed that the ether was moving in a specific direction with a certain speed, so due to the translation of the Earth around the Sun there would be times of the year in which we would have a component of that speed in favor and other times against, reason why they supposed that when we had it in favor, the speed of light would be higher and when we had it against it would be lower. To do this, they measured the speed of light in different seasons of the year and observed that there was no difference. And the most curious thing: that there were not even differences due to the Earth’s own translation speed (30 km / s). In 1905 Albert Einstein gave a satisfactory explanation with his theory of special relativity, in which, in his second postulate, he proposes that the speed of light is isotropic, that is, independent of the relative movement of the observer or the source. To do this, they measured the speed of light in different seasons of the year and observed that there was no difference. And the most curious thing: that there were not even differences due to the Earth’s own translation speed (30 km / s). In 1905 Albert Einstein gave a satisfactory explanation with his theory of special relativity, in which, in his second postulate, he proposes that the speed of light is isotropic, that is, independent of the relative movement of the observer or the source. To do this, they measured the speed of light in different seasons of the year and observed that there was no difference. And the most curious thing: that there were not even differences due to the Earth’s own translation speed (30 km / s). In 1905 Albert Einstein gave a satisfactory explanation with his theory of special relativity, in which, in his second postulate, he proposes that the speed of light is isotropic, that is, independent of the relative movement of the observer or the source.

Spectral distortions

Nebular displacement.

When comparing the spectrum of light from some celestial bodies, with the spectra measured in the laboratory of the same elements as those contained in those bodies, it is observed that they are not the same, since the spectral lines from space are displaced towards positions. of greater wavelength, that is, towards the red side of the spectrum in places of lower energy. Two different types of spectral line displacement have been found:

Nebular displacement

One, the most common, called nebular shift is a systematic shift of spectra from stars and galaxies. Edwin Hubble, after studying the spectral shift of nebulae, interpreted it as the result of the Doppler effect due to the continuous expansion of the universe. Thanks to this, he proposed a formula capable of calculating the distance that separates us from a given body by analyzing the shift of its spectrum: where Δλ is the difference between the wavelengths of the body’s spectrum and the expected one, λ is the expected wavelength and d, the distance in parsecs.

Gravitational displacement

The other, much stranger one is called gravitational displacement or the Einstein effect, observed in extremely dense body spectra. The most famous example is the spectrum of the so-called dark companion of Sirius. The existence of this companion was predicted by Friedrich Bessel in 1844 based on a disturbance he observed in Sirius’ motion, but due to its weak luminosity, it was not discovered until 1861. This companion is a white dwarf that has a mass comparable to that of from the Sun but in a radius approximately one hundred times smaller, so its density is immense (61,000 times that of water). When studying its spectrum, a displacement of 0.3 Å of the ß line of the Balmer series of hydrogen is observed.

Theory of general relativity

Albert Einstein. In order that his previous theory of special relativity would also include gravitational phenomena, Albert Einstein, between 1907 and 1915 developed the theory of general relativity. One of the main conclusions of this theory is that gravity influences the propagation of light, represented in the theory by the gravitational potential Φ, described by: where G is the Universal Gravitational Constant, M the mass and R the distance to object that generates the gravitational field. Einstein found that light, when passing through a gravitational field of potential ría suffered a decrease in its speed, according to the formula: where c0 is the speed of light without a gravitational field and c is the speed with it. The frequency of light emitted by a source in a field == gravitational == is also modified, which explains the gravitational displacement. Another example that experimentally confirms this point in the theory is the spectral lines of the sun, which are redshifted two millionth times when compared to those generated by the same elements on Earth. Finally, in this relationship between light and gravity, this theory predicted that light rays passing near a heavy body deviated by an angle α determined by the effect of its gravitational field, according to the relation: This point of the theory It could be confirmed experimentally by studying the deflection of light caused by the sun, for this reason the scientists studied the position of the stars in the area around the sun taking advantage of an eclipse in 1931.

Paul Dirac. In formulating his wave equation for a free electron, Paul Dirac predicted that it was possible to create a pair of electrons (one positively charged and one negatively charged) from an extremely fast-vibrating electromagnetic field. This theory was quickly confirmed by the experiments of Irene Curie and Frédéric Joliot and by those of James Chadwick, Stuart Blackett and Giuseppe Occhialini when comparing the number of negatively charged electrons and the number of positively charged electrons (the latter called positrons) dislodged by high-frequency γ-rays through thin sheets of lead and discovering that the same amount was obtained from some as from the others. Other ways to create positron-electron pairs were soon found, and a host of methods are known today: Crashing two heavy particles. Passing an electron through the field of an atomic nucleus. The direct collision of two electrons. The direct collision of two photons in a vacuum. The action of the field of an atomic nucleus on a ray γ emitted by the same nucleus. The process in the opposite direction also occurs: when an electron and a positron collide (they tend to join together, since they have opposite electrical charges), they annihilate each other converting all their mass into radiant energy. This radiation is emitted in the form of two γ-ray photons scattered in the same direction, but in a different direction. This relationship between matter-radiation, and vice versa (and especially the conservation of energy in this class of processes) is described in the famous Albert Einstein equation:

Unified field theories

Currently, a theory is sought that is able to explain in a unified way the relationship of light, as an electromagnetic field, with the rest of the fundamental interactions of nature. Early theories attempted to represent electromagnetism and gravitation as aspects of space-time geometry, and although there is some experimental evidence for a connection between electromagnetism and gravitation, only speculative theories have been provided.

Electromagnetic spectrum

The electromagnetic spectrum is made up of all the possible energy levels that light can have. Talking about energy is equivalent to talking about wavelength; Thus, the electromagnetic spectrum also covers all wavelengths that light can have, from thousands of kilometers to femtometers. That is why most schematic representations of the spectrum tend to be logarithmic in scale.

The electromagnetic spectrum is divided into spectral regions, classified according to the methods necessary to generate and detect the various types of radiation. That is why these regions do not have defined limits and there are some overlaps between them.

Visible spectrum

Of the entire spectrum, the portion that the human being is able to see is very small compared to the other spectral regions. Called the visible spectrum, this region comprises wavelengths from 380 nm to 780 nm. The human eye perceives the light of each of these wavelengths as a different color, therefore, in the decomposition of white light in all its wavelengths, by prisms or by rain in the rainbow, the eye sees all colors.