The electromagnetic nature of light is particle-wave dualism. What is wave-particle duality? Double slit experiment

Wave-particle duality of light means that light simultaneously has the properties of continuous electromagnetic waves and the properties of discrete photons. This fundamental conclusion was made by physicists in the 20th century and followed from previous ideas about light. Newton believed that light was a stream of corpuscles, that is, a stream of particles of matter flying in a straight line. This theory explained well the rectilinear propagation of light. But difficulties arose in explaining the laws of reflection and refraction, and the phenomena of diffraction and interference could not be explained at all by the corpuscular theory. Therefore, the wave theory of light arose. This theory explained diffraction and interference, but had difficulty explaining straight light. Only in the 19th century, J. Fresnel, using the discoveries of other physicists, was able to combine the already derived principles into one theory, according to which light is a transverse mechanical wave. Later, Maxwell discovered that light is a type of electromagnetic radiation. But at the beginning of the 20th century, thanks to Einstein’s discoveries, ideas about light changed again. Light came to be understood as a stream of photons. But certain properties of light were perfectly explained by the wave theory. Light has both corpuscular and wave properties. In this case, the following regularities exist: the shorter the wavelength, the brighter the corpuscular properties appear; the longer the wavelength, the brighter the wave properties appear.

According to de Broglie, each microobject is associated, on the one hand, with corpuscular characteristics - energy E and momentum p, and on the other hand, with wave characteristics - frequency and wavelength.

In 1924, the French physicist L. de Broglie put forward a bold hypothesis: wave-particle duality has a universal character, i.e. all particles having a finite momentum P have wave properties. This is how the famous de Broglie formula appeared in physics where m is the mass of the particle, V is its speed, h is Planck’s constant.

So, the corpuscular and wave properties of a micro-object are incompatible with respect to their simultaneous manifestation, however, they equally characterize the object, i.e. complement each other. This idea was expressed by N. Bohr and he formed the basis of the most important methodological principle of modern science, which currently covers not only the physical sciences, but also all of natural science - principle of complementarity (1927). The essence The principle of complementarity according to N. Bohr comes down to the following: no matter how far the phenomena go beyond the classical physical explanation, all experimental data must be described using classical concepts. To fully describe quantum mechanical phenomena, it is necessary to use two mutually exclusive (additional) sets of classical concepts, the combination of which provides the most complete information about these phenomena as a whole.

The principle of complementarity, as a general principle of knowledge, can be formulated as follows: every true natural phenomenon cannot be defined unambiguously using the words of our language and requires for its definition at least two mutually exclusive additional concepts. Such phenomena include, for example, quantum phenomena, life, psyche, etc. Bohr, in particular, saw the need to apply the principle of complementarity in biology, which is due to the extremely complex structure and functions of living organisms, which provide them with almost inexhaustible hidden capabilities.

Over the past hundred years, science has made great strides in studying the structure of our world at both the microscopic and macroscopic levels. The amazing discoveries brought to us by the special and general theories of relativity and quantum mechanics still excite the minds of the public. However, any educated person needs to understand at least the basics of modern scientific achievements. One of the most impressive and important points is wave-particle duality. This is a paradoxical discovery, the understanding of which is beyond the reach of intuitive everyday perception.

Corpuscles and waves

Dualism was first discovered in the study of light, which behaved completely differently depending on conditions. On the one hand, it turned out that light is an optical electromagnetic wave. On the other hand, there is a discrete particle (the chemical action of light). Initially, scientists believed that these two ideas were mutually exclusive. However, numerous experiments have shown that this is not the case. Gradually, the reality of such a concept as wave-particle duality became commonplace. This concept provides the basis for studying the behavior of complex quantum objects that are neither waves nor particles, but only acquire the properties of the latter or the former depending on certain conditions.

Double slit experiment

Photon diffraction is a clear demonstration of dualism. The detector of charged particles is a photographic plate or a fluorescent screen. Each individual photon was marked by illumination or a spot flash. The combination of such marks gave an interference pattern - alternation of weakly and strongly illuminated stripes, which is a characteristic of wave diffraction. This is explained by such a concept as wave-particle duality. The famous physicist and Nobel laureate Richard Feynman said that matter behaves on small scales in such a way that it is impossible to feel the “naturalness” of quantum behavior.

Universal dualism

However, this experience is valid not only for photons. It turned out that dualism is a property of all matter, and it is universal. Heisenberg argued that matter exists in both forms alternately. Today it has been absolutely proven that both properties appear completely simultaneously.

Corpuscular wave

How can we explain this behavior of matter? The wave that is inherent in corpuscles (particles) is called the de Broglie wave, named after the young aristocratic scientist who proposed a solution to this problem. It is generally accepted that de Broglie's equations describe a wave function, which, squared, determines only the probability that a particle is at different points in space at different times. Simply put, the de Broglie wave is a probability. Thus, equality was established between the mathematical concept (probability) and the real process.

Quantum field

What are corpuscles of matter? By and large, these are quanta of wave fields. A photon is a quantum of an electromagnetic field, a positron and an electron are an electron-positron field, a meson is a quantum of a meson field, and so on. The interaction between wave fields is explained by the exchange of certain intermediate particles between them, for example, during electromagnetic interaction there is an exchange of photons. From this directly follows another confirmation that the wave processes described by de Broglie are absolutely real physical phenomena. And particle-wave dualism does not act as a “mysterious hidden property” that characterizes the ability of particles to “reincarnate.” It clearly demonstrates two interrelated actions - the movement of an object and the wave process associated with it.

Tunnel effect

The wave-particle duality of light is associated with many other interesting phenomena. The direction of action of the de Broglie wave appears during the so-called tunnel effect, that is, when photons penetrate through the energy barrier. This phenomenon is caused by the particle momentum exceeding the average value at the moment of the wave antinode. Tunneling has made it possible to develop many electronic devices.

Interference of light quanta

Modern science speaks about the interference of photons in the same mysterious way as about the interference of electrons. It turns out that a photon, which is an indivisible particle, can simultaneously pass along any path open to itself and interfere with itself. If we take into account that the wave-particle duality of the properties of matter and the photon is a wave that covers many structural elements, then its divisibility is not excluded. This contradicts previous views of the particle as an elementary indivisible formation. Possessing a certain mass of movement, the photon forms a longitudinal wave associated with this movement, which precedes the particle itself, since the speed of the longitudinal wave is greater than that of the transverse electromagnetic wave. Therefore, there are two explanations for the interference of a photon with itself: the particle is split into two components, which interfere with each other; The photon wave travels along two paths and forms an interference pattern. It was experimentally discovered that an interference pattern is also created when single charged particles-photons are passed through the interferometer in turn. This confirms the thesis that each individual photon interferes with itself. This is especially clearly seen when taking into account the fact that light (neither coherent nor monochromatic) is a collection of photons that are emitted by atoms in interconnected and random processes.

What is light?

A light wave is an electromagnetic non-localized field that is distributed throughout space. The electromagnetic field of a wave has a volumetric energy density that is proportional to the square of the amplitude. This means that the energy density can change by any amount, that is, it is continuous. On the one hand, light is a stream of quanta and photons (corpuscles), which, thanks to the universality of such a phenomenon as particle-wave duality, represent the properties of an electromagnetic wave. For example, in the phenomena of interference and diffraction and scales, light clearly exhibits the characteristics of a wave. For example, a single photon, as described above, passing through a double slit creates an interference pattern. With the help of experiments, it was proven that a single photon is not an electromagnetic pulse. It cannot be divided into beams with beam splitters, as the French physicists Aspe, Roger and Grangier showed.

Light also has corpuscular properties, which manifest themselves in the Compton effect and the photoelectric effect. A photon can behave like a particle that is absorbed entirely by objects whose dimensions are much smaller than its wavelength (for example, an atomic nucleus). In some cases, photons can generally be considered point objects. It makes no difference from what position we consider the properties of light. In the field of color vision, a stream of light can act as both a wave and a particle-photon as an energy quantum. A spot focused on a retinal photoreceptor, such as the cone membrane, can allow the eye to form its own filtered value as the main spectral rays of light and sort them into wavelengths. According to the quantum energy values, in the brain the object point will be translated into a sensation of color (focused optical image).

If you thought that we had sunk into oblivion with our mind-blowing topics, then we hasten to disappoint you and make you happy: you were mistaken! In fact, all this time we have been trying to find an acceptable method of presenting crazy topics related to quantum paradoxes. We wrote several drafts, but they were all thrown out into the cold. Because when it comes to explaining quantum jokes, we ourselves get confused and admit that we don’t understand a lot (and in general, few people understand this matter, including the world’s cool scientists). Alas, the quantum world is so alien to the philistine worldview that it is not at all a shame to admit your misunderstanding and try a little together to understand at least the basics.

And although, as usual, we will try to talk as clearly as possible with images from Google, the inexperienced reader will need some initial preparation, so we recommend that you look through our previous topics, especially about quanta and matter.
Especially for humanists and other interested people - quantum paradoxes. Part 1.

In this topic we will talk about the most common mystery of the quantum world - wave-particle duality. When we say “the most ordinary,” we mean that physicists have become so tired of it that it doesn’t even seem like a mystery. But this is all because other quantum paradoxes are even more difficult for the average mind to accept.

And it was like this. In the good old days, somewhere in the middle of the 17th century, Newton and Huygens disagreed about the existence of light: Newton shamelessly declared that light is a stream of particles, and old Huygens tried to prove that light is a wave. But Newton was more authoritative, so his statement about the nature of light was accepted as true, and Huygens was laughed at. And for two hundred years light was considered a stream of some unknown particles, the nature of which they hoped to one day discover.

At the beginning of the 19th century, an orientalist named Thomas Young dabbled with optical instruments - as a result, he took and carried out an experiment that is now called Young's experiment, and every physicist considers this experiment sacred.

Thomas Young just directed a beam (of the same color, so that the frequency was approximately the same) of light through two slits in the plate, and placed another screen plate behind it. And showed the result to his colleagues. If light were a stream of particles, then we would see two light stripes in the background.
But, unfortunately for the entire scientific world, a series of dark and light stripes appeared on the plate screen. A common phenomenon called interference is the superposition of two (or more waves) on top of each other.

By the way, it is thanks to interference that we observe rainbow tints on an oil stain or on a soap bubble.

In other words, Thomas Young experimentally proved that light is waves. The scientific world did not want to believe Jung for a long time, and at one time he was so criticized that he even abandoned his ideas of the wave theory. But confidence in their rightness still won, and scientists began to consider light as a wave. True, a wave of what - it was a mystery.
Here, in the picture, is the good old Jung experiment.

It must be said that the wave nature of light did not greatly influence classical physics. Scientists rewrote the formulas and began to believe that soon the whole world would fall at their feet under a single universal formula for everything.
But you already guessed that Einstein, as always, ruined everything. The trouble crept up from the other side - at first scientists got confused in calculating the energy of thermal waves and discovered the concept of quanta (be sure to read about this in our corresponding topic ""). And then, with the help of these same quanta, Einstein struck a blow at physics, explaining the phenomenon of the photoelectric effect.

Briefly: the photoelectric effect (one of the consequences of which is film exposure) is the knocking out of electrons from the surface of certain materials by light. Technically, this knocking out occurs as if light were a particle. Einstein called a particle of light a quantum of light, and later it was given a name - photon.

In 1920, the amazing Compton effect was added to the anti-wave theory of light: when an electron is bombarded with photons, the photon bounces off the electron with a loss of energy (we “shoot” in blue, but the red one flies off), like a billiard ball from another. Compton won the Nobel Prize for this.

This time, physicists were wary of simply abandoning the wave nature of light, but instead thought hard. Science is faced with a terrifying mystery: is light a wave or a particle?

Light, like any wave, has a frequency - and this is easy to check. We see different colors because each color is simply a different frequency of an electromagnetic (light) wave: red is a low frequency, purple is a high frequency.
But it’s amazing: the wavelength of visible light is five thousand times the size of an atom - how does such a “thing” fit into an atom when the atom absorbs this wave? If only the photon is a particle comparable in size to an atom. Is a photon both big and small at the same time?

In addition, the photoelectric effect and the Compton effect clearly prove that light is still a flow of particles: it cannot be explained how a wave transfers energy to electrons localized in space - if light were a wave, then some electrons would be knocked out later than others, and the phenomenon We would not observe the photoelectric effect. But in the case of a flow, a single photon collides with a single electron and, under certain conditions, knocks it out of the atom.

As a result, it was decided: light is both a wave and a particle. Or rather, neither one nor the other, but a new previously unknown form of existence of matter: the phenomena we observe are just projections or shadows of the real state of affairs, depending on how you look at what is happening. When we look at the shadow of a cylinder illuminated from one side, we see a circle, and when illuminated from the other side, we see a rectangular shadow. So it is with the particle-wave representation of light.

But even here everything is not easy. We cannot say that we consider light to be either a wave or a stream of particles. Look out the window. Suddenly, even in cleanly washed glass, we see our own reflection, albeit blurry. What's the catch? If light is a wave, then it is easy to explain reflection in a window - we see similar effects on water when a wave is reflected from an obstacle. But if light is a stream of particles, then reflection cannot be explained so easily. After all, all photons are the same. However, if they are all the same, then the barrier in the form of window glass should have the same effect on them. Either they all pass through the glass, or they are all reflected. But in the harsh reality, some of the photons fly through the glass, and we see the neighboring house and immediately see our reflection.

And the only explanation that comes to mind: photons are on their own. It is impossible to predict with one hundred percent probability how a particular photon will behave - whether it will collide with glass as a particle or as a wave. This is the basis of quantum physics - completely, absolutely random behavior of matter at the micro level without any reason (and in our world of large quantities, we know from experience that everything has a reason). This is a perfect random number generator, unlike a coin toss.

The brilliant Einstein, who discovered the photon, was convinced until the end of his life that quantum physics was wrong, and assured everyone that “God does not play dice.” But modern science increasingly confirms that it does play.

One way or another, one day scientists decided to put an end to the “wave or particle” debate and reproduce Jung’s experience taking into account the technologies of the 20th century. By this time, they had learned to shoot photons one at a time (quantum generators, known among the population as “lasers”), and therefore it was decided to check what would happen on the screen if one shot one particle at two slits: it will finally become clear , what is matter under controlled experimental conditions.

And suddenly - a single quantum of light (photon) showed an interference pattern, that is, the particle flew through both slits at the same time, the photon interfered with itself (in scientific terms). Let's clarify the technical point - in fact, the interference picture was shown not by one photon, but by a series of shots at one particle at intervals of 10 seconds - over time, Young's fringes, familiar to any C student since 1801, appeared on the screen.

From the point of view of the wave, this is logical - the wave passes through the cracks, and now two new waves diverge in concentric circles, overlapping each other.
But from a corpuscular point of view, it turns out that the photon is in two places at the same time when it passes through the slits, and after passing through it mixes with itself. This is generally normal, huh?
It turned out that it was normal. Moreover, since the photon is in two slits at once, it means that it is simultaneously everywhere both before the slits and after flying through them. And in general, from the point of view of quantum physics, the released photon between the start and finish is simultaneously “everywhere and at once.” Physicists call such a finding of a particle “everywhere at once” superposition - a terrible word, which used to be a mathematical pampering, has now become a physical reality.

A certain E. Schrödinger, a well-known opponent of quantum physics, had by this time dug up somewhere a formula that described the wave properties of matter, such as water. And after tinkering with it a little, to my horror, I deduced the so-called wave function. This function showed the probability of finding a photon in a certain location. Note that this is a probability, not an exact location. And this probability depended on the square of the height of the quantum wave crest at a given location (if anyone is interested in the details).

We will devote a separate chapter to the issues of measuring the location of particles.

Further discoveries showed that things with dualism are even worse and more mysterious.
In 1924, a certain Louis de Broglie said that the wave-corpuscular properties of light are the tip of the iceberg. And all elementary particles have this incomprehensible property.
That is, a particle and a wave at the same time are not only particles of the electromagnetic field (photons), but also real particles such as electrons, protons, etc. All matter around us at the microscopic level is waves(and particles at the same time).

And a couple of years later, this was even confirmed experimentally - the Americans drove electrons in cathode ray tubes (which are known to today's old farts under the name "kinescope") - and so observations related to the reflection of electrons confirmed that an electron is also a wave (for ease of understanding, you can say that they placed a plate with two slits in the path of the electron and saw the interference of the electron as it is).

To date, experiments have discovered that atoms also have wave properties, and even some special types of molecules (the so-called “fullerenes”) manifest themselves as waves.

The inquisitive mind of the reader, who has not yet been stunned by our story, will ask: if matter is a wave, then why, for example, is a flying ball not smeared in space in the form of a wave? Why does a jet plane not at all resemble a wave, but is very similar to a jet plane?

De Broglie, the devil, explained everything here: yes, a flying ball or a Boeing is also a wave, but the length of this wave is shorter, the greater the impulse. Momentum is mass times velocity. That is, the greater the mass of matter, the shorter its wavelength. The wavelength of a ball flying at a speed of 150 km/h will be approximately 0.00 meters. Therefore, we are not able to notice how the ball is spread across space as a wave. To us it is solid matter.
An electron is a very light particle and, flying at a speed of 6000 km/sec, it will have a noticeable wavelength of 0.0000000001 meters.

By the way, let’s immediately answer the question why the atomic nucleus is not so “wavelike”. Although it is located in the center of the atom, around which the electron flies crazily and at the same time is smeared, it has a decent momentum associated with the mass of protons and neutrons, as well as high-frequency oscillation (speed) due to the existence of a constant exchange of particles inside the nucleus strong interaction (read the topic). Therefore, the core is more like the solid matter we are familiar with. The electron, apparently, is the only particle with mass that has clearly expressed wave properties, so everyone studies it with delight.

Let's return to our particles. So it turns out: an electron rotating around an atom is both a particle and a wave. That is, the particle rotates, and at the same time, the electron as a wave represents a shell of a certain shape around the nucleus - how can this even be understood by the human brain?

We have already calculated above that a flying electron has a rather huge (for a microcosm) wavelength, and in order to fit around the nucleus of an atom, such a wave needs an indecently large amount of space. This is precisely what explains such large sizes of atoms compared to the nucleus. The wavelengths of the electron determine the size of the atom. The empty space between the nucleus and the surface of the atom is filled by the “accommodation” of the wavelength (and at the same time particle) of the electron. This is a very crude and incorrect explanation - please forgive us - in reality everything is much more complicated, but our goal is to at least allow people who are interested in all this to gnaw off a piece of the granite of science.

Let's be clear again! After some comments on the article [in YP], we realized what an important point this article was missing. Attention! The form of matter we describe is neither a wave nor a particle. It only (simultaneously) has the properties of a wave and the properties of particles. It cannot be said that an electromagnetic wave or an electron wave is like sea waves or sound waves. The waves we are familiar with represent the propagation of disturbances in space filled with some substance.
Photons, electrons and other instances of the microcosm, when moving in space, can be described by wave equations; their behavior is only SIMILAR to a wave, but in no case are they a wave. It’s similar with the corpuscular structure of matter: the behavior of a particle is similar to the flight of small point balls, but these are never balls.
This must be understood and accepted, otherwise all our thoughts will ultimately lead to a search for analogues in the macrocosm and thus the understanding of quantum physics will come to an end, and friarism or charlatan philosophy will begin, such as quantum magic and the materiality of thoughts.

We will consider the remaining terrifying conclusions and consequences of Jung's modernized experiment later in the next part - Heisenberg's uncertainty, Schrödinger's cat, the Pauli exclusion principle and quantum entanglement await the patient and thoughtful reader who will re-read our articles more than once and rummage through the Internet in search of additional information.

Thank you all for your attention. Happy insomnia or cognitive nightmares to everyone!

NB: We diligently remind you that all images are taken from Google (search by images) - authorship is determined there.
Illegal copying of text is prosecuted, suppressed, well, you know...

Wave-particle duality– the property of any microparticle to detect signs of a particle (corpuscle) and a wave. The wave-particle duality is most clearly manifested in elementary particles. An electron, a neutron, a photon, under some conditions, behave like well-localized material objects (particles) in space, moving with certain energies and impulses along classical trajectories, and in others, like waves, which is manifested in their ability to interfere and diffraction. Thus, an electromagnetic wave, scattering on free electrons, behaves like a stream of individual particles - photons, which are quanta of the electromagnetic field (Compton effect), and the momentum of the photon is given by the formula p = h/λ, where λ is the length of the electromagnetic wave, and h is Planck’s constant . This formula in itself is evidence of dualism. In it, on the left is the momentum of an individual particle (photon), and on the right is the wavelength of the photon. The duality of electrons, which we are accustomed to consider as particles, is manifested in the fact that when reflected from the surface of a single crystal, a diffraction pattern is observed, which is a manifestation of the wave properties of electrons. The quantitative relationship between the corpuscular and wave characteristics of an electron is the same as for a photon: р = h/λ (р is the momentum of the electron, and λ is its de Broglie wavelength). Wave-particle duality is the basis of quantum physics.

Wave (fur) is a process always associated with a material environment that occupies a certain volume in space.

64. De Broglie waves. Electron diffraction Wave properties of microparticles.

The development of ideas about the corpuscular-wave properties of matter received in the hypothesis about the wave nature of the movement of microparticles. Louis de Broglie, from the idea of ​​symmetry in nature for particles of matter and light, attributed to any microparticle a certain internal periodic process (1924). Combining the formulas E = hν and E = mc 2, he obtained a relation showing that any particle has its own wavelength : λ B = h/mv = h/p, where p is the momentum of the wave-particle. For example, for an electron with an energy of 10 eV, the de Broglie wavelength is 0.388 nm. Subsequently, it was shown that the state of a microparticle in quantum mechanics can be described by a certain complex wave function coordinates Ψ(q), and the squared modulus of this function |Ψ| 2 defines the probability distribution of coordinate values. This function was first introduced into quantum mechanics by Schrödinger in 1926. Thus, the de Broglie wave does not carry energy, but only reflects the “phase distribution” of some probabilistic periodic process in space. Consequently, the description of the state of microworld objects is probabilistic nature, in contrast to objects of the macroworld, which are described by the laws of classical mechanics.

To prove de Broglie's idea about the wave nature of microparticles, the German physicist Elsasser proposed using crystals to observe electron diffraction (1925). In the USA, K. Davisson and L. Germer discovered the phenomenon of diffraction when an electron beam passes through a plate of nickel crystal (1927). Independently of them, the diffraction of electrons passing through metal foil was discovered by J.P. Thomson in England and P.S. Tartakovsky in the USSR. Thus, de Broglie’s idea about the wave properties of matter found experimental confirmation. Subsequently, diffraction, and therefore wave, properties were discovered in atomic and molecular beams. Not only photons and electrons, but also all microparticles have particle-wave properties.

The discovery of the wave properties of microparticles showed that such forms of matter as field (continuous) and matter (discrete), which from the point of view of classical physics were considered qualitatively different, under certain conditions can exhibit properties inherent in both forms. This speaks of the unity of these forms of matter. A complete description of their properties is possible only on the basis of opposing, but complementary, ideas.

Introduction

Almost simultaneously, two theories of light were put forward: Newton's corpuscular theory and Huygens' wave theory.

According to the corpuscular theory, or theory of outflow, put forward by Newton at the end of the 17th century, luminous bodies emit tiny particles (corpuscles) that fly straight in all directions and, when they enter the eye, cause a sensation of light.

According to the wave theory, a luminous body causes elastic vibrations in a special medium filling the entire cosmic space - the world ether - that propagate in the ether like sound waves in the air.

At the time of Newton and Huygens, most scientists adhered to Newton's corpuscular theory, which quite satisfactorily explained all light phenomena known at that time. The reflection of light was explained similarly to the reflection of elastic bodies upon impact with a plane. The refraction of light was explained by the action of large attractive forces on the corpuscles from a denser medium. Under the influence of these forces, which manifest themselves, according to Newton's theory, when approaching a denser medium, the light corpuscles received acceleration directed perpendicular to the boundary of this medium, as a result of which they changed the direction of movement and at the same time increased their speed. Other light phenomena were explained similarly.

Subsequently, new observations that appeared did not fit into the framework of this theory. In particular, the inconsistency of this theory was discovered when the speed of propagation of light in water was measured. It turned out to be not more, but less than in the air.

At the beginning of the 19th century, Huygens' wave theory, not recognized by his contemporaries, was developed and improved by Young and Fresnel and received universal recognition. In the 60s of the last century, after Maxwell developed the theory of the electromagnetic field, it turned out that light is electromagnetic waves. Thus, the wave mechanistic theory of light was replaced by the wave electromagnetic theory. Light waves (visible spectrum) occupy the range of 0.4–0.7 µm on the electromagnetic wave scale. Maxwell's wave theory of light, which treats radiation as a continuous process, was unable to explain some of the newly discovered optical phenomena. It was supplemented by the quantum theory of light, according to which the energy of a light wave is emitted, distributed and absorbed not continuously, but in certain portions - light quanta, or photons - which depend only on the length of the light wave. Thus, according to modern concepts, light has both wave and corpuscular properties.

Interference of light

Waves that create oscillations at every point in space with a phase difference that does not change over time are called coherent. The phase difference in this case has a constant, but, generally speaking, different value for different points in space. It is obvious that only waves of the same frequency can be coherent.

When several coherent waves propagate in space, the oscillations generated by these waves strengthen each other at some points and weaken each other at others. This phenomenon is called wave interference. Waves of any physical nature can interfere. We will look at the interference of light waves.

Sources of coherent waves are also called coherent. When a certain surface is illuminated by several coherent light sources, alternating light and dark stripes generally appear on this surface.

Two independent light sources, for example two electric lamps, are not coherent. The light waves they emit are the result of the addition of a large number of waves emitted by individual atoms. The emission of waves by atoms occurs randomly, and therefore there are no constant relationships between the phases of the waves emitted by two sources.

When the surface is illuminated by incoherent sources, the pattern of alternating light and dark stripes characteristic of interference does not appear. The illumination at each point turns out to be equal to the sum of the illumination created by each of the sources separately.

Coherent waves are produced by splitting a beam of light from one source into two or more separate beams.

Interference of light can be observed when illuminating a transparent plate of variable thickness, in particular a wedge-shaped plate, with monochromatic (one-color) rays. The observer's eye will receive waves reflected from both the front and back surfaces of the plate. The result of interference is determined by the difference in the phases of these and other waves, which gradually changes with changes in thickness

When a plane-parallel plate is illuminated by a parallel beam, the phase difference of the light waves reflected from its front and rear surfaces is the same at all points - the plate will appear uniformly illuminated.

Around the point of contact of a slightly convex glass with a flat one, when illuminated with monochromatic light, dark and light rings are observed - the so-called Newton's rings. Here, the thinnest layer of air between both glasses plays the role of a reflective film, having a constant thickness along concentric circles.

Diffraction of light.

A light wave does not change the geometric shape of the front when propagating in a homogeneous medium. However, if light propagates in an inhomogeneous medium, in which, for example, there are opaque screens, areas of space with a relatively sharp change in the refractive index, etc., then a distortion of the wave front is observed. In this case, a redistribution of the intensity of the light wave occurs in space. When illuminating, for example, opaque screens with a point source of light at the boundary of the shadow, where, according to the laws of geometric optics, there should be an abrupt transition from shadow to light, a number of dark and light stripes are observed; part of the light penetrates into the region of the geometric shadow. These phenomena relate to the diffraction of light.

So, diffraction of light in the narrow sense is the phenomenon of light bending around the contour of opaque bodies and light entering the region of a geometric shadow; in a broad sense, any deviation in the propagation of light from the laws of geometric optics.

Sommerfeld's definition: diffraction of light is understood as any deviation from rectilinear propagation if it cannot be explained as a result of reflection, refraction or bending of light rays in media with a continuously changing refractive index.

If the medium contains tiny particles (fog) or the refractive index changes noticeably over distances of the order of the wavelength, then in these cases we talk about light scattering and the term “diffraction” is not used.

There are two types of light diffraction. By studying the diffraction pattern at an observation point located at a finite distance from an obstacle, we are dealing with Fresnel diffraction. If the observation point and the light source are located so far from the obstacle that the rays incident on the obstacle and the rays going to the observation point can be considered parallel beams, then we talk about diffraction in parallel rays - Fraunhofer diffraction.

The theory of diffraction considers wave processes in cases where there are any obstacles in the path of wave propagation.

Using the theory of diffraction, problems such as noise protection using acoustic screens, the propagation of radio waves over the Earth's surface, the operation of optical instruments (since the image given by a lens is always a diffraction pattern), surface quality measurements, the study of the structure of matter, and many others are solved. .

Polarization of light

The phenomena of interference and diffraction, which served to substantiate the wave nature of light, do not yet provide a complete picture of the nature of light waves. New features are revealed to us by the experience of passing light through crystals, in particular through tourmaline.

Let's take two identical rectangular tourmaline plates, cut so that one of the sides of the rectangle coincides with a certain direction inside the crystal, called the optical axis. Let's put one plate on top of the other so that their axes coincide in direction, and pass a narrow beam of light from a lantern or the sun through the folded pair of plates. Since tourmaline is a brown-green crystal, the trace of the transmitted beam will appear on the screen as a dark green speck. Let's start rotating one of the plates around the beam, leaving the second one motionless. We will find that the trace of the beam becomes weaker, and when the plate is rotated 90 0, it will completely disappear. With further rotation of the plate, the passing beam will again begin to intensify and reach its previous intensity when the plate rotates 180 0, i.e. when the optical axes of the plates are again parallel. With further rotation of the tourmaline, the beam weakens again.

All observed phenomena can be explained if the following conclusions are drawn.

1) Light vibrations in the beam are directed perpendicular to the line of propagation of light (light waves are transverse).

2) Tourmaline is capable of transmitting light vibrations only when they are directed in a certain way relative to its axis.

3) In the light of a lantern (sun), transverse vibrations of any direction are presented and, moreover, in the same proportion, so that no one direction is predominant.