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Characteristics of light diffraction as a set of phenomena that are caused by the wave nature of light as it propagates in a medium. Violation of the symmetry of the distribution of disturbances in a transverse wave. The essence of diffraction effects and wave polarization.

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Diffraction of light is a set of phenomena that are caused by the wave nature of light and are observed when it propagates in a medium with pronounced inhomogeneities (for example, when passing through holes in opaque screens, near the boundaries of opaque bodies, etc.) In a narrower sense, diffraction is understood as the phenomenon of light bending around small obstacles, i.e. deviations from the laws of geometric optics and, consequently, the penetration of light into the region of geometric shadow.

Fresnel explained the diffraction of light as a result of the interference of secondary waves according to the Huygens-Fresnel principle. [The Huygens-Fresnel principle is an approximate method for solving problems of wave propagation, especially light waves. According to the Huygens-Fresnel principle, each element of the surface that the wave has reached at a given moment is the center of elementary waves, the envelope of which will be the wave surface at the next moment in time. The position of the front of a propagating wave can be represented at any moment in time by the envelope of all secondary (elementary) waves ,Fig.1. The sources of secondary waves are the points to which the front of the primary wave reached at the previous moment in time. It is assumed that secondary waves are emitted only “forward”, i.e. in directions making acute angles with the direction of the outer normal to the front of the primary wave. Huygens' principle allows us to explain the laws of reflection and refraction of light, but it is not sufficient to explain the diffraction pattern.

diffraction light polarization wave

In a broader interpretation, diffraction is associated with a very wide range of phenomena that arise during the propagation of waves in inhomogeneous media, as well as during the propagation of waves limited in space. Diffraction is closely related to the phenomenon of interference - the mutual enhancement or weakening of the amplitude of two or more coherent waves simultaneously propagating in space. Accompanied by alternating maxima and minima of intensity in space. The result of interference (interference pattern - hologram) depends on the phase difference of the overlapping waves. interference in thin films (wavefront division method), in which electromagnetic waves reflected from two surfaces are added. Depending on the relationship between the thickness of the film and the wavelength of the radiation, an increase or decrease in color is observed.

When illuminated with white light (a mixture of different wavelengths), a thickness-dependent coloration of the film appears (for example, rainbow stains on an oil slick in water). The described coloring method is used in nature: the variegated colors of butterfly wings are not due to the presence of a coloring pigment, but to the interference of light in the thin transparent wing scales. In technology, interference coatings are used to create mirrors with a high reflectivity (“dielectric mirrors”) and to brighten optics (attenuate waves reflected from numerous lens surfaces of complex lenses). The high sensitivity of the observed intensity distribution pattern to the path difference of interfering beams underlies a whole class of ultra-precise instruments called interferometers. For example, measuring ultra-low speeds of movement (several centimeters per year): sliding of glaciers, continental drift, etc.

The production of high-quality holograms became possible after the creation of lasers - powerful sources of monochromatic radiation capable of producing a stable interference pattern even with large differences in the path of interfering beams.

Moreover, the phenomenon of diffraction itself is often interpreted as a special case of interference (interference of secondary waves.

Highly sensitive spectral instruments with a diffraction grating as a dispersing element (monochromators, spectrographs, spectrophotometers, etc.) using the phenomenon of light diffraction have become widespread. Diffraction by ultrasonic waves in transparent media makes it possible to determine the elastic constants of a substance, as well as to create acousto-optical light modulators.

The scope of practical application of devices based on quantum optical phenomena is very wide - photocells and photomultipliers, image brightness amplifiers (electron-optical converters), transmitting television tubes, etc. Photocells are used not only for recording radiation, but also as devices that convert the radiant energy of the Sun into electricity to power electrical, radio and other equipment (so-called solar panels). Based on photochromic materials, new systems for recording and storing information are being developed for the needs of computer technology, and protective light filters have been created with an automatic increase in light absorption as its intensity increases. The production of powerful streams of monochromatic laser radiation with different wavelengths opened the way to the development of optical methods for separating isotopes and stimulating the directed occurrence of chemical reactions, and made it possible to find new, unconventional applications in biophysics (the effect of laser light streams on biological objects at the molecular level) and medicine (see. Laser radiation). In technology, the use of lasers has led to the emergence of optical methods for processing materials

Diffraction of waves is observed regardless of their nature and can manifest itself:

· in the transformation of the spatial structure of waves. In some cases, such a transformation can be considered as waves “bending around” obstacles, in other cases - as an expansion of the angle of propagation of wave beams or their deflection in a certain direction;

· in the decomposition of waves according to their frequency spectrum;

Newton introduced the term spectrum into scientific use in 1671-1672 to designate a multi-color band, similar to a rainbow, which is obtained when a solar ray passes through a triangular glass prism. For example, a rainbow occurs when the Sun illuminates a curtain of rain. As the rain subsides and then stops, the rainbow fades and gradually disappears. The colors observed in a rainbow alternate in the same sequence as in the spectrum obtained by passing a beam of sunlight through a prism.

· in the transformation of wave polarization;

Wave polarization is the phenomenon of breaking the symmetry of the distribution of disturbances in a transverse wave (for example, the strengths of electric and magnetic fields in electromagnetic waves) relative to the direction of its propagation. In a longitudinal wave, polarization cannot occur, since disturbances in this type of wave always coincide with the direction of propagation. Most often, this phenomenon is used to create various optical effects, as well as in 3D cinema (IMAX technology), where polarization is used to separate images intended for the right and left eyes.

· in changing the phase structure of waves.

Diffraction effects depend on the relationship between the wavelength and the characteristic size of inhomogeneities in the medium or inhomogeneities in the structure of the wave itself. In nature, an example of diffraction is mirages - these are reflections of some things or phenomena on the surface of hot sand, asphalt, sea, etc. This occurs because the temperature is different in different layers of air, and the temperature difference acts like a mirror. A mirage is something other than reflected objects or phenomena that we accept as reality.

Auroras occur as a result of the bombardment of the upper layers of the atmosphere by charged particles moving towards the Earth along geomagnetic field lines from a region of near-Earth space called the plasma layer. The projection of the plasma layer along geomagnetic field lines onto the earth's atmosphere has the shape of rings surrounding the north and south magnetic poles

Listliterature

Miroshnikov M.M. Theoretical foundations of optical-electronic devices: a textbook for instrument-making universities. - 2nd edition, revised. and additional - St. Petersburg: Mechanical Engineering, 2003 - 696 p.

Born M., Wolf E. Fundamentals of optics. - M.: Nauka, 1970. - 856 p.

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Jung's experience.

Every experiment with the interference of light contains some method of producing two coherent light waves. In the experiment with Fresnel mirrors, as you remember, coherent sources were two images of the same source obtained in both mirrors.

The simplest idea that came to mind first was this. Let's poke two holes in a piece of cardboard and expose it to the sun's rays. These holes will be coherent secondary light sources, since there is only one primary source - the Sun. Consequently, on the screen in the area of overlap of the beams diverging from the holes, we should see an interference pattern.

Such an experiment was carried out long before Jung by the Italian scientist Francesco Grimaldi (who discovered the diffraction of light). However, no interference was observed. Why? This question is not very simple, and the reason is that the Sun is not a point, but an extended source of light (the angular size of the Sun is 30 arc minutes). The solar disk consists of many point sources, each of which produces its own interference pattern on the screen. Overlapping, these individual patterns “smear” each other, and as a result, the screen produces uniform illumination of the area where the beams overlap.

But if the Sun is excessively “big”, then it is necessary to artificially create spot primary source. For this purpose, Young's experiment used a small preliminary hole (Fig. 3).

Rice. 3. Jung's experience diagram

A plane wave falls on the first hole, and a light cone appears behind the hole, expanding due to diffraction. It reaches the next two holes, which become the sources of two coherent light cones. Now - thanks to the point nature of the primary source - an interference pattern will be observed in the area where the cones overlap!

Thomas Young carried out this experiment, measured the width of the interference fringes, derived a formula, and using this formula for the first time calculated the wavelengths of visible light. That is why this experiment is one of the most famous in the history of physics.

Huygens–Fresnel principle.

Let us recall the formulation of Huygens' principle: each point involved in the wave process is a source of secondary spherical waves; these waves propagate from a given point, as if from a center, in all directions and overlap each other.

But a natural question arises: what does “overlap” mean?

Huygens reduced his principle to a purely geometric method of constructing a new wave surface as the envelope of a family of spheres expanding from each point of the original wave surface. Secondary Huygens waves are mathematical spheres, not real waves; their total effect manifests itself only on the envelope, i.e., on the new position of the wave surface.

In this form, Huygens' principle did not answer the question of why a wave traveling in the opposite direction does not arise during the propagation of a wave. Diffraction phenomena also remained unexplained.

The modification of Huygens' principle took place only 137 years later. Augustin Fresnel replaced Huygens' auxiliary geometric spheres with real waves and suggested that these waves interfere together.

Huygens–Fresnel principle. Each point of the wave surface serves as a source of secondary spherical waves. All these secondary waves are coherent due to their common origin from the primary source (and therefore can interfere with each other); the wave process in the surrounding space is the result of the interference of secondary waves.

Fresnel's idea filled Huygens' principle with physical meaning. Secondary waves, interfering, reinforce each other on the envelope of their wave surfaces in the “forward” direction, ensuring further propagation of the wave. And in the “backward” direction, they interfere with the original wave, mutual cancellation is observed, and a backward wave does not arise.

In particular, light propagates where secondary waves are mutually amplified. And in places where secondary waves weaken, we will see dark areas of space.

The Huygens–Fresnel principle expresses an important physical idea: a wave, having moved away from its source, subsequently “lives its own life” and no longer depends on this source. Capturing new areas of space, the wave propagates further and further due to the interference of secondary waves excited at different points in space as the wave passes.

How does the Huygens–Fresnel principle explain the phenomenon of diffraction? Why, for example, does diffraction occur at a hole? The fact is that from the infinite flat wave surface of the incident wave, the screen hole cuts out only a small luminous disk, and the subsequent light field is obtained as a result of the interference of waves from secondary sources located not on the entire plane, but only on this disk. Naturally, the new wave surfaces will no longer be flat; the path of the rays is bent, and the wave begins to propagate in different directions that do not coincide with the original one. The wave goes around the edges of the hole and penetrates into the geometric shadow area.

Secondary waves emitted by different points of the cut out light disk interfere with each other. The result of interference is determined by the phase difference of the secondary waves and depends on the angle of deflection of the rays. As a result, an alternation of interference maxima and minima occurs - which is what we saw in Fig. 2.

Fresnel not only supplemented Huygens' principle with the important idea of coherence and interference of secondary waves, but also came up with his famous method for solving diffraction problems, based on the construction of so-called Fresnel zones. The study of Fresnel zones is not included in the school curriculum - you will learn about them in a university physics course. Here we will only mention that Fresnel, within the framework of his theory, managed to provide an explanation of our very first law of geometric optics - the law of rectilinear propagation of light.

Diffraction grating.

A diffraction grating is an optical device that allows you to decompose light into spectral components and measure wavelengths. Diffraction gratings are transparent and reflective.

We will consider a transparent diffraction grating. It consists of a large number of slots of width , separated by intervals of width (Fig. 4). Light only passes through slits; the gaps do not allow light to pass through. The quantity is called the lattice period.

Rice. 4. Diffraction grating

The diffraction grating is made using a so-called dividing machine, which applies streaks to the surface of glass or transparent film. In this case, the strokes turn out to be opaque spaces, and the untouched places serve as cracks. If, for example, a diffraction grating contains 100 lines per millimeter, then the period of such a grating will be equal to: d = 0.01 mm = 10 microns.

First, we will look at how monochromatic light, that is, light with a strictly defined wavelength, passes through the grating. An excellent example of monochromatic light is the beam of a laser pointer with a wavelength of about 0.65 microns).

In Fig. In Fig. 5 we see such a beam falling on one of the standard set of diffraction gratings. The grating slits are located vertically, and periodically located vertical stripes are observed on the screen behind the grating.

As you already understood, this is an interference pattern. A diffraction grating splits the incident wave into many coherent beams, which propagate in all directions and interfere with each other. Therefore, on the screen we see an alternation of interference maxima and minima - light and dark stripes.

The theory of diffraction gratings is very complex and in its entirety is far beyond the scope of the school curriculum. You should know only the most basic things related to one single formula; this formula describes the positions of the maximum illumination of the screen behind the diffraction grating.

So, let a plane monochromatic wave fall on a diffraction grating with a period (Fig. 6). The wavelength is .

Rice. 6. Diffraction by grating

To make the interference pattern clearer, you can place a lens between the grating and the screen, and place the screen in the focal plane of the lens. Then the secondary waves, traveling in parallel from different slits, will converge at one point on the screen (the side focus of the lens). If the screen is located far enough away, then there is no special need for a lens - the rays arriving at a given point on the screen from various slits will already be almost parallel to each other.

Let's consider secondary waves deviating by an angle. The path difference between two waves coming from adjacent slits is equal to the small leg of a right triangle with the hypotenuse; or, which is the same thing, this path difference is equal to the leg of the triangle. But the angle is equal to the angle since these are acute angles with mutually perpendicular sides. Therefore, our path difference is equal to .

Interference maxima are observed in cases where the path difference is equal to an integer number of wavelengths:

(1)

If this condition is met, all waves arriving at a point from different slits will add up in phase and reinforce each other. In this case, the lens does not introduce an additional path difference - despite the fact that different rays pass through the lens along different paths. Why does this happen? We will not go into this issue, since its discussion goes beyond the scope of the Unified State Exam in physics.

Formula (1) allows you to find the angles that specify the directions to the maxima:

. (2)

When we get it central maximum, or zero order maximum.The difference in the path of all secondary waves traveling without deviation is equal to zero, and at the central maximum they add up with a zero phase shift. The central maximum is the center of the diffraction pattern, the brightest of the maximums. The diffraction pattern on the screen is symmetrical relative to the central maximum.

When we get the angle:

This angle sets the directions for first order maxima. There are two of them, and they are located symmetrically relative to the central maximum. The brightness in the first-order maxima is somewhat less than in the central maximum.

Similarly, at we have the angle:

He gives directions to second order maxima. There are also two of them, and they are also located symmetrically relative to the central maximum. The brightness in the second-order maxima is somewhat less than in the first-order maxima.

An approximate picture of the directions to the maxima of the first two orders is shown in Fig. 7.

Rice. 7. Maxima of the first two orders

In general, two symmetrical maxima k-order are determined by the angle:

. (3)

When small, the corresponding angles are usually small. For example, at μm and μm, the first-order maxima are located at an angle. Brightness of the maxima k-order gradually decreases with growth k. How many maxima can you see? This question is easy to answer using formula (2). After all, sine cannot be greater than one, therefore:

Using the same numerical data as above, we get: . Therefore, the highest possible maximum order for a given lattice is 15.

Look again at Fig. 5 . On the screen we can see 11 maxima. This is the central maximum, as well as two maxima of the first, second, third, fourth and fifth orders.

Using a diffraction grating, you can measure an unknown wavelength. We direct a beam of light onto the grating (the period of which we know), measure the angle at the maximum of the first
order, we use formula (1) and get:

Diffraction grating as a spectral device.

Above we considered the diffraction of monochromatic light, which is a laser beam. Often you have to deal with non-monochromatic radiation. It is a mixture of various monochromatic waves that make up range of this radiation. For example, white light is a mixture of waves throughout the visible range, from red to violet.

The optical device is called spectral, if it allows you to decompose light into monochromatic components and thereby study the spectral composition of the radiation. The simplest spectral device is well known to you - it is a glass prism. Spectral devices also include a diffraction grating.

Let us assume that white light is incident on a diffraction grating. Let's return to formula (2) and think about what conclusions can be drawn from it.

The position of the central maximum () does not depend on the wavelength. At the center of the diffraction pattern they will converge with zero path difference All monochromatic components of white light. Therefore, at the central maximum we will see a bright white stripe.

But the positions of the order maxima are determined by the wavelength. The smaller the , the smaller the angle for a given . Therefore, to the maximum k The th-order monochromatic waves are separated in space: the violet stripe will be closest to the central maximum, the red stripe will be the farthest.

Consequently, in each order, white light is laid out by a lattice into a spectrum.
The first-order maxima of all monochromatic components form a first-order spectrum; then there are spectra of the second, third, and so on orders. The spectrum of each order has the form of a color band, in which all the colors of the rainbow are present - from violet to red.

Diffraction of white light is shown in Fig. 8 . We see a white stripe in the central maximum, and on the sides there are two first-order spectra. As the deflection angle increases, the color of the stripes changes from purple to red.

But a diffraction grating not only allows one to observe spectra, that is, to carry out a qualitative analysis of the spectral composition of radiation. The most important advantage of a diffraction grating is the possibility of quantitative analysis - as mentioned above, with its help we can to measure wavelengths. In this case, the measuring procedure is very simple: in fact, it comes down to measuring the direction angle to the maximum.

Natural examples of diffraction gratings found in nature are bird feathers, butterfly wings, and the mother-of-pearl surface of a sea shell. If you squint and look at the sunlight, you can see a rainbow color around the eyelashes. Our eyelashes act in this case like a transparent diffraction grating in Fig. 6, and the lens is the optical system of the cornea and lens.

The spectral decomposition of white light, given by a diffraction grating, is most easily observed by looking at an ordinary compact disc (Fig. 9). It turns out that the tracks on the surface of the disk form a reflective diffraction grating!

Diffraction of light is the phenomenon of deviation of light from linear propagation in a medium with sharp inhomogeneities, i.e. light waves bend around obstacles, but provided that the dimensions of the latter are comparable to the length of the light wave. For red light, the wavelength is λкр≈8∙10 -7 m, and for violet light - λ f ≈4∙10 -7 m. The phenomenon of diffraction is observed at distances l from an obstacle, where D is the linear size of the obstacle, λ is the wavelength. So, to observe the phenomenon of diffraction, it is necessary to fulfill certain requirements for the size of obstacles, the distances from the obstacle to the light source, as well as the power of the light source. In Fig. Figure 1 shows photographs of diffraction patterns from various obstacles: a) a thin wire, b) a round hole, c) a round screen.

Rice. 1

To solve diffraction problems - finding the distribution on the screen of the intensities of a light wave propagating in a medium with obstacles - approximate methods based on the Huygens and Huygens-Fresnel principles are used.

Huygens principle: each point S 1, S 2,…,S n of the AB wave front (Fig. 2) is a source of new, secondary waves. New position of wave front A 1 B 1 after time
represents the envelope surface of secondary waves.

Huygens-Fresnel principle: all secondary sources S 1, S 2,…,S n located on the surface of the wave are coherent with each other, i.e. have the same wavelength and constant phase difference. The amplitude and phase of the wave at any point in M space is the result of the interference of waves emitted by secondary sources (Fig. 3).

Rice. 2

Rice. 3

The rectilinear propagation of a beam SM (Fig. 3) emitted by a source S in a homogeneous medium is explained by the Huygens-Fresnel principle. All secondary waves emitted by secondary sources located on the surface of the AB wave front are canceled out as a result of interference, except for waves from sources located in a small section of the segment ab, perpendicular to SM. Light travels along a narrow cone with a very small base, i.e. almost straight forward.

Diffraction grating.

The phenomenon of diffraction is the basis for the design of a remarkable optical device - a diffraction grating. Diffraction grating in optics is a collection of a large number of obstacles and holes concentrated in a limited space on which light diffraction occurs.

The simplest diffraction grating is a system of N identical parallel slits in a flat opaque screen. A good grating is made using a special dividing machine, which produces parallel strokes on a special plate. The number of strokes reaches several thousand per 1 mm; the total number of strokes exceeds 100,000 (Fig. 4).

Fig.5

Rice. 4

If the width of the transparent spaces (or reflective stripes) b, and the width of the opaque spaces (or light-scattering stripes) a, then the value d=b+a called constant (period) of the diffraction grating(Fig. 5).

According to the Huygens-Fresnel principle, each transparent gap (or slit) is a source of coherent secondary waves that can interfere with each other. If a beam of parallel light rays falls on a diffraction grating perpendicular to it, then at a diffraction angle φ on the screen E (Fig. 5), located in the focal plane of the lens, a system of diffraction maxima and minima will be observed, resulting from the interference of light from various slits.

Let us find the condition under which the waves coming from the slits reinforce each other. For this purpose, let us consider waves propagating in the direction determined by the angle φ (Fig. 5). The path difference between the waves from the edges of adjacent slits is equal to the length of the segment DK=d∙sinφ. If this segment contains an integer number of wavelengths, then the waves from all the slits, adding up, will reinforce each other.

Major Highs during diffraction by a grating are observed at an angle φ, satisfying the condition d∙sinφ=mλ, Where m=0,1,2,3… is called the order of the main maximum. Magnitude δ=DK=d∙sinφ is the optical path difference between similar rays B.M. And DN, coming from neighboring cracks.

Major lows on a diffraction grating are observed at such diffraction angles φ for which the light from different parts of each slit is completely extinguished as a result of interference. The condition of the main maxima coincides with the condition of attenuation at one slit d∙sinφ=nλ (n=1,2,3…).

A diffraction grating is one of the simplest, fairly accurate devices for measuring wavelengths. If the grating period is known, then determining the wavelength is reduced to measuring the angle φ corresponding to the direction to the maximum.

To observe phenomena caused by the wave nature of light, in particular, diffraction, it is necessary to use radiation that is highly coherent and monochromatic, i.e. laser radiation. A laser is a source of plane electromagnetic wave.

Double-slit diffraction

Diffraction- a phenomenon that occurs when waves propagate (for example, light and sound waves). The essence of this phenomenon is that the wave is able to bend around obstacles. This results in the wave motion being observed in an area behind the obstacle where the wave cannot reach directly. The phenomenon is explained by the interference of waves at the edges of opaque objects or inhomogeneities between different media along the path of wave propagation. An example would be the appearance of colored light stripes in the shadow area from the edge of an opaque screen.

Diffraction manifests itself well when the size of the obstacle in the path of the wave is comparable to its length or less.

Acoustic diffraction- deviation from straight-line propagation of sound waves.

1. Slit diffraction

Scheme of the formation of regions of light and shadow during diffraction by a slit

In the case when a wave falls on a screen with a slit, it penetrates due to diffraction, but a deviation from the rectilinear propagation of the rays is observed. The interference of waves behind the screen leads to the appearance of dark and light areas, the location of which depends on the direction in which the observation is being made, the distance from the screen, etc.

2. Diffraction in nature and technology

Diffraction of sound waves is often observed in everyday life as we hear sounds that reach us from behind obstacles. It is easy to observe the waves on the water going around small obstacles.

The scientific and technical uses of the diffraction phenomenon are varied. Diffraction gratings are used to split light into a spectrum and to create mirrors (for example, for semiconductor lasers). X-ray, electron, and neutron diffraction is used to study the structure of crystalline solids.

Diffraction time imposes limitations on the resolution of optical instruments, such as microscopes. Objects whose dimensions are smaller than the wavelength of visible light (400-760 nm) cannot be viewed with an optical microscope. A similar limitation exists in the lithography method, which is widely used in the semiconductor industry for the production of integrated circuits. Therefore, it is necessary to use light sources in the ultraviolet region of the spectrum.

3. Diffraction of light

The phenomenon of light diffraction clearly confirms the theory of the corpuscular-wave nature of light.

It is difficult to observe the diffraction of light, since the waves deviate from the interference at noticeable angles only under the condition that the size of the obstacles is approximately equal to the wavelength of the light, and it is very small.

For the first time, having discovered interference, Young performed an experiment on the diffraction of light, with the help of which the wavelengths corresponding to light rays of different colors were studied. The study of diffraction was completed in the works of O. Fresnel, who constructed the theory of diffraction, which in principle allows one to calculate the diffraction pattern that arises as a result of light bending around any obstacles. Fresnel achieved such success by combining Huygens' principle with the idea of interference of secondary waves. The Huygens-Fresnel principle is formulated as follows: diffraction occurs due to the interference of secondary waves.

A light breeze came, and ripples (a wave of small length and amplitude) ran along the surface of the water, encountering various obstacles on its way, above the surface of the water, plant stems, tree branches. On the leeward side behind the branch, the water is calm, there is no disturbance, and the wave bends around the plant stems.

WAVE DIFFRACTION (from lat. difractus– broken) waves bending around various obstacles. Wave diffraction is characteristic of any wave motion; occurs if the dimensions of the obstacle are smaller than the wavelength or comparable to it.

Diffraction of light is the phenomenon of deviation of light from the rectilinear direction of propagation when passing near obstacles. During diffraction, light waves bend around the boundaries of opaque bodies and can penetrate into the region of geometric shadow.
An obstacle can be a hole, a gap, or the edge of an opaque barrier.

Diffraction of light manifests itself in the fact that light penetrates into the region of a geometric shadow in violation of the law of rectilinear propagation of light. For example, passing light through a small round hole, we find a larger bright spot on the screen than would be expected with linear propagation.

Due to the short wavelength of light, the angle of deflection of light from the direction of rectilinear propagation is small. Therefore, to clearly observe diffraction, it is necessary to use very small obstacles or place the screen far from the obstacles.

Diffraction is explained on the basis of the Huygens–Fresnel principle: each point on the wave front is a source of secondary waves. The diffraction pattern results from the interference of secondary light waves.

The waves formed at points A and B are coherent. What is observed on the screen at points O, M, N?

Diffraction is clearly observed only at distances

where R is the characteristic dimensions of the obstacle. At shorter distances, the laws of geometric optics apply.

The phenomenon of diffraction imposes a limitation on the resolution of optical instruments (for example, a telescope). As a result, a complex diffraction pattern is formed in the focal plane of the telescope.

Diffraction grating – is a collection of a large number of narrow, parallel, close to each other transparent to light areas (slits) located in the same plane, separated by opaque spaces.

Diffraction gratings can be either reflective or transmitting light. The principle of their operation is the same. The grating is made using a dividing machine that makes periodic parallel strokes on a glass or metal plate. A good diffraction grating contains up to 100,000 lines. Let's denote:

a– the width of the slits (or reflective stripes) transparent to light;
b– the width of the opaque spaces (or light-scattering areas).
Magnitude d = a + b is called the period (or constant) of the diffraction grating.

The diffraction pattern created by the grating is complex. It exhibits main maxima and minima, secondary maxima, and additional minima due to diffraction by the slit.
The main maxima, which are narrow bright lines in the spectrum, are of practical importance when studying spectra using a diffraction grating. If white light falls on a diffraction grating, the waves of each color included in its composition form their own diffraction maxima. The position of the maximum depends on the wavelength. Zero highs (k = 0 ) for all wavelengths are formed in the directions of the incident beam (β = 0 ), therefore there is a central bright band in the diffraction spectrum. To the left and right of it, color diffraction maxima of different orders are observed. Since the diffraction angle is proportional to the wavelength, red rays are deflected more than violet rays. Note the difference in the order of colors in the diffraction and prismatic spectra. Thanks to this, a diffraction grating is used as a spectral apparatus, along with a prism.

When passing through a diffraction grating, a light wave with a length λ the screen will give a sequence of minimums and maximums of intensity. Intensity maxima will be observed at angle β:

where k is an integer called the order of the diffraction maximum.

Basic summary:

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An example of the manifestation of diffraction is. School encyclopedia

Characteristics of light diffraction as a set of phenomena that are caused by the wave nature of light as it propagates in a medium. Violation of the symmetry of the distribution of disturbances in a transverse wave. The essence of diffraction effects and wave polarization.

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