Themes Unified State Exam codifier: light diffraction, diffraction grating.

If an obstacle appears in the path of the wave, then diffraction - deviation of the wave from rectilinear propagation. This deviation cannot be reduced to reflection or refraction, as well as the curvature of the path of rays due to a change in the refractive index of the medium. Diffraction consists of the fact that the wave bends around the edge of the obstacle and enters the region of the geometric shadow.

Let, for example, a plane wave fall on a screen with sufficient narrow gap(Fig. 1). A diverging wave appears at the exit from the slit, and this divergence increases as the slit width decreases.

In general, diffraction phenomena are expressed more clearly the smaller the obstacle. Diffraction is most significant in cases where the size of the obstacle is smaller or on the order of the wavelength. It is precisely this condition that the slot width in Fig. 1 must satisfy. 1.

Diffraction, like interference, is characteristic of all types of waves - mechanical and electromagnetic. There is visible light special case electromagnetic waves; it is not surprising, therefore, that one can observe
diffraction of light.

So, in Fig. Figure 2 shows the diffraction pattern obtained as a result of passing a laser beam through a small hole with a diameter of 0.2 mm.

We see, as expected, a central bright spot; Very far from the spot there is a dark area - a geometric shadow. But around the central spot - instead of a clear boundary of light and shadow! - there are alternating light and dark rings. The farther from the center, the less bright the light rings become; they gradually disappear into the shadow area.

Reminds me of interference, doesn't it? This is what she is; these rings are interference maxima and minima. What waves are interfering here? Soon we will deal with this issue, and at the same time we will find out why diffraction is observed in the first place.

But first, one cannot fail to mention the very first classical experiment on the interference of light - Young's experiment, in which the phenomenon of diffraction was significantly used.

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 ( angular size of the Sun is equal to 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 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 the 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 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 large number 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 a diffraction grating is very complex and in its entirety is far beyond the scope of 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 this point screen from different slits will be almost parallel to each other.

Let us consider secondary waves deviating by an angle . The path difference between two waves coming from adjacent slits is equal to a small leg right triangle with 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 because it is sharp corners 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, there cannot be a sine more than one, That's why:

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 less, the smaller angle for this . 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 makes it possible to observe spectra, i.e., to conduct qualitative analysis spectral composition of radiation. The most important advantage of a diffraction grating is the ability 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!

Often a wave encounters small (compared to its length) obstacles on its path. The relationship between wavelength and the size of obstacles mainly determines the behavior of the wave.

Waves can bend around the edges of obstacles. When the size of the obstacles is small, the waves, going around the edges of the obstacles, close behind them. Thus, sea waves freely bend around a stone protruding from the water if its dimensions are less than the wavelength or comparable to it. Behind the stone, the waves propagate as if it were not there at all (small stones in Fig. 127). In exactly the same way, the wave from a stone thrown into a pond bends around a twig sticking out of the water. Only behind an obstacle of a large size compared to the wavelength (the large stone in Fig. 127) is a “shadow” formed: the waves do not penetrate beyond it.

Sound waves also have the ability to bend around obstacles. You can hear a car honking around the corner of the house when the car itself is not visible. In the forest, trees obscure your comrades. To avoid losing them, you start screaming. Sound waves Unlike the light, they freely bend around tree trunks and carry your voice to your comrades. Deviation from the rectilinear propagation of waves, the bending of waves around obstacles, is called diffraction. Diffraction is inherent in any wave process to the same extent as interference. Diffraction causes curvature of wave surfaces at the edges of obstacles.

Wave diffraction manifests itself especially clearly in cases where the size of obstacles is smaller than the wavelength or comparable to it.

The phenomenon of wave diffraction on the surface of water can be observed if a screen with a narrow slit, the dimensions of which is smaller than the wavelength, is placed in the path of the waves (Fig. 128). It will be clearly visible that a circular wave is propagating behind the screen, as if an oscillating body, the source of the waves, was located in the opening of the screen. According to Huygens' principle, this should be the case. Secondary sources in a narrow slit are located so close to each other that they can be considered as one point source.


If the size of the slit is large compared to the wavelength, then the pattern of wave propagation behind the screen is completely different (Fig. 129). The wave passes through the slit, almost without changing its shape. Only at the edges can you notice slight curvatures of the wave surface, due to which the wave partially penetrates into the space behind the screen. Huygens' principle allows us to understand why diffraction occurs. Secondary waves emitted by sections of the medium penetrate the edges of an obstacle located in the path of wave propagation.

DIFFRACTION OF LIGHT

If light is a wave process, then, in addition to interference, diffraction of light should also be observed. After all, diffraction - the bending of waves around obstacles - is inherent in any wave motion. But observing the diffraction of light is not easy. The fact is that the waves noticeably bend around obstacles, the dimensions of which are comparable to the wavelength, and the length of the light wave is very small.

By passing a thin beam of light through a small hole, one can observe a violation of the law of rectilinear propagation of light. The bright spot opposite the hole will be larger than what would be expected if the light travels in a straight line.

Jung's experience. In 1802, Young, who discovered the interference of light, performed a classical experiment on diffraction (Fig. 203). In the opaque screen he pierced two small holes B and C with a pin at a short distance from each other.

These holes were illuminated by a narrow beam of light, which in turn passed through a small hole A in another screen. It was this detail, which was very difficult to think of at that time, that decided the success of the experiment. Only coherent waves interfere. Arising in accordance with Huygens' principle spherical wave from hole A excited coherent oscillations in holes B and C. Due to diffraction, two light cones emerged from holes B and C, which partially overlapped. As a result of the interference of light waves, alternating light and dark stripes appeared on the screen. By closing one of the holes, Young discovered that the interference fringes disappeared. It was with the help of this experiment that Young first measured the wavelengths corresponding to light rays of different colors, and quite accurately.

Fresnel's theory. The study of diffraction was completed in the works of Fresnel. Fresnel not only studied various cases of diffraction experimentally in more detail, but also constructed a quantitative theory of diffraction, which makes it possible, in principle, to calculate the diffraction pattern that arises when light bends around any obstacles. He was the first to explain the rectilinear propagation of light in a homogeneous medium on the basis of wave theory.

Fresnel achieved these successes by combining Huygens' principle with the idea of ​​interference of secondary waves. This has already been briefly mentioned in Chapter Four.

In order to calculate the amplitude of a light wave at any point in space, you need to mentally surround the light source with a closed surface. The interference of waves from secondary sources located on this surface determines the amplitude at the point in space under consideration.

This kind of calculations made it possible to understand how light from a point source S, emitting spherical waves, reaches an arbitrary point in space B (Fig. 204).

If we consider secondary sources on a spherical wave surface of radius R, then the result of the interference of secondary waves from these sources at point B turns out to be the same as if only secondary sources on a small spherical segment ab sent light to point B. Secondary waves emitted by sources located on the rest of the surface cancel each other out as a result of interference. Therefore, everything happens as if the light propagated only along the straight line SB, i.e. rectilinearly.

At the same time, Fresnel quantitatively examined diffraction by various types of obstacles.

A curious incident occurred at a meeting of the French Academy of Sciences in 1818. One of the scientists present at the meeting drew attention to the fact that Fresnel’s theories entailed facts that clearly contradicted common sense. For certain hole sizes and certain distances from the hole to the light source and the screen, there should be a dark spot in the center of the light spot. Behind the small opaque disk, on the contrary, there should be a light spot in the center of the shadow. Imagine the surprise of the scientists when the experiments performed proved that this was actually the case.

Diffraction patterns from various obstacles. Due to the fact that the wavelength of light is very short, the angle of deflection of light from the direction of rectilinear propagation is small. Therefore, for a clear observation of diffraction (in particular, in those cases that have just been discussed), the distance between the obstacle, which is bent by light, and the screen must be large.

Figure 205 shows how diffraction patterns from various obstacles look in photographs: a) a thin wire; b) round hole; c) round screen.

Fresnel zones for a three-centimeter wave

Zone plate for three-centimeter waves

Three-centimeter waves: Poisson's spot

Three-centimeter waves: phase zone plate

Round hole. Geometric optics - Fresnel diffraction

Round hole. Fresnel diffraction - Fraunhofer diffraction

Comparison of diffraction patterns: iris diaphragm and circular hole

Poisson's spot

In physics, light diffraction is the phenomenon of deviation from the laws of geometric optics during the propagation of light waves.

The term " diffraction" comes from Latin diffractus, which literally means “waves bending around an obstacle.” Initially, the phenomenon of diffraction was considered exactly this way. In fact, this is a much broader concept. Although the presence of an obstacle in the path of a wave always causes diffraction, in some cases the waves can bend around it and penetrate into the region of the geometric shadow, in others they are only deflected in a certain direction. The decomposition of waves along the frequency spectrum is also a manifestation of diffraction.

How does light diffraction manifest itself?

In a transparent homogeneous medium, light travels in a straight line. Let's place an opaque screen with a small circle-shaped hole in the path of the light beam. On the observation screen located behind him at a sufficiently large distance, we will see diffraction picture: alternating light and dark rings. If the hole in the screen has the shape of a slit, the diffraction pattern will be different: instead of circles, we will see parallel alternating light and dark stripes. What causes them to appear?

Huygens-Fresnel principle

They tried to explain the phenomenon of diffraction back in the time of Newton. But to do this on the basis of the existing at that time corpuscular theory there was no light.

Christiaan Huygens

In 1678, the Dutch scientist Christiaan Huygens derived the principle named after him, according to which each point of the wave front(surface reached by the wave) is the source of a new secondary wave. And the envelope of the surfaces of secondary waves shows the new position of the wave front. This principle made it possible to determine the direction of movement of a light wave and to construct wave surfaces in different cases. But he could not explain the phenomenon of diffraction.

Augustin Jean Fresnel

Many years later, in 1815 French physicistAugustin Jean Fresnel developed Huygens' principle by introducing the concepts of coherence and wave interference. Having supplemented Huygens' principle with them, he explained the cause of diffraction by the interference of secondary light waves.

What is interference?

Interference called the superposition phenomenon coherent(having the same vibration frequency) waves against each other. As a result of this process, the waves either strengthen each other or weaken each other. We observe the interference of light in optics as alternating light and dark stripes. A striking example interference of light waves - Newton's rings.

The sources of secondary waves are part of the same wave front. Therefore, they are coherent. This means that interference will be observed between the emitted secondary waves. At those points in space where light waves intensify, we see light (maximum illumination), and where they cancel each other out, we see darkness (minimum illumination).

In physics, two types of light diffraction are considered: Fresnel diffraction (diffraction by a hole) and Fraunhofer diffraction (diffraction by a slit).

Fresnel diffraction

Such diffraction can be observed if an opaque screen with a narrow round hole (aperture) is placed in the path of the light wave.

If light propagated in a straight line, we would see a bright spot on the observation screen. In fact, as light passes through the hole, it diverges. On the screen you can see concentric (having a common center) alternating light and dark rings. How are they formed?

According to the Huygens-Fresnel principle, the front of a light wave, reaching the plane of the hole in the screen, becomes a source of secondary waves. Since these waves are coherent, they will interfere. As a result, at the observation point we will observe alternating light and dark circles (maxima and minima of illumination).

Its essence is as follows.

Let's imagine that a spherical light wave propagates from a source S 0 to the observation point M . Through the point S a spherical wave surface passes through. Let's divide it into ring zones so that the distance from the edges of the zone to the point M differed by ½ wavelength of light. The resulting annular zones are called Fresnel zones. And the partitioning method itself is called Fresnel zone method .

Distance from point M to the wave surface of the first Fresnel zone is equal to l + ƛ/2 , to the second zone l + 2ƛ/2 etc.

Each Fresnel zone is considered as a source of secondary waves of a certain phase. Two adjacent Fresnel zones are in antiphase. This means that secondary waves arising in adjacent zones will attenuate each other at the observation point. A wave from the second zone will dampen the wave from the first zone, and a wave from the third zone will strengthen it. The fourth wave will again weaken the first, etc. As a result, the total amplitude at the observation point will be equal to A = A 1 - A 2 + A 3 - A 4 + ...

If an obstacle is placed in the path of light that will open only the first Fresnel zone, then the resulting amplitude will be equal to A 1 . This means that the radiation intensity at the observation point will be much higher than in the case when all zones are open. And if you close all even-numbered zones, the intensity will increase many times, since there will be no zones that weaken it.

Even or odd zones can be blocked using a special device, which is a glass plate on which concentric circles are engraved. This device is called Fresnel plate.

For example, if the inner radii of the dark rings of the plate coincide with the radii of the odd Fresnel zones, and the outer radii with the radii of the even ones, then in this case the even zones will be “turned off”, which will cause increased illumination at the observation point.

Fraunhofer diffraction

A completely different diffraction pattern will appear if an obstacle in the form of a screen with a narrow slit is placed in the path of a flat monochromatic light wave perpendicular to its direction. Instead of light and dark concentric circles on the observation screen, we will see alternating light and dark stripes. The brightest stripe will be located in the center. As you move away from the center, the brightness of the stripes will decrease. This diffraction is called Fraunhofer diffraction. It occurs when a parallel beam of light falls on the screen. To obtain it, the light source is placed in the focal plane of the lens. The observation screen is located in the focal plane of another lens located behind the slit.

If light propagated rectilinearly, then on the screen we would observe a narrow light strip passing through point O (the focus of the lens). But why do we see a different picture?

According to the Huygens-Fresnel principle, secondary waves are formed at each point of the wave front that reaches the slit. Rays coming from secondary sources change their direction and deviate from the original direction by an angle φ . They gather at a point P focal plane of the lens.

Let us divide the slit into Fresnel zones in such a way that the optical path difference between the rays emanating from neighboring zones is equal to half the wavelength ƛ/2 . If an odd number of such zones fits into the gap, then at the point R we will observe maximum illumination. And if it’s even, then the minimum.

b · sin φ= + 2 m ·ƛ/2 - minimum intensity condition;

b · sin φ= + 2( m +1)·ƛ/2 - condition of maximum intensity,

Where m - number of zones, ƛ - wavelength, b - width of the gap.

The deflection angle depends on the slot width:

sin φ= m ·ƛ/ b

The wider the slit, the more the positions of the minima are shifted towards the center, and the brighter the maximum in the center will be. And the narrower this slit is, the wider and blurrier the diffraction pattern will be.

Diffraction grating

The phenomenon of light diffraction is used in an optical device called diffraction grating . We will obtain such a device if we place parallel slits or protrusions of the same width on any surface at equal intervals or apply strokes to the surface. The distance between the centers of the slots or protrusions is called period of the diffraction grating and is designated by the letter d . If per 1 mm of grating there are N streaks or crevices, then d = 1/ N mm.

Light reaching the surface of the grating is broken up by streaks or slits into separate coherent beams. Each of these beams is subject to diffraction. As a result of interference, they are strengthened or weakened. And on the screen we see rainbow stripes. Since the angle of deflection depends on the wavelength, and each color has its own wavelength, white light, passing through a diffraction grating, is decomposed into a spectrum. Moreover, light with a longer wavelength is deflected by a larger angle. That is, red light is deflected most strongly in a diffraction grating, unlike a prism, where the opposite happens.

A very important characteristic of a diffraction grating is angular dispersion:

Where ∆ φ - the difference between the interference maxima of two waves,

∆ƛ - the amount by which the lengths of two waves differ.

k - serial number diffraction maximum, measured from the center of the diffraction image.

Diffraction gratings are divided into transparent and reflective. In the first case, slits are cut in a screen made of opaque material or strokes are applied to a transparent surface. In the second, strokes are applied to the mirror surface.

The compact disc, familiar to all of us, is an example of a reflective diffraction grating with a period of 1.6 microns. The third part of this period (0.5 microns) is the recess (sound track) where the recorded information is stored. It scatters light. The remaining 2/3 (1.1 microns) reflect light.

Diffraction gratings are widely used in spectral instruments: spectrographs, spectrometers, spectroscopes for precise measurements of wavelength.

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 are observed different order. 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:

White and any complex light can be considered as a superposition of monochromatic waves with different wavelengths, which behave independently when diffraction by a grating. Accordingly, conditions (7), (8), (9) for each wavelength will be satisfied at different angles, i.e. the monochromatic components of the light incident on the grating will appear spatially separated. The set of main diffraction maxima of the mth order (m≠0) for all monochromatic components of light incident on the grating is called the mth order diffraction spectrum.

The position of the main diffraction maximum of zero order (central maximum φ=0) does not depend on the wavelength, and for white light it will look like a white stripe. The diffraction spectrum of the mth order (m≠0) for incident white light has the form of a colored band in which all the colors of the rainbow occur, and for complex light in the form of a set of spectral lines corresponding to the monochromatic components of complex light incident on a diffraction grating (Fig. 2).

A diffraction grating as a spectral device has the following main characteristics: resolution R, angular dispersion D and dispersion region G.

The smallest difference in wavelengths of two spectral lines δλ, at which the spectral apparatus resolves these lines, is called the spectral resolvable distance, and the value is the resolution of the apparatus.

Spectral resolution condition (Rayleigh criteria):

Spectral lines with close wavelengths λ and λ’ are considered resolved if the main maximum of the diffraction pattern for one wavelength coincides in position with the first diffraction minimum in the same order for another wave.

Using the Rayleigh criterion we obtain:

, (10)

where N is the number of grating lines (slits) involved in diffraction, m is the order of the diffraction spectrum.

And the maximum resolution:

, (11)

where L is the total width of the diffraction grating.

Angular dispersion D is a quantity defined as the angular distance between directions for two spectral lines that differ in wavelength by 1

And
.

From the condition of the main diffraction maximum

(12)

Dispersion region G – the maximum width of the spectral interval Δλ, at which there is no overlap of diffraction spectra of neighboring orders

, (13)

where λ is the initial boundary of the spectral interval.

Description of installation.

The task of determining the wavelength using a diffraction grating comes down to measuring diffraction angles. These measurements in this work are made with a goniometer (protractor).

The goniometer (Fig. 3) consists of the following main parts: a base with a table (I), on which the main scale in degrees is printed (dial –L); a collimator (II) rigidly fixed to the base and an optical tube (III) mounted on a ring that can rotate about an axis passing through the center of the stage. There are two verniers N located opposite each other on the ring.

The collimator is a tube with a lens F1, in the focal plane of which there is a narrow slit S, about 1 mm wide, and a movable eyepiece O with an index thread H.

Installation data:

The price of the smallest division of the main scale of the goniometer is 1 0.

The vernier division price is 5.

Diffraction grating constant
, [mm].

A mercury lamp (DRSh 250 – 3), which has a discrete emission spectrum, is used as a light source in laboratory work. The work measures the wavelengths of the brightest spectral lines: blue, green and two yellow (Fig. 2b).