When a light beam hits the interface between two media, light is reflected: the beam changes its direction and returns to the original medium.
In fig. 4.2 shows the incident beam AO, the reflected beam OB, as well as the perpendicular OC drawn to the reflecting surface KL at the point of incidence O.
Rice. 4.2. Reflection law
The angle AOC is called the angle of incidence. Please note and remember: the angle of incidence is measured from the perpendicular to the reflecting surface, and not from the surface itself! Likewise, the angle of reflection is the angle BOC formed by the reflected beam and perpendicular to the surface.
4.2.1 Law of reflection
Now we will formulate one of the most ancient laws of physics. He was known to the Greeks in antiquity!
The law of reflection.
1) The incident beam, the reflected beam and the perpendicular to the reflecting surface, drawn at the point of incidence, lie in the same plane.
2) The angle of reflection is equal to the angle of incidence.
Thus, \ AOC = \ BOC, as shown in Fig. 4.2.
The law of reflection has one simple but very important geometric consequence. Let's take a look at fig. 4.3. Let a light ray emanate from point A. Construct a point A0 symmetric to point A with respect to the reflecting surface KL.
Rice. 4.3. The reflected ray comes out of point A0
It is clear from the symmetry of the points A and A0 that \ AOK = \ A0 OK. Also, \ AOK + \ AOC = 90. Therefore, \ A0 OB = 2 (\ AOK + \ AOC) = 180, and therefore the points A0, O and B are collinear! The reflected ray OB, as it were, leaves point A0, symmetric to point A
relative to the reflective surface. This fact will be extremely useful to us in the very near future.
The law of reflection describes the path of individual light rays of narrow beams of light. But in many cases the beam is wide enough, that is, it consists of many parallel beams. The reflection pattern of a wide beam of light will depend on the properties of the reflecting surface.
If the surface is uneven, then after reflection, the parallelism of the rays will be violated. As an example, Fig. 4.4 shows the reflection from a wavy surface. The reflected rays, as we can see, go in very different directions.
Rice. 4.4. Reflection from a wavy surface
But what does an “unbalanced” surface mean? Which surfaces are "smooth"? The answer is this: a surface is considered uneven if the dimensions of its unevenness are not less than the length of light waves. So, in fig. 4.4 the characteristic size of irregularities is several orders of magnitude greater than the wavelengths of visible light.
A surface with microscopic irregularities comparable to the wavelengths of visible light is called matte. As a result of the reflection of a parallel beam from a matte surface, scattered light is obtained, the rays of such light go in all possible directions3. The reflection itself from a matte surface is therefore called diffuse or diffuse4.
If the size of the surface irregularities is less than the wavelength of the light wave, then such a surface is called specular. When reflected from a mirror surface, the parallelism of the beam is preserved: the reflected rays also go parallel (Fig. 4.5).
Rice. 4.5. Reflection from a mirror surface
A roughly mirrored surface is the smooth surface of water, glass, or polished metal. Reflection from a specular surface is called specular, respectively. We will be interested in a simple but important special case of specular reflection, reflection in a flat mirror.
4.2.2 Plane mirror
A flat mirror is the part of a plane that mirrors light. A flat mirror is a common thing; there are several such mirrors in your home. But now we can figure out why, looking in the mirror, you see in it a reflection of yourself and the objects next to you.
Point light source S in Fig. 4.6 emits rays in different directions; let's take two close beams falling on a flat mirror. We already know that reflected rays
3 That is why we see the surrounding objects: they reflect the diffused light, which we observe from any angle.
4 The Latin word di usio just means spreading, spreading, scattering.
will go as if they are emanating from the point S0, which is symmetrical to the point S with respect to the plane of the mirror.
Rice. 4.6. Image of a light source in a flat mirror
The fun begins when the diverging reflected rays hit our eye. The peculiarity of our consciousness is that the brain completes the diverging beam, continuing it behind the mirror until it intersects at the point S0. It seems to us that the reflected rays emanate from the point S0, we see a luminous point there!
This point serves as an image of the light source S. Of course, in reality nothing shines behind the mirror, no energy is concentrated there, this is an illusion, an illusion of sight, a product of our consciousness. Therefore, the point S0 is called an imaginary image of the source S. At the point S0, it is not the light rays themselves that intersect, but their mental extensions “in the looking glass”.
It is clear that the S0 image will exist regardless of the size of the mirror and whether the source is directly above the mirror or not (Figure 4.7). It is only important that the rays reflected from the mirror fall into the eye, and the eye itself will form an image of the source.
Rice. 4.7. The source is not above the mirror: the image is still there
The field of vision, the spatial area from which the source image is visible, depends on the location of the source and the size of the mirror. The field of view is defined by the edges K and L of the mirror KL. The construction of the field of vision of the image S0 is clear from Fig.4.8; the desired area of vision is highlighted with a gray background.
MOU "Secondary School No. 87"
Light reflection
Performed:
Ziziko Julia
9B grade student
Supervisor:
Physics teacher
Eremina S.N.
ZATO Seversk
1. Introduction
2. Reflection of light.
3. Reflection of light in any mirrors.
4. Periscope.
5. Conclusion.
6. References.
Introduction.
My work is called “The Phenomenon of Reflection of Light. Periscope".
I took this topic because it is interesting because it explains many of the facts of light reflection from a scientific point of view. When I take a mirror and look directly into it, then I see my own reflection, and when I look into it from the side, I do not observe my reflection. From this we can conclude that the mirror surface has many interesting properties, and I would like to know more about them. For example, why when you change the position of the mirror, objects in it are reflected in different ways and why flat surfaces reflect better than rough ones.
In addition, I was interested in how an object is reflected in two mirrors directed by reflective surfaces to each other or at a slight angle. This property of mirrors is used in the periscope. I wanted to create my own periscope and see it confirmed
whether in practice my assumptions.
Reflection of light.
The law of light reflection is a physical phenomenon in which light falling from one medium to the interface with another medium returns back to the first medium.
| A person sees a light source when a beam emanating from this source hits the eye. If the body is not a source, then the eye can perceive rays from any source, reflected by this body, that is, falling on the surface of this body and changing the direction of further propagation. A body that reflects rays becomes a source of reflected light. The rays falling on the surface of the body change the direction of further propagation. When reflected, light returns to the same medium from which it fell onto the surface of the body. A body that reflects rays becomes a source of reflected light. |
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Fig. 1. Fermat's principle and the law of reflection
Indeed, in Fig. 1 DADC = DFDC, then according to Heron's postulate:
min (AC + CB) = min (FC + CВ) = FВ = FO + OB = AO + OB => a = b
It is taken into account here that the shortest path between two points (F and B) will be along the straight line FB through point O.
Note that in a similar way, the law of refraction of light can be derived from Fermat's principle.
The law of light reflection.
The incident ray, the normal to the reflecting surface and the reflected ray lie in the same plane (Fig. 2), and the angles between the rays and the normal are equal to each other: the angle of incidence i is equal to the angle of reflection i. " polished metal surfaces (mirrors), known already in a very distant era.
Rice. 2 The law of reflection. Rice. 3 The law of refraction. The law of refraction of light.
Refraction of light - a change in the direction of propagation of optical radiation (light) when it passes through the interface of homogeneous isotropic transparent (non-absorbing) media with refractive indices n 1 and n 2. Refraction of light is determined by the following two laws: the refracted ray lies in the plane passing through the incident ray and the normal (perpendicular) to the interface; angles of incidence φ and refraction χ (Fig. 3) are related Snell's law of refraction:| |
Reflection of light in any mirrors.
SPHERICAL MIRRORS
Proceeding from the law of reflection, it is also possible to solve problems about crooked mirrors, not only those that are hung in the room of laughter, but about spherical mirrors used in transport, in flashlights and searchlights, the mirror of engineer Garin's hyperboloid.
In fig. 3, 4 show examples of constructing an image of an object in the form of an arrow in concave and convex spherical mirrors. Imaging techniques are similar to those used for thin lenses. So, for example, a parallel beam of rays incident on a concave mirror is collected at one point - the focus, which is located at the focal distance f from the lens, equal to half the radius of curvature R of the mirror.
Rice. 3. Construction of an image in a concave spherical mirror
In a concave mirror, the actual image is inverted, it can be enlarged or reduced depending on the distance between the object and the mirror, and the imaginary one is direct and enlarged, as in a collecting lens. In a convex mirror, the image is always imaginary, direct and reduced, as in a diffusing lens.
Rice. 4. Construction of an image in a convex spherical mirror
For spherical mirrors, a formula similar to that of a thin lens applies:
1 / a + 1 / b = 1 / f = 2 / R,
1 / a-1 / b = -1 / f = -2 / R,
where a and b are the distances from the object and image to the lens. The first of these formulas is true for a concave mirror, the second for a convex one.
ELLIPTIC MIRROR
The parabolic mirror is the main element 
telescopes-reflectors
With the help of such telescopes, it is possible to study the most remote corners of the universe.
Spiral galaxies in the constellation Andromeda.
To locate the planets of the solar system, radars are used, which are based on 
parabolic mirror.
Radar makes it possible to "probe" the surface relief of planets, even shrouded in thick clouds, through which the surface is not visible through an ordinary telescope.
Radar map of Venus.
FLAT MIRROR
Flat mirrors are used in a device such as a periscope.
Periscope
(from the Greek periskopéo - look around, inspect), an optical device for observation from shelters (trenches, dugouts, etc.), tanks, submarines. Many detectors allow you to measure horizontal and vertical angles on the ground and determine the distance to the observed objects. The device and optical characteristics of the P. are determined by its purpose, the place of installation, and the depth of the shelter from which the observation is conducted. The simplest is a vertical periscope, consisting of a vertical telescope and 2 mirrors installed at an angle of 45 ° to the axis of the tube and forming an optical system that refracts light rays coming from the observed object and directs them into the eye of the observer. Prismatic periscopes are widespread, in the tube of which, instead of mirrors, rectangular prisms are installed, as well as a telescopic lens system and an inverting system, with the help of which an enlarged direct image can be obtained. The periscope field of view at low magnification (up to 1.5 times) is about 40 °; it usually decreases with increasing increase. Some types of periscope allow all-round visibility.
Optical layout of the periscope
For the first time, the prototype of the periscope was used by Iosif Nikolaevich Livchak. Livchak Iosif Nikolaevich, Russian inventor in the field of printing, military affairs and transport. From 1863 he lived in Vienna, where he published the satirical magazine "Strakhopud" (1863-68), and also participated in the publication of the magazines "Golden Letter" (1864-1868) and "Slavyanskaya Zarya" (1867-68). L. called for the liberation of the Slavic lands from the rule of Austria-Hungary and their unification around Russia. In the early 70s. moved to Russia, where he took up inventive activity. He created a matrix-punching typesetting machine, which in 1875 was used to type the newspaper "Vilensky Vestnik". He invented a sighting machine (1886), an optical device, a diascope (a prototype of a periscope), awarded with a large gold medal of the Paris Academy. Constructed an indicator of the path and speed of the locomotive; for this work, the Russian Technical Society was awarded the gold medal. A.P. Borodin (1903).
Conclusion.
Having studied the scientific literature and created my own model of the periscope, I believe that I managed to achieve my goals.
I also believe that it is very important to know and apply knowledge about reflection in a flat mirror in everyday life. Now I am much better at reflecting light. Now it will be much easier for me to study the topic "Optics" in the 11th grade.
Bibliography.
1. Myakishev G.Ya. Physics: Textbook for 11th grade. OU - M.: Education, 2004.
2. Pinsky A.A. Physics. In-depth study of physics: textbook. allowance. - M.: Education, 1994.
3. Khilkevich S.S. Physics around us. - M .: Nauka, 1985
4. Sivukhin D.V. General course of physics. Optics. - M .: Nauka, 1980
5. Educational reference book for the student. - Moscow, Bustard, 2005
6.http: //www.edu.yar.ru:8100/~pcollege/discover/99/s8/1b.html
The laws of reflection and refraction of light. Total internal light reflection
The laws of light reflection were found experimentally as early as the 3rd century BC by the ancient Greek scientist Euclid. Also, these laws can be obtained as a consequence of the Huygens principle, according to which each point of the medium, to which the disturbance has reached, is a source of secondary waves. The wave surface (wave front) at the next moment is a tangent surface to all secondary waves. Huygens principle is purely geometric.
A plane wave is incident on a smooth reflective surface of a CM (Fig. 1), that is, a wave whose wave surfaces are stripes.
Rice. 1 Construction of Huygens.
А 1 А and В 1 В - rays of the incident wave, АС - wave surface of this wave (or wave front).
Bye wave front from point C it will move in time t to point B, from point A a secondary wave will propagate along the hemisphere at a distance AD = CB, since AD = vt and CB = vt, where v is the wave propagation speed.
The wave surface of the reflected wave is the straight line BD tangent to the hemispheres. Further, the wave surface will move parallel to itself in the direction of the reflected beams AA 2 and BB 2.
Rectangular triangles ΔACB and ΔADB have a common hypotenuse AB and equal legs AD = CB. Therefore, they are equal.
The angles CAB = α and DBA = γ are equal, because they are angles with mutually perpendicular sides. And from the equality of triangles it follows that α = γ.
It also follows from the construction of Huygens that the incident and reflected rays lie in the same plane with the perpendicular to the surface, reconstructed at the point of incidence of the ray.
The laws of reflection are valid for the opposite direction of the path of the light rays. Due to the reversibility of the path of light rays, we have that a ray propagating along the path of the reflected one is reflected along the path of the incident one.
Most bodies only reflect the radiation incident on them, without being a source of light. Illuminated objects are visible from all sides, since light is reflected from their surface in different directions, scattering.
This phenomenon is called diffuse reflection or diffuse reflection... Diffuse reflection of light (Fig. 2) occurs from all rough surfaces. To determine the path of the reflected ray of such a surface, a plane tangent to the surface is drawn at the point of incidence of the ray, and the angles of incidence and reflection are plotted in relation to this plane.
Rice. 2. Diffuse reflection of light.
For example, 85% of white light is reflected from the surface of the snow, 75% from white paper, 0.5% from black velvet. Diffuse reflection of light does not cause unpleasant sensations in the human eye, unlike mirror reflection.
Specular light reflection- this is when the rays of light falling on a smooth surface at a certain angle are reflected mainly in one direction (Fig. 3.). The reflective surface in this case is called mirror(or mirror surface). Mirror surfaces can be considered optically smooth if the dimensions of irregularities and inhomogeneities on them do not exceed the wavelength of the light wave (less than 1 μm). For such surfaces, the law of light reflection is fulfilled.
Rice. 3. Specular reflection of light.
Flat mirror Is a mirror, the reflecting surface of which is a plane. A flat mirror makes it possible to see objects in front of it, and these objects seem to be located behind the mirror plane. In geometric optics, each point of the light source S is considered the center of the diverging beam of rays (Fig. 4.). Such a beam of rays is called homocentric... An image of a point S in an optical device is called the center S 'of a homocentric reflected and refracted beam of rays in various media. If the light scattered by the surfaces of various bodies hits a flat mirror, and then, reflected from it, falls into the eye of the observer, then images of these bodies are visible in the mirror.
Rice. 4. An image produced by a flat mirror.
The image S 'is called real if at the point S 1 the reflected (refracted) rays of the beam themselves intersect. An image S 1 is called imaginary if it is not the reflected (refracted) rays themselves that intersect, but their extensions. Light energy does not enter this point. In fig. 4 shows the image of the luminous point S, which appears with the help of a flat mirror.
The SO beam falls on the CM mirror at an angle of 0 °, therefore, the reflection angle is 0 °, and this beam, after reflection, follows the path OS. From the whole set of rays falling from point S onto a flat mirror, we select a ray SO 1.
The SO 1 beam falls on the mirror at an angle α and is reflected at an angle γ (α = γ). If we continue the reflected rays behind the mirror, they will converge at point S 1, which is an imaginary image of point S in a flat mirror. Thus, it seems to a person that the rays go out from the point S 1, although in fact there are no rays that go out from this point and enter the eye. The image of point S 1 is located symmetrically to the most luminous point S relative to the CM mirror. Let's prove it.
Beam SB, incident on the mirror at an angle of 2 (Fig. 5.), according to the law of light reflection, is reflected at an angle of 1 = 2.
Rice. 5. Reflection from a flat mirror.
From fig. 1.8 it can be seen that angles 1 and 5 are equal - as vertical. The sums of the angles are 2 + 3 = 5 + 4 = 90 °. Therefore angles 3 = 4 and 2 = 5.
Rectangular triangles ΔSOB and ΔS 1 OB have a common leg OB and equal acute angles 3 and 4, therefore, these triangles are equal in side and two angles adjacent to the leg. This means that SO = OS 1, that is, point S 1 is located symmetrically to point S relative to the mirror.
In order to find the image of the object AB in a flat mirror, it is enough to lower the perpendiculars from the extreme points of the object to the mirror and, extending them beyond the mirror, postpone behind it a distance equal to the distance from the mirror to the extreme point of the object (Fig. 6.). This image will be ghost and life size. The sizes and relative position of objects are preserved, but at the same time in the mirror the left and right sides of the image are reversed in comparison with the object itself. The parallelism of light rays incident on a flat mirror after reflection is also not violated.
Rice. 6. Image of an object in a flat mirror.
In technology, mirrors with a complex curved reflective surface are often used, for example, spherical mirrors. Spherical mirror Is the surface of the body, which has the shape of a spherical segment and reflects light in a specular way. The parallelism of the rays when reflected from such surfaces is violated. The mirror is called concave if the rays are reflected from the inner surface of the spherical segment.
Parallel light rays after reflection from such a surface are collected at one point, therefore a concave mirror is called collecting... If the rays are reflected from the outer surface of the mirror, then it will convex... Parallel light beams are scattered in different directions, therefore convex mirror are called scattering.
| Refraction At the interface between two media, the incident light flux is divided into two parts: one part is reflected, the other is refracted. | |
| W. Snell (Snell) before H. Huygens and I. Newton in 1621 experimentally discovered the law of refraction of light, but did not receive a formula, but expressed it in the form of tables, since by this time, the functions sin and cos were not yet known in mathematics. | |
| Refraction of light obeys the law: 1. The incident ray and the refracted ray lie in the same plane with the perpendicular, which is located at the point of incidence of the ray to the interface between the two media. 2. The ratio of the sine of the angle of incidence to the sine of the angle of refraction for these two media is a constant value (for monochromatic light). |  |
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| Refraction is caused by the difference in wave propagation velocities in different media. | |
| A quantity equal to the ratio of the speed of light in a vacuum to the speed of light in a given medium is called the absolute refractive index of the medium. This is a tabular value - a characteristic of a given environment. | |
A value equal to the ratio of the speed of light in one medium to the speed of light in another is called the relative refractive index of the second medium relative to the first.  | |
| Proof of the law of refraction. Propagation of incident and refracted rays: MM "- the interface between two media. Beams А 1 А and В 1 В - incident rays; α - angle of incidence; AC - wave surface at the moment when ray А 1 А reaches the interface between the media. using the Huygens principle, we construct a wave surface at the moment when ray B 1 B reaches the interface between the media. Let us construct refracted rays AA 2 and BB 2. β is the angle of refraction. AB is the common side of triangles ABC and ABD. Because rays and wave surfaces are mutually are perpendicular, then the angle ABD = α and the angle BAC = β. Then we get: |
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| In a prism or plane-parallel plate, refraction occurs on each face in accordance with the law of refraction of light. Remember that there is always a reflection. In addition, the actual path of the rays depends on both the refractive index and the refractive angle - the angle at the apex of the prism.) |   |
| Total reflection If light falls from an optically denser medium into an optically less dense one, then at a certain angle of incidence for each medium, the refracted ray disappears. Only refraction is observed. This phenomenon is called total internal reflection. |  |
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The angle of incidence, which corresponds to the angle of refraction of 90 °, is called the limiting angle of total internal reflection (a 0). From the law of refraction it follows that when light passes from any medium to vacuum (or air)  |  |
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| If we try to look from under the water at what is in the air, then at a certain value of the angle at which we look, we can see the bottom reflected from the surface of the water. It is important to take this into account in order not to lose orientation. | ||
| In jewelry, the cut of stones is selected so that full reflection is observed on each face. This explains the "play of stones". | ||
| The phenomenon of the mirage is also explained by complete internal reflection. |  |
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Everything that we see in the surrounding space either emits light or reflects it.
Radiated color
Is the light emitted by an active source. Examples of such sources are the sun, a light bulb, or a monitor screen. Their action is usually based on heating metal bodies or chemical or thermonuclear reactions. The color of any emitter depends on the spectral composition of the radiation. If the source emits light waves in the entire visible range, then its color will be perceived by our eyes as white. The predominance in its spectral composition of wavelengths of a certain range (for example, 400 - 450 nm) will give us a feeling of the dominant color in it (in this case, blue-violet). Finally, the presence in the emitted light of light components from different regions of the visible spectrum (for example, red and green) gives us the perception of the resulting color (in this case, yellow). But at the same time, in any case, the emitted color entering our eye retains all the colors from which it was created.
Reflected light
arises when some object (or rather, its surface) reflects light waves incident on it from a light source. The mechanism of color reflection depends on the color type of the surface, which can be roughly divided into two groups:
· Achromatic;
· Chromatic.
The first group consists of achromatic (otherwise colorless) colors: black, white and all gray (from the darkest to the lightest) (Fig. 4). They are often referred to as neutral. In the extreme case, such surfaces either reflect all rays incident on them without absorbing anything (ideally white surface), or completely absorb the rays without reflecting anything (ideal black surface). All other variants (gray surfaces) absorb light waves of different lengths evenly. The color reflected from them does not change its spectral composition, only its intensity changes.
The second group is formed by surfaces painted in chromatic colors, which reflect light with different wavelengths in different ways. So, if you light a piece of green paper with white, the paper will look green, because its surface absorbs all light waves except for the green component of white. What happens if you shine red or blue on green paper? The paper will be perceived as black, because it does not reflect the red and blue colors falling on it. If you shine green light on a green object, it will make it stand out from the surrounding objects of a different color.
The process of light reflection is accompanied not only by the associated absorption process in the near-surface layer. In the presence of translucent objects, part of the incident light passes through them (see Fig. 4). The effect of camera filters is based on this property, which cuts out the desired color range from the visible spectrum (otherwise, they cut off the unwanted color spectrum).
Rice. 4 Mechanisms of reflection by surfaces: a - green, b - yellow, c-white, d - black surfaces
To better understand this effect, press a plate of colored plexiglass against the surface of the light bulb. As a result, our eye "sees" the color that has not been absorbed by the plastic.
Each object has spectral characteristics of reflection and transmission. These characteristics determine how an object reflects and transmits light at certain wavelengths (Figure 5).
Spectral reflection curve
is determined by measuring the reflected light when illuminating an object with a standard source.
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At the interface between two different media, if this interface significantly exceeds the wavelength, a change in the direction of propagation of light occurs: part of the light energy returns to the first medium, that is reflected, and part of it penetrates into the second environment and at the same time refracts... Ray AO is called incident ray, and ray OD - reflected beam(see fig. 1.3). The relative position of these rays is determined laws of reflection and refraction of light.
Rice. 1.3. Reflection and refraction of light.
The angle α between the incident ray and the perpendicular to the interface, restored to the surface at the point of incidence of the ray, is called angle of incidence.
The angle γ between the reflected beam and the same perpendicular is called reflection angle.
Each environment to a certain extent (that is, in its own way) reflects and absorbs light radiation. The quantity that characterizes the reflectivity of the surface of a substance is called reflection coefficient... The reflection coefficient shows what part of the energy brought by the radiation to the surface of the body is the energy carried away from this surface by the reflected radiation. This coefficient depends on many reasons, for example, on the composition of the radiation and on the angle of incidence. Light is completely reflected from a thin film of silver or liquid mercury deposited on a sheet of glass.
Light reflection laws
The laws of light reflection were found experimentally as early as the 3rd century BC by the ancient Greek scientist Euclid. Also, these laws can be obtained as a consequence of the Huygens principle, according to which each point of the medium, to which the disturbance has reached, is a source of secondary waves. The wave surface (wave front) at the next moment is a tangent surface to all secondary waves. Huygens principle is purely geometric.
A plane wave is incident on a smooth reflective surface of a CM (Fig. 1.4), that is, a wave whose wave surfaces are stripes.
Rice. 1.4. Construction of Huygens.
А 1 А and В 1 В - rays of the incident wave, АС - wave surface of this wave (or wave front).
Bye wave front from point C it will move in time t to point B, from point A a secondary wave will propagate along the hemisphere at a distance AD = CB, since AD = vt and CB = vt, where v is the wave propagation speed.
The wave surface of the reflected wave is the straight line BD tangent to the hemispheres. Further, the wave surface will move parallel to itself in the direction of the reflected beams AA 2 and BB 2.
Rectangular triangles ΔACB and ΔADB have a common hypotenuse AB and equal legs AD = CB. Therefore, they are equal.
The angles CAB = = α and DBA = = γ are equal, because they are angles with mutually perpendicular sides. And from the equality of triangles it follows that α = γ.
It also follows from the construction of Huygens that the incident and reflected rays lie in the same plane with the perpendicular to the surface, reconstructed at the point of incidence of the ray.
The laws of reflection are valid for the opposite direction of the path of the light rays. As a consequence of the reversibility of the path of light rays, we have that a ray propagating along the path of the reflected one is reflected along the path of the incident one.
Most bodies only reflect the radiation incident on them, without being a source of light. Illuminated objects are visible from all sides, since light is reflected from their surface in different directions, scattering. This phenomenon is called diffuse reflection or diffuse reflection... Diffuse reflection of light (Fig. 1.5) occurs from all rough surfaces. To determine the path of the reflected ray of such a surface, a plane tangent to the surface is drawn at the point of incidence of the ray, and the angles of incidence and reflection are plotted in relation to this plane.
Rice. 1.5. Diffuse reflection of light.
For example, 85% of white light is reflected from the surface of the snow, 75% from white paper, 0.5% from black velvet. Diffuse reflection of light does not cause unpleasant sensations in the human eye, unlike mirror reflection.
- this is when the rays of light falling on a smooth surface at a certain angle are reflected mainly in one direction (Fig. 1.6). The reflective surface in this case is called mirror(or mirror surface). Mirror surfaces can be considered optically smooth if the dimensions of irregularities and inhomogeneities on them do not exceed the wavelength of the light wave (less than 1 μm). For such surfaces, the law of light reflection is fulfilled.
Rice. 1.6. Specular reflection of light.
Flat mirror Is a mirror, the reflecting surface of which is a plane. A flat mirror makes it possible to see objects in front of it, and these objects seem to be located behind the mirror plane. In geometric optics, each point of the light source S is considered the center of the diverging beam of rays (Fig. 1.7). Such a beam of rays is called homocentric... An image of a point S in an optical device is called the center S 'of a homocentric reflected and refracted beam of rays in various media. If the light scattered by the surfaces of various bodies hits a flat mirror, and then, reflected from it, falls into the eye of the observer, then images of these bodies are visible in the mirror.
Rice. 1.7. An image produced by a flat mirror.
The image S 'is called valid if at the point S' the reflected (refracted) rays of the beam themselves intersect. An image S 'is called imaginary if it is not the reflected (refracted) rays themselves that intersect, but their extensions. Light energy does not enter this point. In fig. 1.7 shows the image of the luminous point S, which appears with the help of a flat mirror.
The SO beam falls on the CM mirror at an angle of 0 °, therefore, the reflection angle is 0 °, and this beam, after reflection, follows the path OS. From the whole set of rays falling from point S onto a flat mirror, we select a ray SO 1.
The SO 1 beam falls on the mirror at an angle α and is reflected at an angle γ (α = γ). If we continue the reflected rays behind the mirror, they will converge at point S 1, which is an imaginary image of point S in a flat mirror. Thus, it seems to a person that the rays go out from the point S 1, although in fact there are no rays that go out from this point and enter the eye. The image of point S 1 is located symmetrically to the most luminous point S relative to the CM mirror. Let's prove it.
The SB beam, incident on the mirror at an angle of 2 (Fig. 1.8), according to the law of light reflection, is reflected at an angle of 1 = 2.
Rice. 1.8. Reflection from a flat mirror.
From fig. 1.8 it can be seen that angles 1 and 5 are equal - as vertical. The sums of the angles are 2 + 3 = 5 + 4 = 90 °. Therefore angles 3 = 4 and 2 = 5.
Rectangular triangles ΔSOB and ΔS 1 OB have a common leg OB and equal acute angles 3 and 4, therefore, these triangles are equal in side and two angles adjacent to the leg. This means that SO = OS 1, that is, point S 1 is located symmetrically to point S relative to the mirror.
In order to find the image of the object AB in a flat mirror, it is enough to lower the perpendiculars from the extreme points of the object to the mirror and, continuing them outside the mirror, postpone behind it a distance equal to the distance from the mirror to the extreme point of the object (Fig. 1.9). This image will be ghost and life size. The sizes and relative position of objects are preserved, but at the same time in the mirror the left and right sides of the image are reversed in comparison with the object itself. The parallelism of light rays incident on a flat mirror after reflection is also not violated.
Rice. 1.9. An image of an object in a flat mirror.
In technology, mirrors with a complex curved reflective surface are often used, for example, spherical mirrors. Spherical mirror Is the surface of the body, which has the shape of a spherical segment and reflects light in a specular way. The parallelism of the rays when reflected from such surfaces is violated. The mirror is called concave if the rays are reflected from the inner surface of the spherical segment. Parallel light rays after reflection from such a surface are collected at one point, therefore a concave mirror is called collecting... If the rays are reflected from the outer surface of the mirror, then it will convex... Parallel light beams are scattered in different directions, therefore convex mirror are called scattering.