Most of the objects around you - houses, trees, your classmates, etc. - are not sources of light. But you see them. The answer to the question "Why so?" you will find in this paragraph.

Rice. 11.1. In the absence of a light source, nothing can be seen. If there is a light source, we see not only the source itself, but also objects that reflect the light coming from the source.

Finding out why we see bodies that are not light sources

You already know that light travels in a straight line in a homogeneous transparent medium.

But what happens if there is some body in the path of the light beam? Part of the light can pass through the body if it is transparent, part will be absorbed, and part will be reflected from the body. Some of the reflected rays will hit our eyes, and we will see this body (Fig. 11.1).

Establishing the laws of light reflection

To establish the laws of light reflection, we will use a special device - an optical washer*. We fix a mirror in the center of the washer and direct a narrow beam of light at it so that it gives a light strip on the surface of the washer. We see that the beam of light reflected from the mirror also gives a light strip on the surface of the washer (see Fig. 11.2).

The direction of the incident light beam will be set by the CO beam (Fig. 11.2). This beam is called the incident beam. The direction of the reflected beam of light will be set by the beam OK. This ray is called the reflected ray.

From the point O of incidence of the beam, we draw a perpendicular OB to the surface of the mirror. Let us pay attention to the fact that the incident ray, the reflected ray and the perpendicular lie in the same plane - in the plane of the washer surface.

The angle α between the incident beam and the perpendicular drawn from the point of incidence is called the angle of incidence; the angle β between the reflected ray and the given perpendicular is called the angle of reflection.

By measuring the angles α and β, we can verify that they are equal.

If you move the light source along the edge of the disk, the angle of incidence of the light beam will change and the angle of reflection will change accordingly, and each time the angle of incidence and the angle of reflection of the light will be equal (Fig. 11.3). So, we have established the laws of light reflection:

Rice. 11.3. As the angle of incidence of light changes, the angle of reflection also changes. The angle of reflection is always equal to the angle of incidence

Rice. 11.5. Demonstration of the reversibility of light rays: the reflected beam follows the path of the incident beam

rice. 11.6. Approaching the mirror, we see our “double” in it. Of course, there is no “double” there - we see our reflection in the mirror

1. The incident beam, the reflected beam and the perpendicular to the reflection surface, drawn from the point of incidence of the beam, lie in the same plane.

2. The angle of reflection is equal to the angle of incidence: β = α.

The laws of light reflection were established by the ancient Greek scientist Euclid as early as the 3rd century BC. BC e.

In what direction should the professor turn the mirror in order to " sunbeam»hit the boy (Fig. 11.4)?

Using a mirror on an optical washer, one can also demonstrate the reversibility of light rays: if the incident beam is directed along the path of the reflected one, then the reflected the beam will go along the path of the falling person (Fig. 11.5).

We study the image in a flat mirror

Consider how an image is created in a flat mirror (Fig. 11.6).

Let a divergent beam of light fall from a point light source S onto the surface of a flat mirror. From this beam we select the rays SA, SB and SC. Using the laws of light reflection, we construct the reflected rays LL b BB 1 and CC 1 (Fig. 11.7, a). These rays will go in a divergent beam. If you extend them in the opposite direction (behind the mirror), they will all intersect at one point - S 1 located behind the mirror.

If some of the rays reflected from the mirror enter your eye, it will seem to you that the reflected rays come from point S 1, although in reality there is no light source at point S 1. Therefore, the point S 1 is called the imaginary image of the point S. A flat mirror always gives a virtual image.

Find out how the object and its image are located relative to the mirror. To do this, we turn to geometry. Consider, for example, a ray SC that falls on a mirror and is reflected from it (Fig. 11.7, b).

From the figure we see that Δ SOC = Δ S 1 OC are right-angled triangles having a common side CO and equal acute angles (because according to the law of light reflection α = β). From the equality of triangles, we have that SO \u003d S 1 O, that is, the point S and its image S 1 are symmetrical with respect to the surface of a flat mirror.

The same can be said about the image of an extended object: the object and its image are symmetrical with respect to the surface of a flat mirror.

So, we have installed General characteristics images in flat mirrors.

1. A flat mirror gives a virtual image of an object.

2. The image of an object in a flat mirror and the object itself are symmetrical with respect to the surface of the mirror, and this means:

1) the image of the object is equal in size to the object itself;

2) the image of the object is located at the same distance from the surface of the mirror as the object itself;

3) the segment connecting the point on the object and the corresponding point on the image is perpendicular to the surface of the mirror.

Distinguish between specular and diffuse reflection of light

In the evening, when the light is on in the room, we can see our image in window glass. But the image disappears if the curtains are drawn: we will not see our image on the fabric. And why? The answer to this question is related to at least two physical phenomena.

The first such physical phenomenon is the reflection of light. In order for an image to appear, the light must be reflected from the surface in a specular manner: after the specular reflection of the light coming from a point source S, the continuation of the reflected rays will intersect at one point S 1, which will be the image of the point S (Fig. 11.8, a). Such reflection is possible only from very smooth surfaces. They are called so - mirror surfaces. In addition to the usual mirror, examples of mirror surfaces are glass, polished furniture, calm water surface, etc. (Fig. 11.8, b, c).

If light is reflected from a rough surface, such a reflection is called scattered (diffuse) (Fig. 11.9). In this case, the reflected rays propagate in different directions (which is why we see the illuminated object from any direction). It is clear that there are much more surfaces that scatter light than mirror ones.

Look around and name at least ten surfaces that reflect light diffusely.

Rice. 11.8. Specular reflection of light is the reflection of light from a smooth surface.

Rice. 11.9. Scattered (diffuse) reflection of light is the reflection of light from a rough surface

The second physical phenomenon that affects the ability to see an image is the absorption of light. After all, light is not only reflected from physical bodies, but also absorbed by them. The best light reflectors are mirrors: they can reflect up to 95% of the incident light. Bodies are good reflectors of light. white color, but the black surface absorbs almost all the light falling on it.

When snow falls in autumn, the nights become much lighter. Why? Learning to solve problems

Task. On fig. 1 schematically shows the object BC and the mirror NM. Find graphically the area from which the image of the object BC is completely visible.

Analysis of a physical problem. In order to see the image of a certain point of an object in a mirror, it is necessary that at least part of the rays falling from this point onto the mirror be reflected into the observer's eye. It is clear that if the rays emanating from the extreme points of the object are reflected into the eye, then the rays emanating from all points of the object are also reflected into the eye.

Solution, analysis of results

1. Let's construct point B 1 - the image of point B in a flat mirror (Fig. 2, a). The area bounded by the surface of the mirror and the rays reflected from the extreme points of the mirror will be the area from which the image B 1 of point B in the mirror is visible.

2. Having similarly constructed the image C 1 of point C, we determine the area of ​​its vision in the mirror (Fig. 2, b).

3. The observer can see the image of the entire object only if the rays that give both images - B 1 and C 1 (Fig. 2, c) enter his eye. Hence, the area highlighted in Fig. 2, in orange, is the area from which the image of the object is completely visible.

Analyze the result obtained, once again consider Fig. 2 to the problem and offer an easier way to find the area of ​​vision of an object in a flat mirror. Check your assumptions by plotting the field of view of several objects in two ways.

Summing up

All visible bodies reflect light. When light is reflected, two laws of light reflection are fulfilled: 1) the incident beam, the reflected beam and the perpendicular to the reflection surface, drawn from the point of incidence of the beam, lie in the same plane; 2) the angle of reflection is equal to the angle of incidence.

The image of an object in a flat mirror is imaginary, equal in size to the object itself and located at the same distance from the mirror as the object itself.

Distinguish between specular and diffuse reflections of light. In the case of specular reflection, we can see a virtual image of an object in a reflective surface; in the case of diffuse reflection, no image appears.

Control questions

1. Why do we see surrounding bodies? 2. What angle is called the angle of incidence? reflection angle? 3. Formulate the laws of light reflection. 4. What device can be used to verify the validity of the laws of light reflection? 5. What is the property of reversibility of light rays? 6. In what case is the image called imaginary? 7. Describe the image of an object in a flat mirror. 8. How is diffuse reflection of light different from specular?

Exercise number 11

1. A girl stands at a distance of 1.5 m from a flat mirror. How far is her reflection from the girl? Describe it.

2. The driver of the car, looking in the rearview mirror, saw a passenger sitting on back seat. Can the passenger at this moment, looking in the same mirror, see the driver?

3. Transfer the pic. 1 in a notebook, for each case construct an incident (or reflected) ray. Label the angles of incidence and reflection.

4. The angle between the incident and reflected rays is 80°. What is the angle of incidence of the beam?

5. The object was at a distance of 30 cm from a flat mirror. Then the object was moved 10 cm from the mirror in a direction perpendicular to the surface of the mirror, and 15 cm parallel to it. What was the distance between the object and its reflection? What did it become?

6. You are moving towards the mirror shop window at a speed of 4 km/h. How fast is your reflection approaching you? By how much will the distance between you and your reflection decrease when you walk 2 m?

7. A sunbeam is reflected from the surface of the lake. The angle between the incident ray and the horizon is twice as large as the angle between the incident and reflected rays. What is the angle of incidence of the beam?

8. The girl looks into a mirror hanging on the wall at a slight angle (Fig. 2).

1) Build the reflection of the girl in the mirror.

2) Find graphically what part of her body the girl sees; the area from which the girl sees herself completely.

3) What changes will be observed if the mirror is gradually covered with an opaque screen?

9. At night, in the light of car headlights, a puddle on the pavement seems to the driver dark spot on a lighter road background. Why?

10. In fig. 3 shows the path of the rays in the periscope - a device whose operation is based on the rectilinear propagation of light. Explain how this device works. Use additional sources of information and find out where it is used.

LAB #3

Subject. Investigation of light reflection using a flat mirror.

Purpose: experimentally check the laws of light reflection.

equipment: a light source (a candle or an electric lamp on a stand), a flat mirror, a screen with a slit, several blank white sheets of paper, a ruler, a protractor, a pencil.

instructions for work

preparation for the experiment

1. Before doing work, remember: 1) safety requirements when working with glass objects; 2) laws of reflection of light.

2. Assemble the experimental setup (Fig. 1). For this:

1) install the screen with a slot on a white sheet of paper;

2) by moving the light source, get a strip of light on paper;

3) place a flat mirror at a certain angle to the strip of light and perpendicular to the sheet of paper so that the reflected beam of light also gives a clearly visible strip on the paper.

Experiment

Strictly follow the safety instructions (see the flyleaf of the textbook).

1. With a well-sharpened pencil, draw a line along the mirror on paper.

2. Put three points on a sheet of paper: the first one is in the middle of the incident light beam, the second one is in the middle of the reflected light beam, the third one is in the place where the light beam hits the mirror (Fig. 2).

3. Repeat the above steps a few more times (on different sheets paper), setting the mirror at different angles to the incident light beam.

4. By changing the angle between the mirror and the sheet of paper, make sure that in this case you will not see the reflected beam of light.

Processing the results of the experiment

For each experience:

1) build the beam incident on the mirror and the reflected beam;

2) through the point of incidence of the beam, draw a perpendicular to the line drawn along the mirror;

3) Label and measure the angle of incidence (α) and the angle of reflection (β) of the light. Enter the measurement results in the table.

Analysis of the experiment and its results

Analyze the experiment and its results. Make a conclusion in which indicate: 1) what is the ratio between the angle of incidence of the light beam and the angle of its reflection you have set; 2) whether the results of the experiments turned out to be absolutely accurate, and if not, what are the reasons for the error.

creative task

Using fig. 3, think over and write down a plan for conducting an experiment to determine the height of a room using a flat mirror; indicate the required equipment.

Experiment if possible.

Task "with an asterisk"

At the interface between two different media, if this interface significantly exceeds the wavelength, there is a change in the direction of light propagation: part of the light energy returns to the first medium, that is reflected, and part penetrates into the second medium and at the same time refracted. The AO beam is called incident beam, and the ray OD is reflected beam(see fig. 1.3). The mutual arrangement of these rays is determined by laws of reflection and refraction of light.

Rice. 1.3. Reflection and refraction of light.

The angle α between the incident beam and the perpendicular to the interface, restored to the surface at the point of incidence of the beam, is called angle of incidence.

The angle γ between the reflected ray and the same perpendicular is called reflection angle.

Each medium to a certain extent (that is, in its own way) reflects and absorbs light radiation. The value that characterizes reflectivity the surface of matter is called reflection coefficient. The reflection coefficient shows what part of the energy brought by radiation to the surface of a body is the energy carried away from this surface by reflected radiation. This coefficient depends on many factors, for example, on the composition of the radiation and on the angle of incidence. Light is completely reflected from thin film silver or liquid mercury deposited on a sheet of glass.

Laws of light reflection

The laws of light reflection were found experimentally back in 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 perturbation 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 falls on a smooth reflective surface of the CM (Fig. 1.4), that is, a wave whose wave surfaces are strips.

Rice. 1.4. Huygens construction.

A 1 A and B 1 B are the rays of the incident wave, AC is the wave surface of this wave (or the wave front).

Bye wave front from point C it will move in time t to point B, from point A the secondary wave will propagate along the hemisphere to the distance AD ​​= CB, since AD ​​= vt and CB = vt, where v is the speed of wave propagation.

The wave surface of the reflected wave is a 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 .

Right triangles ΔACB and ΔADB have a common hypotenuse AB and equal legs AD = CB. Therefore, they are equal.

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 Huygens construction that the incident and reflected rays lie in the same plane with the perpendicular to the surface restored at the point of incidence of the ray.

The laws of reflection are valid for the reverse direction of the light rays. Due to the reversibility of the course 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, as 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 beam of such a surface, a plane tangent to the surface is drawn at the point of incidence of the beam, and the angles of incidence and reflection are plotted with respect 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 discomfort in the human eye, as opposed to the mirror.

- this is when 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 sizes of irregularities and inhomogeneities on them do not exceed the light wavelength (less than 1 μm). For such surfaces, the law of light reflection is satisfied.

Rice. 1.6. Mirror reflection of light.

flat mirror is a mirror whose reflecting surface 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. The image of a point S in an optical device is the center S' of a homocentric reflected and refracted beam of rays in various media. If light scattered by surfaces 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 real if the reflected (refracted) rays of the beam themselves intersect at the point S'. The image S' is called imaginary if it is not the reflected (refracted) rays themselves that intersect in it, but their continuations. Light energy does not enter this point. On fig. 1.7 shows the image of a luminous point S, which appears with the help of a flat mirror.

The beam SO falls on the mirror KM at an angle of 0°, therefore, the angle of reflection is 0°, and this beam after reflection follows the path OS. From the entire set of rays falling from point S to a flat mirror, we select the ray SO 1.

Beam SO 1 falls on the mirror at an angle α and is reflected at an angle γ (α = γ ). If we continue the reflected rays beyond the mirror, then they will converge at the point S 1, which is an imaginary image of the point S in a flat mirror. Thus, it seems to a person that the rays come out of the point S 1, although in reality there are no rays coming out of this point and entering the eye. The image of the point S 1 is located symmetrically to the most luminous point S relative to the KM mirror. Let's prove it.

The beam SB, incident on the mirror at an angle of 2 (Fig. 1.8), according to the law of reflection of light, 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 sum of the angles 2 + 3 = 5 + 4 = 90°. Therefore, angles 3 = 4 and 2 = 5.

Right-angled 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, the point S 1 is located symmetrically to the point S with respect to the mirror.

In order to find the image of an 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 beyond the mirror, set aside a distance behind it equal to the distance from the mirror to the extreme point of the object (Fig. 1.9). This image will be imaginary and in life size. The dimensions and relative position of objects are preserved, but at the same time, in the mirror, the left and right side the images are reversed in comparison with the object itself. The parallelism of light rays incident on a flat mirror after reflection is also not disturbed.

Rice. 1.9. Image of an object in a flat mirror.

In engineering, mirrors with a complex curved reflective surface, such as spherical mirrors, are often used. spherical mirror- this is the surface of the body, which has the shape of a spherical segment and reflects light specularly. The parallelism of the rays upon reflection from such surfaces is violated. The mirror is called concave if rays are reflected from inner surface spherical segment. Parallel light rays after reflection from such a surface are collected at one point, so a concave mirror is called gathering. If the rays are reflected from the outer surface of the mirror, then it will convex. Parallel light rays scatter in different sides, That's why convex mirror called scattering.

It should be noted that the image that we see on the other side of the mirror is not created by the rays themselves, but by their mental continuation. Such an image is called imaginary. It can be seen with the eye, but it is impossible to get it on the screen, since it was created not by rays, but by their mental continuation.

When reflecting, the principle of the shortest propagation time of light is also observed. In order to get after reflection into the eye of the observer, the light must come exactly the way that the law of reflection indicates to it. It is by propagating along such a path that the light will spend on its path least time of all possible options.

Law of refraction of light

As we already know, light can propagate not only in vacuum, but also in other transparent media. In this case, the light will experience refraction. When passing from a less dense medium to a denser one, the ray of light during refraction is pressed against the perpendicular drawn to the point of incidence, and when passing from a denser medium to a less dense one, it is vice versa: it deviates from the perpendicular.

There are two laws of refraction:

The incident ray, the refracted ray and the perpendicular drawn to the point of incidence lie in the same plane.

2. The ratio of the sines of the angles of incidence and refraction is equal to the inverse ratio of the refractive indices:

sin a = n2

sin g n1

Of interest is the passage of a beam of light through a trihedral prism. In this case, in any case, there is a deviation of the beam after passing through the prism from the original direction:

Different transparent bodies have different refractive indices. For gases, it differs very little from unity. With increasing pressure, it increases, therefore, the refractive index of gases also depends on temperature. Recall that if you look at distant objects through the hot air rising from the fire, we see that everything that is in the distance looks like a swaying haze. In liquids, the refractive index depends not only on the liquid itself, but also on the concentration of substances dissolved in it. Below is a small table of the refractive indices of some substances.

Total internal reflection of light.

fiber optics

It should be noted that the light beam, propagating in space, has the property of reversibility. This means that along which path the beam propagates from the source in space, it will follow the same path back if the source and the observation point are interchanged.

Imagine that a beam of light propagates from an optically denser medium to an optically less dense one. Then, according to the law of refraction, it must come out during refraction, deviating from the perpendicular. Consider the rays emanating from a point source of light located in an optically denser medium, for example, in water.

It can be seen from this figure that the first beam is incident on the interface perpendicularly. In this case, the beam from the original direction does not deviate. Often its energy is reflected from the interface and returned to the source. The rest of his energy goes out. The rest of the rays are partially reflected, partially go out. As the angle of incidence increases, so does the angle of refraction, which corresponds to the law of refraction. But when the angle of incidence takes such a value that, according to the law of refraction, the beam exit angle should be 90 degrees, then the beam will not reach the surface at all: all 100% of the beam energy will be reflected from the interface. All other rays incident on the interface at an angle greater than this will be completely reflected from the interface. This corner is called limiting angle, and the phenomenon is called total internal reflection. That is, the interface in this case acts as a perfect mirror. The value of the limiting angle for the boundary with vacuum or air can be calculated by the formula:

Sin apr = 1/n Here n is the refractive index of the denser medium.

The phenomenon of total internal reflection is widely used in various optical devices. In particular, it is used in a device for determining the concentration of dissolved substances in water (refractometer). There, the limiting angle of total internal reflection is measured, by which the refractive index is determined, and then the concentration of dissolved substances is determined from the table.

The phenomenon of total internal reflection is especially pronounced in fiber optics. The figure below shows one fiberglass in section:

Let's take a thin glass fiber and launch a beam of light into one of the ends. Since the fiber is very thin, any beam that enters the end of the fiber will fall on its side surface at an angle that significantly exceeds the limiting angle and will be completely reflected. Thus, the incoming beam will be repeatedly reflected from the side surface and will exit the opposite end with little or no loss. Outwardly, it will look as if the opposite end of the fiber glows brightly. In addition, it is not at all necessary that the fiberglass be straight. It can bend as you like, and no bends will affect the propagation of light through the fiber.

In this regard, scientists came up with the idea: what if we take not one fiber, but a whole bunch of them. But at the same time, it is necessary that all fibers in the bundle are in strict mutual order and on both sides of the bundle the ends of all fibers are in the same plane. And if, at the same time, an image is applied to one end of the bundle using a lens, then each fiber individually will transmit one small particle of the image to the opposite end of the bundle. All together, the fibers at the opposite end of the bundle will reproduce the same image that was created by the lens. Moreover, the image will be in natural light. Thus, a device was created, later named fibrogastroscope. With this device, you can examine the inner surface of the stomach without making surgical intervention. A fibrogastroscope is inserted through the esophagus into the stomach and the inner surface of the stomach is examined. In principle, this device can examine not only the stomach, but also other organs from the inside. This device is used not only in medicine, but also in various fields of technology to examine inaccessible areas. And at the same time, the harness itself can have all kinds of bends, which in this case do not affect the image quality in any way. The only drawback of this device is the raster structure of the image: that is, the image consists of individual dots. In order for the image to be sharper, you need to have even more glass fibers, and they must be even thinner. And this significantly increases the cost of the device. But with the further development of technical capabilities this problem will soon be resolved.

Lens

First, let's look at the lens. The lens is transparent body bounded either by two spherical surfaces or by a spherical surface and a plane.

Consider lenses in cross section. The lens bends the light beam passing through it. If the beam, after passing through the lens, will be collected at a point, then such a lens is called collecting. If the incident parallel light beam diverges after passing through the lens, then such a lens is called scattering.

Converging and diverging lenses and their conventions:

It can be seen from this figure that all rays incident parallel to the lens converge at one point. This point is called focus(F) lenses. The distance from the focus to the lens itself is called focal length lenses. It is measured in SI units in meters. But there is another unit that characterizes the lens. This value is called the optical power and is the reciprocal of the focal length and is called diopter. (Dp). Denoted by letter D. D = 1/F. For a converging lens, the optical power value has a plus sign. If the lens is exposed to light reflected from some extended object, then each element of the object will be displayed in the plane passing through the focus in the form of an image. This will invert the image. Since this image will be created by the rays themselves, it will be called valid.

This phenomenon is used in modern cameras. The actual image is created on photographic film.

A diverging lens acts in the opposite way to a converging lens. If a parallel beam of light falls on it along the normal, then after passing through the lens, the beam of light will diverge as if all the rays come out of some imaginary point located on the other side of the lens. This point is called the imaginary focus and the focal length will be with a minus sign. Hence, optical power such a lens will also be expressed in diopter, but its value will be with a minus sign. When viewing surrounding objects through a diverging lens, all objects visible through the lens will appear reduced in size.

Light is an important part of our life. Without it, life on our planet is impossible. At the same time, many phenomena that are associated with light are actively used today in various fields of human activity, from the production of electrical appliances to spacecraft. One of the fundamental phenomena in physics is the reflection of light.

reflection of light

The law of reflection of light is studied at school. What you need to know about him, and much more useful information our article can tell you.

Fundamentals of knowledge about light

As a rule, physical axioms are among the most understandable, since they have a visual manifestation that can be easily observed at home. The law of reflection of light implies a situation where light rays change direction when they collide with different surfaces.

Note! The boundary of refraction significantly increases such a parameter as the wavelength.

During the refraction of rays, part of their energy will return back to the primary medium. When some of the rays penetrate into another medium, their refraction is observed.
To understand all these physical phenomena, you need to know the relevant terminology:

• the flux of light energy in physics is defined as falling when it hits the interface between two substances;
• part of the energy of light, which in a given situation returns to the primary medium, is called reflected;

Note! There are several formulations of the reflection rule. No matter how you formulate it, it will still describe the relative position of the reflected and incident rays.

• angle of incidence. This refers to the angle that is formed between the perpendicular line of the media boundary and the light incident on it. It is determined at the point of incidence of the beam;

Beam angles

• reflection angle. It is formed between the reflected beam and the perpendicular line that was restored at the point of its incidence.

In addition, it is necessary to know that light can propagate in a homogeneous medium exclusively in a straight line.

Note! Different media can reflect and absorb light radiation in different ways.

This is where the reflection coefficient comes from. This is a value that characterizes the reflectivity of objects and substances. It means how much radiation brought by the light flux to the surface of the medium will be the energy that will be reflected from it. This ratio depends on a number of factors, including highest value have radiation composition and angle of incidence.
Full reflection of the light flux is observed when the beam falls on substances and objects that have a reflective surface. For example, the reflection of a beam can be observed when it hits glass, liquid mercury or silver.

A small historical excursion

The laws of refraction and reflection of light were formed and systematized as early as the 3rd century. BC e. They were designed by Euclid.

All laws (refraction and reflection) that relate to this physical phenomenon have been established experimentally and can easily be confirmed by Huygens' geometric principle. According to this principle, any point of the medium, to which a disturbance can reach, acts as a source of secondary waves.
Let's take a closer look at the laws that exist today.

Laws are the basis of everything

The law of reflection of the light flux is defined as a physical phenomenon, during which the light directed from one medium to another, at their section, will be partially returned back.

Reflection of light at the interface

The visual analyzer of a person observes light at the moment when the beam coming from its source enters the eyeball. In a situation where the body does not act as a source, the visual analyzer can perceive rays from another source that are reflected from the body. In this case, the light radiation incident on the surface of an object can change the direction of its further propagation. As a result, the body that reflects the light will act as its source. When reflected, part of the stream will return to the first medium from which it was originally directed. Here the body that reflects it will become the source of the already reflected flow.
There are several laws for this physical phenomenon:

• the first law says: the reflecting and incident beam, together with the perpendicular line that appears at the interface between the media, as well as at the restored point of incidence of the light flux, must be located in the same plane;

Note! This implies that a plane wave is incident on the reflective surface of an object or substance. Its wave surfaces are stripes.

First and second law

• second law. Its formulation is as follows: the angle of reflection of the light flux will be equal to the angle of incidence. This is due to the fact that they have mutually perpendicular sides. Taking into account the principles of equality of triangles, it becomes clear where this equality comes from. Using these principles, it is easy to prove that these angles are in the same plane as the perpendicular line drawn, which was restored at the boundary of the separation of two substances at the point of incidence of the light beam.

These two laws in optical physics are fundamental. Moreover, they are also valid for a beam that has a reverse motion. As a result of the reversibility of the beam energy, the flow propagating along the path of the previously reflected one will be reflected similarly to the path of the incident one.

The Law of Reflection in Practice

It is possible to verify the implementation of this law in practice. To do this, you need to direct a thin beam to any reflective surface. For this purpose, a laser pointer is perfect and ordinary mirror.

The effect of the law in practice

Aim the laser pointer at the mirror. As a result laser ray bounce off the mirror and propagate further into given direction. In this case, the angles of the incident and reflected beams will be equal even with a normal look at them.

Note! Light from such surfaces will be reflected at an obtuse angle and then propagate along a low path, which is located close enough to the surface. But the beam, which will fall almost vertically, will be reflected at an acute angle. At the same time, its further path will be almost similar to the falling one.

As we see, key point this rule is the fact that the angles must be measured from the perpendicular to the surface at the point of incidence of the light flux.

Note! This law obeys not only light, but also any kind of electromagnetic waves (microwave, radio, x-ray waves, etc.).

Features of diffuse reflection

Many objects can only reflect the light radiation incident on their surface. Well-lit objects are clearly visible from different directions, as their surface reflects and scatters light in different directions.

diffuse reflection

This phenomenon is called diffuse (diffuse) reflection. This phenomenon is formed when radiation hits various rough surfaces. Thanks to him, we are able to distinguish between objects that do not have the ability to emit light. If the scattering of light radiation is equal to zero, then we will not be able to see these objects.

Note! Diffuse reflection does not cause discomfort in a person.

The absence of discomfort is explained by the fact that not all the world, according to above rule, returns to the primary environment. Moreover, this parameter different surfaces will be different:

• near snow - about 85% of the radiation is reflected;
• for white paper - 75%;
• for black and velor - 0.5%.

If the reflection comes from rough surfaces, then the light will be directed towards each other randomly.

Mirroring Features

The specular reflection of light radiation differs from the previously described situations. This is due to the fact that as a result of the flow falling on a smooth surface at a certain angle, they will be reflected in the same direction.

Mirror reflection

This phenomenon can be easily reproduced using an ordinary mirror. When pointing the mirror at Sun rays, it will act as an excellent reflective surface.

Note! TO mirror surfaces can be attributed whole line tel. For example, this group includes all smooth optical objects. But such a parameter as the size of irregularities and inhomogeneities in these objects will be less than 1 micron. The wavelength of light is approximately 1 µm.

All such mirror reflective surfaces obey the previously described laws.

The use of law in technology

Today, mirrors or mirror objects with a curved reflective surface are often used in technology. These are the so-called spherical mirrors.
Such objects are bodies that have the shape of a spherical segment. Such surfaces are characterized by a violation of the parallelism of the rays.
On this moment There are two types of spherical mirrors:

• concave. They are able to reflect light radiation from the inner surface of their sphere segment. When reflected, the rays are collected here at one point. Therefore, they are often also called "gatherers";

concave mirror

• convex. Such mirrors are characterized by reflection of radiation from the outer surface. During this, dispersion to the sides occurs. For this reason, such objects are called "scattering".

convex mirror

In this case, there are several options for the behavior of the rays:

• burning almost parallel to the surface. In this situation, it only slightly touches the surface, and is reflected at a very obtuse angle. Then he goes on a fairly low trajectory;
• when falling back, the rays are repelled at an acute angle. In this case, as we said above, the reflected beam will follow a path very close to the incident one.

As you can see, the law is fulfilled in all cases.

Conclusion

The laws of reflection of light radiation are very important to us because they are fundamental physical phenomena. They have found wide application in various fields human activity. The study of the fundamentals of optics takes place in high school, which once again proves the importance of such basic knowledge.

How to make angel eyes for a vase yourself?

surface light beam (Fig. 3.1) (`vecS_1` - vector directed along the incident beam). At the point `O`, where the ray rests against the plane, we construct to the plane external normal `vecN` (i.e. perpendicular) and, finally, through the ray `vecS_1` and the normal `vecN` draw the plane `P`. This plane is called plane of incidence. Whatever substance our chosen surface consists of, some part of the incident radiation will be reflected. In which direction will the reflected beam `vecS_2` go?

It would be strange if it deviated from the plane of incidence, for example, to the right or to the left: after all, the properties of space on both sides of this plane are the same. Fortunately, this does not happen.

Sharp corner between the ray `vecS_1` and the outward normal `vecN` is called the angle of incidence. Let's denote this corner with the symbol `varphi_1`. The sharp angle formed by the reflected ray `vecS_2` and the normal (let's denote it `varphi_2`) is called the angle of reflection. Numerous observations and measurements allow us to formulate the following postulate of geometric optics:

Postulate 3

The incident ray `vecS_1`, the normal `vecN` and the reflected ray `vecS_2` always lie in the same plane, called the plane of incidence. The angle of reflection is equal to the angle of incidence, i.e.

`varphi_2=varphi_1`. (3.1)

Let's introduce one more definition. The angle `delta`, formed by the continuation of the beam incident on a flat mirror, and the beam reflected from the mirror, will be called the deflection angle. The deflection angle is always less than or equal to `180^@`. The concept of the angle of deviation can be interpreted much broader. In what follows, we will call this the angle formed by the continuation of a ray entering an arbitrary optical system and the ray leaving this system.

Determine the angle of deflection of a beam incident on a plane mirror. Angle of incidence `varphi_1=30^@`.

The angle `alpha` formed by the incident and reflected rays is equal to the sum of the angles of incidence and reflection, i.e. `alpha=60^@`. The `alpha` and `delta` angles are adjacent. Hence,

`delta=180^@-60^@=120^@`.

A smooth surface that reflects almost all of the radiation incident on it is called a specular surface. This begs the question: why “almost everything” and not “everything”? The answer is simple: perfect mirrors does not occur in nature. For example, the mirrors that you meet in everyday life reflect up to `90%` of the incident light, and the remaining `10%` partially pass through and partially absorb.

Modern lasers use mirrors that reflect up to `99%` of radiation and even more (although in a rather narrow region of the spectrum, but we will talk about this when you are in grade 11). For the manufacture of such mirrors, a whole scientific theory was developed and a special production was organized.

Pure transparent water also reflects part of the radiation incident on its surface. When light falls along the normal to the surface, a little less than `2%` of the energy of the incident radiation is reflected. As the angle of incidence increases, the proportion of reflected radiation increases. At an angle of incidence close to `90^@` ( sliding fall), nearly all `100%` of the incident energy is reflected.

Let's briefly touch on one more question. There are no perfectly smooth surfaces. When enough high magnification On the surface of the mirror, you can see microcracks, chips, irregularities, the plane of which is inclined relative to the plane of the mirror. The more irregularities, the more dull the reflection of objects in the mirror seems. Surface white writing paper so heavily dotted with microscopic irregularities that it practically does not give any specular reflection. Such a surface is said to reflect diffusely , i.e., different tiny areas of the surface of the paper reflect light in different directions. But such a surface is clearly visible from different places. In general, most objects reflect light diffusely. Diffuse reflective surfaces are used as screens.

However, it is possible to get a mirror image of bright objects from paper. To do this, you need to look at the surface of the paper almost along its surface. It is best to observe the reflection of a glowing light bulb or the Sun. Do this experiment!

When constructing an image of some point `S` in a flat mirror, it is necessary to use, according to at least,two arbitrary beam. The construction technique is clear from Fig. 3.2. From a practical point of view, it is expedient to let one of the rays (in the figure, it is ray 1) along the normal to the plane of the mirror.

It is customary to call the image of an object obtained as a result of the intersection of reflected rays, valid, and the image obtained by mentally crossing the continuations of these rays in the opposite direction - imaginary. Thus, `S_1` is a virtual image of the source `S` in a flat mirror (Fig. 3.2).

Example 3.1

Bulb table lamp is located at a distance of `l_1=0.6` m from the table surface and `L_2=1.8` m from the ceiling. The filament of a light bulb can be considered a point source of light. On the table lies a fragment of a flat mirror in the form of a triangle with sides `5` cm, `6` cm and `7` cm (Fig. 3.3).

1) At what distance from the ceiling is the image of the filament of the light bulb given by the mirror?

2) Find the shape and dimensions of the "bunny" obtained from a fragment of a mirror on the ceiling (MIPT, 1996).

Let's make a drawing explaining the meaning of the task (Fig. 3.3). Pay attention to two things:

a) the mirror is on the table at some arbitrary distance from the lamp;

b) the image can be constructed using any rays "reflected" from the plane coinciding with the plane of the mirror (for example, rays `3^"` and `4^"`). It is easy to show that `SC=CS_1`, i.e. `L_3=L_1`. Therefore, the distance

`x=2L_1+L_2=>x=2*0.6+1.8=3` m.

To determine the shape and size of the "bunny", it is convenient to consider the rays "emanating" from the image `S_1`. Since the plane of the mirror and the ceiling are parallel, the shape of the "bunny" will be similar to a mirror. Let's find the similarity coefficient. If the length of the side of the mirror is `h`, and the length of the side of the "bunny" corresponding to it is `H`, then you can write the proportion:

`h/H=L_3/x=(0.6 "m")/(3 "m")=1/5=>H=5h`.

Thus, the lengths of the sides of the "bunny" are `25` cm, `30` cm and `35` cm, respectively.

Example 3.2

In the first room, there is a flower `(F)` on the table and a mirror `(M)` hanging on the wall near the door `(D)`. Malvina `(G)` is in the next room (Fig. 3.4). Choose the correct statement.

A. From her place, Malvina cannot see the imaginary image of the flower `(F)` in the mirror.

B. From her place, Malvina can see her image in the mirror.

V. From his place, Malvina cannot see in the mirror actual image flower `(F)`.

Let's make an explanatory drawing (Fig. 3.5). To do this, we will build an image `F^"` of a flower. It will be imaginary.

The straight line `F^"G` is not blocked by obstacles, therefore, Malvina can see the imaginary image of the flower `(F^")`. So answer A is not correct. She cannot see her image. So answer B is not valid either. Since the image of the flower is imaginary, Malvina cannot see the real image of the flower.

The correct answer is B.