It plays an important role in the study of seismic waves. Reflection is observed on surface waves in water bodies. Reflection is observed with many types of electromagnetic waves, not only for visible light. The reflection of VHF and higher frequency radio waves is essential to radio transmissions and radar. Even hard x-rays and gamma rays can be reflected at small angles to the surface by specially made mirrors. In medicine, the reflection of ultrasound at the interfaces between tissues and organs is used in ultrasound diagnostics.
Story
For the first time the law of reflection is mentioned in Euclid's Catoptrika, dated to about 200 BC. e.
Laws of reflection. Fresnel formulas
The law of light reflection - establishes a change in the direction of the light beam as a result of a meeting with a reflective (mirror) surface: the incident and reflected rays lie in the same plane with the normal to the reflecting surface at the point of incidence, and this normal divides the angle between the rays into two equal parts. The widely used but less accurate formulation "angle of incidence equals angle of reflection" does not indicate the exact direction of reflection of the beam. However, it looks like this:
This law is a consequence of the application of Fermat's principle to a reflecting surface and, like all laws of geometric optics, is derived from wave optics. The law is valid not only for perfectly reflecting surfaces, but also for the boundary of two media, partially reflecting light. In this case, as well as the law of refraction of light, it does not state anything about the intensity of the reflected light.
Fedorov shift
Types of reflection
Reflection of light can be mirror(that is, as observed when using mirrors) or diffuse(in this case, during reflection, the path of the rays from the object is not preserved, but only the energy component of the light flux) depending on the nature of the surface.
Mirror reflection
Specular reflection of light is distinguished by a certain relationship between the positions of the incident and reflected rays: 1) the reflected ray lies in a plane passing through the incident ray and the normal to the reflecting surface, restored at the point of incidence; 2) the angle of reflection is equal to the angle of incidence. The intensity of the reflected light (characterized by the reflection coefficient) depends on the angle of incidence and polarization of the incident beam of rays (see Polarization of light), as well as on the ratio of the refractive indices n 2 and n 1 of the 2nd and 1st media. Quantitatively, this dependence (for a reflective medium - a dielectric) is expressed by the Fresnel formulas. From them, in particular, it follows that when light is incident along the normal to the surface, the reflection coefficient does not depend on the polarization of the incident beam and is equal to
In an important special case of normal incidence from air or glass to their interface (refractive index of air = 1.0; glass = 1.5), it is 4%.
Total internal reflection
With an increase in the angle of incidence, the angle of refraction also increases, while the intensity of the reflected beam increases, and that of the refracted beam decreases (their sum is equal to the intensity of the incident beam). At a certain critical value, the intensity of the refracted beam becomes zero and total reflection of the light occurs. The value of the critical angle of incidence can be found by setting the angle of refraction equal to 90° in the law of refraction:
Diffuse reflection of light
When light is reflected from an uneven surface, the reflected rays diverge in different directions (see Lambert's Law). For this reason, you cannot see your reflection when looking at a rough (matte) surface. Diffuse reflection becomes when the surface is uneven on the order of a wavelength or more. Thus, the same surface can be matte, diffusely reflective for visible or ultraviolet radiation, but smooth and specularly reflective for infrared radiation.
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In the second series of the program “Academy of Entertaining Sciences. Physics ”Professor Quark will tell the children about the physics of a mirror. It turns out that the mirror has many interesting features, and with the help of physics, you can unravel why this happens. Why does a mirror reflect everything in reverse? Why do objects in the mirror appear further than they are? How to make a mirror reflect objects correctly? You will learn the answers to these and many other questions by watching a video tutorial on mirror physics.
Mirror physics
A mirror is a smooth surface designed to reflect light. The invention of the real glass mirror can be traced back to 1279, when the Franciscan John Pecamum described a way to cover glass with a thin layer of lead. The physics of a mirror is not that complicated. The course of the rays reflected from the mirror is simple if the laws of geometric optics are applied. A ray of light falls on a mirror surface at an angle alpha to the normal (perpendicular) drawn to the point where the ray hits the mirror. The angle of the reflected beam will be equal to the same alpha value. A ray incident on a mirror at right angles to the plane of the mirror will be reflected back to itself. For the simplest - flat - mirror, the image will be located behind the mirror symmetrically to the object relative to the plane of the mirror, it will be imaginary, direct and the same size as the object itself. This is easy to establish using the law of reflection of light. Reflection is a physical process of interaction of waves or particles with a surface, a change in the direction of a wave front at the boundary of two media with different properties, in which the wave front returns to the medium from which it came. Simultaneously with the reflection of waves at the interface between media, as a rule, refraction of waves occurs (with the exception of cases of total internal reflection). The law of light reflection establishes a change in the direction of the light beam as a result of a meeting with a reflective (mirror) surface: the incident and reflected rays lie in the same plane with the normal to the reflecting surface at the point of incidence, and this normal divides the angle between the rays into two equal parts. The widely used, but less accurate, formulation "the angle of reflection equals the angle of incidence" does not indicate the exact direction of the reflection of the beam. Mirror physics allows you to do various interesting tricks based on optical illusions. Daniil Edisonovich Quark will demonstrate some of these tricks to viewers in his laboratory.
Subscribe to the channel "Academy of Entertaining Sciences" and watch new lessons: http://www.youtube.com/user/AcademiaNauk?sub_confirmation=1 Academy of Entertaining Sciences. Physics. Lesson 2. Mirror physics. Video lessons of physics. In the second series of the program “Academy of Entertaining Sciences. Physics ”Professor Quark will tell the children about the physics of a mirror. It turns out that the mirror has many interesting features, and with the help of physics, you can unravel why this happens. Why does a mirror reflect everything in reverse? Why do objects in the mirror appear further than they are? How to make a mirror reflect objects correctly? You will learn the answers to these and many other questions by watching a video tutorial on mirror physics. The Physics of a Mirror A mirror is a smooth surface designed to reflect light. The invention of the real glass mirror can be traced back to 1279, when the Franciscan John Pecamum described a way to cover glass with a thin layer of lead. The physics of a mirror is not that complicated. The course of the rays reflected from the mirror is simple if the laws of geometric optics are applied. A ray of light falls on a mirror surface at an angle alpha to the normal (perpendicular) drawn to the point where the ray hits the mirror. The angle of the reflected beam will be equal to the same alpha value. A ray incident on a mirror at right angles to the plane of the mirror will be reflected back to itself. For the simplest - flat - mirror, the image will be located behind the mirror symmetrically to the object relative to the plane of the mirror, it will be imaginary, direct and the same size as the object itself. This is easy to establish using the law of reflection of light. Reflection is a physical process of interaction of waves or particles with a surface, a change in the direction of a wave front at the boundary of two media with different properties, in which the wave front returns to the medium from which it came. Simultaneously with the reflection of waves at the interface between media, as a rule, refraction of waves occurs (with the exception of cases of total internal reflection). The law of light reflection - establishes a change in the direction of the light beam as a result of a meeting with a reflective (mirror) surface: the incident and reflected rays lie in the same plane with the normal to the reflecting surface at the point of incidence, and this normal divides the angle between the rays into two equal parts. The widely used, but less accurate, formulation "the angle of reflection equals the angle of incidence" does not indicate the exact direction of the reflection of the beam. Mirror physics allows you to do various interesting tricks based on optical illusions. Daniil Edisonovich Quark will demonstrate some of these tricks to viewers in his laboratory.
Well-known modern mirrors, as a rule, are nothing more than a sheet of glass with a thin metal layer applied to the inside. It seems that mirrors have always been around, in one form or another, but in their current form, they appeared relatively recently. As early as a thousand years ago, mirrors were polished copper or bronze discs that cost more than most people of that era could afford. The peasant, who wanted to see his own reflection, went to look into the pond. Full length mirrors are an even more recent invention. They are only about 400 years old.
Mirrors present us with truth and illusion at the same time. Perhaps this paradox makes mirrors the center of attraction for magic and science.
Mirrors in history
When people started making simple mirrors around 600 BC, they used polished obsidian as a reflective surface. Eventually, they began to produce more elaborate mirrors made from copper, bronze, silver, gold, and even lead.
However, given the weight of the material, these mirrors were tiny by our standards. They rarely reached 20 cm in diameter and were mainly used as decoration. It was especially chic to wear a mirror attached to the belt with a chain.
One of the exceptions was the Pharos lighthouse, one of the seven wonders of the world, whose large bronze mirror reflected the fire of a huge fire at night.
Modern mirrors appeared only at the end of the Middle Ages, but in those days their production was difficult and expensive. One problem was that the glass sand contained too many impurities to create true transparency. In addition, the thermal shock caused by the addition of molten metal to create a reflective surface almost always shattered the glass.
During the Renaissance, when the Florentines invented a way to make a low-temperature lead back, modern mirrors made their debut. These mirrors were finally clean, which allowed them to be used in art. For example, architect Filippo Brunelleschi created a linear perspective with mirrors to give the illusion of depth. In addition, mirrors founded a new art form - the self-portrait. The Venetian masters of the mirror business have reached the heights in glass technology. Their secrets were so precious, and the mirror trade so lucrative, that treacherous craftsmen who tried to sell their knowledge abroad were often killed.
At this time, mirrors were still only available to the wealthy, but scientists began to look for alternative ways to use them. In the early 1660s, mathematicians noted that mirrors could potentially be used in telescopes instead of lenses. James Bradley used this knowledge to build the first reflecting telescope in 1721.
A modern mirror is made by silvering - spraying a thin layer of silver or aluminum onto the wrong side of a sheet of glass. Justus von Leibig invented this process in 1835. Most mirrors made today are made by the more advanced process of heating aluminum in a vacuum, which then sticks to cooler glass. Silver can still be used for household mirrors, but silver has a significant drawback - it quickly oxidizes and absorbs atmospheric sulfur, creating dark areas. Aluminum is less prone to darkening because the thin layer of aluminum oxide remains transparent. Mirrors are now used for everything from LCD projection to car headlights and lasers.
Mirror physics
To understand the physics of a mirror, we must first understand the physics of light. AT reflection law it is said that when a beam of light hits a surface, it bounces off in a certain way, like a ball thrown at a wall. The incoming corner, called angle of incidence, is always equal to the angle at which the ray leaves the surface, or reflection angle.
Light itself is invisible until it bounces off something and enters our eyes. A beam of light propagating through space is not visible from the outside until it enters a medium that scatters it, such as a cloud of hydrogen. This scattering is known as diffuse reflection and is how our eyes interpret what happens when light strikes an uneven surface. The law of reflection still applies, but instead of hitting one smooth surface, light hits many microscopic surfaces.
Mirrors, having a smooth surface, reflect light without disturbing incoming images. It is called mirror reflection. The image in the mirror is imaginary, since it is formed not by the intersection of the reflected light rays themselves, but by their “continuations through the mirror.” Many people have a curious question - why do mirrors always show images rotated “from left to right” and not “correct”? The fact is that the mirror image looks like a "light stamp", and not a view of the object from the point of view of the mirror. At the same time, both the distance to the object and the size of the object in a flat mirror remain the same as in the original.
Mirror types
An easy way to change how a mirror works is to warp it. Curved mirrors exist in two basic versions: convex and concave.
Reflection of a parallel beam of rays from a convex mirror. F is the imaginary focus of the mirror, O is the optical center; OP - main optical axis
convex a mirror in which the center is arched outward reflects a wide angle near its edges, producing a slightly distorted image that is smaller than the actual size. Convex mirrors have many uses. The smaller the image size, the more you can see in such a mirror. Convex mirrors are used in automotive rear-view mirrors. Some department stores install vertically convex dressing room mirrors because they make customers look taller and thinner than they really are.
Reflection of a parallel beam of rays from a concave spherical mirror. Points O - optical center, P - pole, F - main focus of the mirror; OP is the main optical axis, R is the radius of curvature of the mirror
Concave or spherical mirrors with inward curvature look like fragments of a sphere. With these mirrors, light is reflected in a certain area in front of them. This area is called focal point. From a distance, objects in such a mirror will appear upside down, but if you get closer to the mirror than the focal point, the image turns upside down. Concave mirrors are used everywhere, for example, to light the Olympic Flame.
The focal lengths of spherical mirrors are assigned a certain sign:
for a concave mirror for a convex one where R is the radius of curvature of the mirror.
Now that you know the main types of mirrors, you can think of other, more unusual types. Here is a short list:
1. Non-reversing mirror: The non-reversing mirror was patented in 1887 when John Derby created it by placing two mirrors perpendicular to each other.
2. Acoustic mirrors: Acoustic mirrors in the form of huge concrete dishes are built to reflect and propagate sound, not light. The British military used them before the invention radar as an early warning system for air attacks.
3. Two-sided mirrors: These mirrors are made by coating one side of a sheet of glass with a very thin layer of reflective material through which bright light can pass. Such mirrors are installed in interrogation rooms. On one side of such a mirror is a dark room for watching police officers, on the other, a brightly lit interrogation room. Observers from a dark room see the interrogated person in a bright room, and he sees only his mirror image in such a mirror. Ordinary window glass is also a weak reflective material. For this reason, it is difficult to see something on the street at night when the light is on in the room.
Mirrors in literature and superstition
There are plenty of magical mirrors in literature, from the ancient story of the handsome Narcissus, in love and longing for his own reflection in a pool of water, to Alice's journey through the Looking Glass. In Chinese mythology, there is a story about the Kingdom of Mirrors, where creatures are bound by the magic of a dream, but one day they will be resurrected to fight with our world.
Mirrors also have close ties to the concept of the soul. This gives rise to many wild superstitions. For example, if you break a mirror, you will allegedly earn seven whole years of bad luck. The explanation is that your soul, renewed every seven years, is destroyed along with the broken mirror. From the same theory it follows that vampires who do not have a soul become invisible in the mirror. Looking in the mirror is also dangerous for babies whose souls are not developed or they begin to stutter.
Perfumes are often associated with mirrors. Mirrors are covered with cloth out of respect for the memory of those who died during Jewish mourning, but in many countries this is also customary. According to superstition, a mirror can trap the soul of a dying person. A woman who is in labor and looks in a mirror will soon see ghostly faces peeking out from behind her reflection. Moreover, if you look in the mirror on Christmas Eve with a candle in your hand and call the name of the deceased in a loud voice, then the power of the mirror will show you the face of that person. Also common are girlish fortune telling about the “betrothed”, in which, according to the plan of the fortuneteller, the mirror should show the face of the future groom.
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 the reflectivity of the surface of a substance 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 a thin film of 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).
Till 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 a 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, in contrast to the specular reflection.
- 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 fulfilled.
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 a 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 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 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 life size. The dimensions 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 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 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, 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 directions, so convex mirror called scattering.
