Ticket 75.
Law of Light Reflection: the incident and reflected rays, as well as the perpendicular to the interface between the two media, reconstructed at the point of incidence of the ray, lie in the same plane (plane of incidence). The angle of reflection γ is equal to the angle of incidence α.
Law of light refraction: the incident and refracted rays, as well as the perpendicular to the interface between the two media, reconstructed at the point of incidence of the ray, lie in the same plane. The ratio of the sine of the angle of incidence α to the sine of the angle of refraction β is a constant value for two given media:
The laws of reflection and refraction are explained in wave physics. According to wave concepts, refraction is a consequence of changes in the speed of propagation of waves when passing from one medium to another. Physical meaning of the refractive index is the ratio of the speed of propagation of waves in the first medium υ 1 to the speed of their propagation in the second medium υ 2:
Figure 3.1.1 illustrates the laws of reflection and refraction of light.
A medium with a lower absolute refractive index is called optically less dense.
When light passes from an optically denser medium to an optically less dense medium n 2< n 1 (например, из стекла в воздух) можно наблюдать total reflection phenomenon, that is, the disappearance of the refracted ray. This phenomenon is observed at angles of incidence exceeding a certain critical angle α pr, which is called limiting angle of total internal reflection(see Fig. 3.1.2).
For the angle of incidence α = α pr sin β = 1; value sin α pr = n 2 / n 1< 1.
If the second medium is air (n 2 ≈ 1), then it is convenient to rewrite the formula in the form
The phenomenon of total internal reflection is used in many optical devices. The most interesting and practically important application is the creation of optical fibers, which are thin (from several micrometers to millimeters) arbitrarily curved threads made of optically transparent material (glass, quartz). Light incident on the end of the light guide can travel along it over long distances due to total internal reflection from the side surfaces (Figure 3.1.3). The scientific and technical direction involved in the development and application of optical light guides is called fiber optics.
Dispersion of light (decomposition of light)- this is a phenomenon caused by the dependence of the absolute refractive index of a substance on the frequency (or wavelength) of light (frequency dispersion), or, the same thing, the dependence of the phase speed of light in a substance on the wavelength (or frequency). It was discovered experimentally by Newton around 1672, although theoretically quite well explained much later.
Spatial dispersion is called the dependence of the dielectric constant tensor of the medium on the wave vector. This dependence causes a number of phenomena called spatial polarization effects.
One of the most clear examples of dispersion - white light decomposition when passing through a prism (Newton's experiment). The essence of the dispersion phenomenon is the difference in the speed of propagation of light rays of different wavelengths in a transparent substance - an optical medium (while in a vacuum the speed of light is always the same, regardless of wavelength and therefore color). Typically, the higher the frequency of a light wave, the higher the refractive index of the medium for it and the lower the speed of the wave in the medium:
Newton's experiments Experiment on the decomposition of white light into a spectrum: Newton directed a beam of sunlight through a small hole onto a glass prism. When hitting the prism, the beam was refracted and on the opposite wall gave an elongated image with a rainbow alternation of colors - a spectrum. Experiment on the passage of monochromatic light through a prism: Newton placed red glass in the path of a solar ray, behind which he received monochromatic light (red), then a prism and observed on the screen only a red spot from the light ray. Experience in the synthesis (production) of white light: First, Newton directed a ray of sunlight onto a prism. Then, having collected the colored rays emerging from the prism using a collecting lens, Newton received a white image of a hole on a white wall instead of a colored stripe. Newton's conclusions:- a prism does not change light, but only decomposes it into its components - light rays that differ in color differ in the degree of refraction; Violet rays refract most strongly, red ones less strongly - red light, which refracts less, has the highest speed, and violet has the least, which is why the prism decomposes the light. The dependence of the refractive index of light on its color is called dispersion.
Conclusions:- a prism decomposes light - white light is complex (composite) - violet rays are refracted more strongly than red ones. The color of a light beam is determined by its vibration frequency. When moving from one medium to another, the speed of light and wavelength change, but the frequency that determines the color remains constant. The boundaries of the ranges of white light and its components are usually characterized by their wavelengths in vacuum. White light is a collection of waves with lengths from 380 to 760 nm.
Ticket 77.
Absorption of light. Bouguer's law
The absorption of light in a substance is associated with the conversion of the energy of the electromagnetic field of the wave into the thermal energy of the substance (or into the energy of secondary photoluminescent radiation). The law of light absorption (Bouguer's law) has the form:
I=I 0 exp(- x),(1)
Where I 0 , I-light intensity at the input (x=0) and leaving the layer of medium thickness X, - absorption coefficient, it depends on .
For dielectrics =10 -1 10 -5 m -1 , for metals =10 5 10 7 m -1 , Therefore, metals are opaque to light.
Dependency ( ) explains the color of absorbing bodies. For example, glass that absorbs red light poorly will appear red when illuminated with white light.
Scattering of light. Rayleigh's law
Diffraction of light can occur in an optically inhomogeneous medium, for example in a turbid environment (smoke, fog, dusty air, etc.). By diffracting on inhomogeneities of the medium, light waves create a diffraction pattern characterized by a fairly uniform distribution of intensity in all directions.
This diffraction by small inhomogeneities is called scattering of light.
This phenomenon is observed when a narrow beam of sunlight passes through dusty air, scatters on dust particles and becomes visible.
If the sizes of inhomogeneities are small compared to the wavelength (no more than 0,1 ), then the intensity of the scattered light turns out to be inversely proportional to the fourth power of the wavelength, i.e.
I diss ~ 1/ 4 , (2)
this dependence is called Rayleigh's law.
Light scattering is also observed in clean media that do not contain foreign particles. For example, it can occur on fluctuations (random deviations) of density, anisotropy or concentration. This type of scattering is called molecular scattering. It explains, for example, the blue color of the sky. Indeed, according to (2), blue and blue rays are scattered more strongly than red and yellow ones, because have a shorter wavelength, thereby causing the blue color of the sky.
Ticket 78.
Polarization of light- a set of wave optics phenomena in which the transverse nature of electromagnetic light waves is manifested. Transverse wave- particles of the medium oscillate in directions perpendicular to the direction of propagation of the wave ( Fig.1).
Fig.1 Transverse wave
Electromagnetic light wave plane polarized(linear polarization), if the directions of oscillation of vectors E and B are strictly fixed and lie in certain planes ( Fig.1). A plane polarized light wave is called plane polarized(linearly polarized) light. Unpolarized(natural) wave - an electromagnetic light wave in which the directions of oscillation of the vectors E and B in this wave can lie in any planes perpendicular to the velocity vector v. Unpolarized light- light waves in which the directions of oscillations of the vectors E and B change chaotically so that all directions of oscillations in planes perpendicular to the ray of wave propagation are equally probable ( Fig.2).
Fig.2 Unpolarized light
Polarized waves- in which the directions of the vectors E and B remain unchanged in space or change according to a certain law. Radiation in which the direction of vector E changes chaotically - unpolarized. An example of such radiation is thermal radiation (chaotically distributed atoms and electrons). Plane of polarization- this is a plane perpendicular to the direction of oscillations of the vector E. The main mechanism for the occurrence of polarized radiation is the scattering of radiation by electrons, atoms, molecules, and dust particles.
1.2. Types of polarization There are three types of polarization. Let's give them definitions. 1. Linear Occurs if the electric vector E maintains its position in space. It seems to highlight the plane in which vector E oscillates. 2. Circular This is polarization that occurs when the electric vector E rotates around the direction of wave propagation with an angular velocity equal to the angular frequency of the wave, while maintaining its absolute value. This polarization characterizes the direction of rotation of the vector E in a plane perpendicular to the line of sight. An example is cyclotron radiation (a system of electrons rotating in a magnetic field). 3. Elliptical It occurs when the magnitude of the electric vector E changes so that it describes an ellipse (rotation of the vector E). Elliptical and circular polarization can be right-handed (vector E rotates clockwise when looking towards the propagating wave) and left-handed (vector E rotates counter-clockwise when looking towards the propagating wave).
In reality, it occurs most often partial polarization (partially polarized electromagnetic waves). Quantitatively, it is characterized by a certain quantity called degree of polarization R, which is defined as: P = (Imax - Imin) / (Imax + Imin) Where Imax,Immin- the highest and lowest density of electromagnetic energy flux through the analyzer (Polaroid, Nicolas prism...). In practice, radiation polarization is often described by Stokes parameters (they determine radiation fluxes with a given polarization direction).
Ticket 79.
If natural light falls on the interface between two dielectrics (for example, air and glass), then part of it is reflected, and part of it is refracted and spreads in the second medium. By installing an analyzer (for example, tourmaline) in the path of the reflected and refracted rays, we make sure that the reflected and refracted rays are partially polarized: when the analyzer is rotated around the rays, the light intensity periodically increases and weakens (complete quenching is not observed!). Further studies showed that in the reflected beam, vibrations perpendicular to the plane of incidence predominate (they are indicated by dots in Fig. 275), while in the refracted beam, vibrations parallel to the plane of incidence (depicted by arrows) predominate.
The degree of polarization (the degree to which light waves are separated with a certain orientation of the electric (and magnetic) vector) depends on the angle of incidence of the rays and the refractive index. Scottish physicist D. Brewster(1781-1868) installed law, according to which at the angle of incidence i B (Brewster angle), determined by the relation
(n 21 - refractive index of the second medium relative to the first), the reflected beam is plane polarized(contains only vibrations perpendicular to the plane of incidence) (Fig. 276). The refracted ray at the angle of incidencei B polarized to the maximum, but not completely.
If light strikes an interface at the Brewster angle, then the reflected and refracted rays mutually perpendicular(tg i B = sin i B/cos i B, n 21 = sin i B / sin i 2 (i 2 - angle of refraction), whence cos i B=sin i 2). Hence, i B + i 2 = /2, but i’ B= i B (law of reflection), therefore i’ B+ i 2 = /2.
The degree of polarization of reflected and refracted light at different angles of incidence can be calculated from Maxwell’s equations, if we take into account the boundary conditions for the electromagnetic field at the interface between two isotropic dielectrics (the so-called Fresnel formulas).
The degree of polarization of refracted light can be significantly increased (by multiple refraction, provided that the light is incident each time on the interface at the Brewster angle). If, for example, for glass ( n= 1.53) the degree of polarization of the refracted beam is 15%, then after refraction into 8-10 glass plates superimposed on each other, the light emerging from such a system will be almost completely polarized. Such a collection of plates is called foot. The foot can be used to analyze polarized light both during its reflection and during its refraction.
Ticket 79 (for Spur)
As experience shows, during the refraction and reflection of light, the refracted and reflected light turns out to be polarized, and the reflection. light can be completely polarized at a certain angle of incidence, but incidentally. light is always partially polarized. Based on Frinell's formulas, it can be shown that reflection. Light is polarized in a plane perpendicular to the plane of incidence and refracted. the light is polarized in a plane parallel to the plane of incidence.
The angle of incidence at which the reflection light is completely polarized is called Brewster's angle. Brewster's angle is determined from Brewster's law: - Brewster's law. In this case, the angle between the reflections. and refraction. rays will be equal. For an air-glass system, the Brewster angle is equal. To obtain good polarization, i.e. , when refracting light, many edible surfaces are used, which are called Stoletov’s Stop.
Ticket 80.
Experience shows that when light interacts with matter, the main effect (physiological, photochemical, photoelectric, etc.) is caused by oscillations of the vector, which in this regard is sometimes called the light vector. Therefore, to describe the patterns of light polarization, the behavior of the vector is monitored.
The plane formed by the vectors and is called the plane of polarization.
If vector oscillations occur in one fixed plane, then such light (ray) is called linearly polarized. It is conventionally designated as follows. If the beam is polarized in a perpendicular plane (in the plane xoz, see fig. 2 in the second lecture), then it is designated.
Natural light (from ordinary sources, the sun) consists of waves that have different, chaotically distributed planes of polarization (see Fig. 3).
Natural light is sometimes conventionally designated as such. It is also called non-polarized.
If, as the wave propagates, the vector rotates and the end of the vector describes a circle, then such light is called circularly polarized, and the polarization is called circular or circular (right or left). There is also elliptical polarization.
There are optical devices (films, plates, etc.) - polarizers, which extract linearly polarized light or partially polarized light from natural light.
Polarizers used to analyze the polarization of light are called analyzers.
The polarizer (or analyzer) plane is the plane of polarization of light transmitted by the polarizer (or analyzer).
Let linearly polarized light with amplitude fall on a polarizer (or analyzer) E 0 . The amplitude of the transmitted light will be equal to E=E 0 cos j, and intensity I=I 0 cos 2 j.
This formula expresses Malus's law:
The intensity of linearly polarized light passing through the analyzer is proportional to the square of the cosine of the angle j between the plane of oscillation of the incident light and the plane of the analyzer.
Ticket 80 (for spur)
Polarizers are devices that make it possible to obtain polarized light. Analyzers are devices that can be used to analyze whether light is polarized or not. Structurally, a polarizer and an analyzer are one and the same. Zn Malus. Let intensity light fall on the polarizer, if the light is natural -th then all directions of vector E are equally probable. Each vector can be decomposed into two mutually perpendicular components: one of which is parallel to the plane of polarization of the polarizer, and the other is perpendicular to it.
Obviously, the intensity of the light emerging from the polarizer will be equal. Let us denote the intensity of the light emerging from the polarizer by (). If an analyzer is placed on the path of the polarized light, the main plane of which makes an angle with the main plane of the polarizer, then the intensity of the light emerging from the analyzer is determined by the law.
Ticket 81.
While studying the glow of a solution of uranium salts under the influence of radium rays, the Soviet physicist P. A. Cherenkov drew attention to the fact that the water itself also glows, in which there are no uranium salts. It turned out that when rays (see Gamma radiation) are passed through pure liquids, they all begin to glow. S. I. Vavilov, under whose leadership P. A. Cherenkov worked, hypothesized that the glow was associated with the movement of electrons knocked out of atoms by radium quanta. Indeed, the glow strongly depended on the direction of the magnetic field in the liquid (this suggested that it was caused by the movement of electrons).
But why do electrons moving in a liquid emit light? The correct answer to this question was given in 1937 by Soviet physicists I.E. Tamm and I.M. Frank.
An electron, moving in a substance, interacts with the atoms surrounding it. Under the influence of its electric field, atomic electrons and nuclei are displaced in opposite directions - the medium is polarized. Polarized and then returning to their original state, the atoms of the medium located along the electron trajectory emit electromagnetic light waves. If the speed of the electron v is less than the speed of light propagation in the medium (the refractive index), then the electromagnetic field will overtake the electron, and the substance will have time to polarize in space ahead of the electron. The polarization of the medium in front of and behind the electron is opposite in direction, and the radiations of oppositely polarized atoms, “added”, “quench” each other. When atoms that have not yet been reached by an electron do not have time to polarize, and radiation appears directed along a narrow conical layer with an apex coinciding with the moving electron and an angle at the apex c. The appearance of the light "cone" and the radiation condition can be obtained from the general principles of wave propagation.
Rice. 1. Mechanism of wavefront formation
Let the electron move along the axis OE (see Fig. 1) of a very narrow empty channel in a homogeneous transparent substance with a refractive index (the empty channel is needed so that collisions of the electron with atoms are not taken into account in the theoretical consideration). Any point on the line OE successively occupied by an electron will be the center of light emission. Waves emanating from successive points O, D, E interfere with each other and are amplified if the phase difference between them is zero (see Interference). This condition is satisfied for a direction that makes an angle of 0 with the trajectory of the electron. Angle 0 is determined by the relation:.
Indeed, let us consider two waves emitted in a direction at an angle of 0 to the electron velocity from two points of the trajectory - point O and point D, separated by a distance . At point B, lying on line BE, perpendicular to OB, the first wave at - after time To point F, lying on line BE, a wave emitted from the point will arrive at the moment of time after the wave is emitted from point O. These two waves will be in phase, i.e. the straight line will be a wave front if these times are equal:. That gives the condition of equality of times. In all directions for which, the light will be extinguished due to the interference of waves emitted from sections of the trajectory separated by a distance D. The value of D is determined by the obvious equation, where T is the period of light oscillations. This equation always has a solution if.
If , then the direction in which the emitted waves, when interfering, are amplified, does not exist and cannot be greater than 1.
Rice. 2. Distribution of sound waves and the formation of a shock wave during body movement
Radiation is observed only if .
Experimentally, electrons fly in a finite solid angle, with some spread in speed, and as a result, radiation propagates in a conical layer near the main direction determined by the angle.
In our consideration, we neglected the electron slowdown. This is quite acceptable, since the losses due to Vavilov-Cerenkov radiation are small and, to a first approximation, we can assume that the energy lost by the electron does not affect its speed and it moves uniformly. This is the fundamental difference and unusualness of the Vavilov-Cherenkov radiation. Typically, charges emit while experiencing significant acceleration.
An electron outpacing its light is similar to an airplane flying at a speed greater than the speed of sound. In this case, a conical shock sound wave also propagates in front of the aircraft (see Fig. 2).
Light refraction- a phenomenon in which a ray of light, passing from one medium to another, changes direction at the boundary of these media.
Refraction of light occurs according to the following law:
The incident and refracted rays and the perpendicular drawn to the interface between the two media at the point of incidence of the ray lie in the same plane. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant value for two media:
,
Where α
- angle of incidence,
β
- refraction angle,
n - a constant value independent of the angle of incidence.
When the angle of incidence changes, the angle of refraction also changes. The greater the angle of incidence, the greater the angle of refraction.
If light comes from an optically less dense medium to a more dense medium, then the angle of refraction is always less than the angle of incidence: β < α.
A ray of light directed perpendicular to the interface between two media passes from one medium to another without refraction.
absolute refractive index of a substance- a value equal to the ratio of the phase speeds of light (electromagnetic waves) in vacuum and in a given environment n=c/v
The quantity n included in the law of refraction is called the relative refractive index for a pair of media.
The value n is the relative refractive index of medium B with respect to medium A, and n" = 1/n is the relative refractive index of medium A with respect to medium B.
This value, other things being equal, is greater than unity when the beam passes from a denser medium to a less dense medium, and less than unity when the beam passes from a less dense medium to a denser medium (for example, from a gas or from a vacuum to a liquid or solid). There are exceptions to this rule, and therefore it is customary to call a medium optically more or less dense than another.
A ray falling from airless space onto the surface of some medium B is refracted more strongly than when falling on it from another medium A; The refractive index of a ray incident on a medium from airless space is called its absolute refractive index.
(Absolute - relative to vacuum.
Relative - relative to any other substance (the same air, for example).
The relative indicator of two substances is the ratio of their absolute indicators.)
Total internal reflection- internal reflection, provided that the angle of incidence exceeds a certain critical angle. In this case, the incident wave is completely reflected, and the value of the reflection coefficient exceeds its highest values for polished surfaces. The reflectance of total internal reflection is independent of wavelength.
In optics, this phenomenon is observed for a wide range of electromagnetic radiation, including the X-ray range.
In geometric optics, the phenomenon is explained within the framework of Snell's law. Considering that the angle of refraction cannot exceed 90°, we find that at an angle of incidence whose sine is greater than the ratio of the lower refractive index to the larger index, the electromagnetic wave must be completely reflected into the first medium.
In accordance with the wave theory of the phenomenon, the electromagnetic wave still penetrates into the second medium - the so-called “non-uniform wave” propagates there, which decays exponentially and does not carry energy with it. The characteristic depth of penetration of an inhomogeneous wave into the second medium is of the order of the wavelength.
Laws of light refraction.
From all that has been said we conclude:
1 . At the interface between two media of different optical densities, a light ray changes its direction when passing from one medium to another.
2. When a light beam passes into a medium with a higher optical density, the angle of refraction is less than the angle of incidence; When a light ray passes from an optically denser medium to a less dense medium, the angle of refraction is greater than the angle of incidence.
The refraction of light is accompanied by reflection, and with an increase in the angle of incidence, the brightness of the reflected beam increases, and the refracted beam weakens. This can be seen by carrying out the experiment shown in the figure. Consequently, the reflected beam carries with it more light energy, the greater the angle of incidence.
Let MN- the interface between two transparent media, for example, air and water, JSC- incident ray, OB- refracted ray, - angle of incidence, - angle of refraction, - speed of light propagation in the first medium, - speed of light propagation in the second medium.
When solving problems in optics, you often need to know the refractive index of glass, water, or another substance. Moreover, in different situations, both absolute and relative values of this quantity can be used.
Two types of refractive index
First, let’s talk about what this number shows: how the direction of light propagation changes in one or another transparent medium. Moreover, an electromagnetic wave can come from a vacuum, and then the refractive index of glass or other substance will be called absolute. In most cases, its value lies in the range from 1 to 2. Only in very rare cases the refractive index is greater than two.
If in front of the object there is a medium denser than vacuum, then they speak of a relative value. And it is calculated as the ratio of two absolute values. For example, the relative refractive index of water-glass will be equal to the quotient of the absolute values for glass and water.
In any case, it is denoted by the Latin letter “en” - n. This value is obtained by dividing the same values by each other, therefore it is simply a coefficient that has no name.
What formula can you use to calculate the refractive index?
If we take the angle of incidence as “alpha” and the angle of refraction as “beta”, then the formula for the absolute value of the refractive index looks like this: n = sin α/sin β. In English-language literature you can often find a different designation. When the angle of incidence is i, and the angle of refraction is r.
There is another formula for how to calculate the refractive index of light in glass and other transparent media. It is related to the speed of light in a vacuum and the same, but in the substance under consideration.
Then it looks like this: n = c/νλ. Here c is the speed of light in a vacuum, ν is its speed in a transparent medium, and λ is the wavelength.
What does the refractive index depend on?
It is determined by the speed at which light propagates in the medium under consideration. Air in this regard is very close to a vacuum, so light waves propagate in it practically without deviating from their original direction. Therefore, if the refractive index of glass-air or any other substance bordering air is determined, then the latter is conventionally taken as a vacuum.
Every other environment has its own characteristics. They have different densities, they have their own temperature, as well as elastic stresses. All this affects the result of light refraction by the substance.
The characteristics of light play an important role in changing the direction of wave propagation. White light is made up of many colors, from red to violet. Each part of the spectrum is refracted in its own way. Moreover, the value of the indicator for the wave of the red part of the spectrum will always be less than that of the rest. For example, the refractive index of TF-1 glass varies from 1.6421 to 1.67298, respectively, from the red to violet part of the spectrum.
Examples of values for different substances
Here are the values of absolute values, that is, the refractive index when a beam passes from a vacuum (which is equivalent to air) through another substance.
These figures will be needed if it is necessary to determine the refractive index of glass relative to other media.
What other quantities are used when solving problems?
Total reflection. It is observed when light passes from a denser medium to a less dense one. Here, at a certain angle of incidence, refraction occurs at a right angle. That is, the beam slides along the boundary of two media.
The limiting angle of total reflection is its minimum value at which light does not escape into a less dense medium. Less of it means refraction, and more means reflection into the same medium from which the light moved.
Task No. 1
Condition. The refractive index of glass has a value of 1.52. It is necessary to determine the limiting angle at which light is completely reflected from the interface of surfaces: glass with air, water with air, glass with water.
You will need to use the refractive index data for water given in the table. It is taken equal to unity for air.
The solution in all three cases comes down to calculations using the formula:
sin α 0 /sin β = n 1 /n 2, where n 2 refers to the medium from which the light propagates, and n 1 where it penetrates.
The letter α 0 denotes the limit angle. The value of angle β is 90 degrees. That is, its sine will be one.
For the first case: sin α 0 = 1 /n glass, then the limiting angle turns out to be equal to the arcsine of 1 /n glass. 1/1.52 = 0.6579. The angle is 41.14º.
In the second case, when determining the arcsine, you need to substitute the value of the refractive index of water. The fraction 1 /n of water will take the value 1/1.33 = 0.7519. This is the arcsine of the angle 48.75º.
The third case is described by the ratio of n water and n glass. The arcsine will need to be calculated for the fraction: 1.33/1.52, that is, the number 0.875. We find the value of the limiting angle by its arcsine: 61.05º.
Answer: 41.14º, 48.75º, 61.05º.
Problem No. 2
Condition. A glass prism is immersed in a vessel with water. Its refractive index is 1.5. A prism is based on a right triangle. The larger leg is located perpendicular to the bottom, and the second is parallel to it. A ray of light falls normally on the upper face of the prism. What must be the smallest angle between a horizontal leg and the hypotenuse for light to reach the leg located perpendicular to the bottom of the vessel and exit the prism?
In order for the ray to exit the prism in the manner described, it needs to fall at a maximum angle onto the inner face (the one that is the hypotenuse of the triangle in the cross section of the prism). This limiting angle turns out to be equal to the desired angle of the right triangle. From the law of light refraction, it turns out that the sine of the limiting angle divided by the sine of 90 degrees is equal to the ratio of two refractive indices: water to glass.
Calculations lead to the following value for the limiting angle: 62º30´.
Processes that are associated with light are an important component of physics and surround us everywhere in our everyday life. The most important in this situation are the laws of reflection and refraction of light, on which modern optics is based. The refraction of light is an important part of modern science.
Distortion effect
This article will tell you what the phenomenon of light refraction is, as well as what the law of refraction looks like and what follows from it.
Basics of a physical phenomenon
When a beam falls on a surface that is separated by two transparent substances that have different optical densities (for example, different glasses or in water), some of the rays will be reflected, and some will penetrate into the second structure (for example, they will propagate in water or glass). When moving from one medium to another, a ray typically changes its direction. This is the phenomenon of light refraction.
The reflection and refraction of light is especially visible in water.
Distortion effect in water
Looking at things in water, they appear distorted. This is especially noticeable at the boundary between air and water. Visually, underwater objects appear to be slightly deflected. The described physical phenomenon is precisely the reason why all objects appear distorted in water. When the rays hit the glass, this effect is less noticeable.
Refraction of light is a physical phenomenon that is characterized by a change in the direction of movement of a solar ray at the moment it moves from one medium (structure) to another.
To improve our understanding of this process, consider an example of a beam hitting water from air (similarly for glass). By drawing a perpendicular line along the interface, the angle of refraction and return of the light beam can be measured. This index (angle of refraction) will change as the flow penetrates the water (inside the glass).
Note! This parameter is understood as the angle formed by a perpendicular drawn to the separation of two substances when a beam penetrates from the first structure to the second.
Beam Passage
The same indicator is typical for other environments. It has been established that this indicator depends on the density of the substance. If the beam falls from a less dense to a denser structure, then the angle of distortion created will be greater. And if it’s the other way around, then it’s less.
At the same time, a change in the slope of the decline will also affect this indicator. But the relationship between them does not remain constant. At the same time, the ratio of their sines will remain a constant value, which is reflected by the following formula: sinα / sinγ = n, where:
- n is a constant value that is described for each specific substance (air, glass, water, etc.). Therefore, what this value will be can be determined using special tables;
- α – angle of incidence;
- γ – angle of refraction.
To determine this physical phenomenon, the law of refraction was created.
Physical law
The law of refraction of light fluxes allows us to determine the characteristics of transparent substances. The law itself consists of two provisions:
- First part. The beam (incident, modified) and the perpendicular, which was restored at the point of incidence on the boundary, for example, of air and water (glass, etc.), will be located in the same plane;
- The second part. The ratio of the sine of the angle of incidence to the sine of the same angle formed when crossing the boundary will be a constant value.
Description of the law
In this case, at the moment the beam exits the second structure into the first (for example, when the light flux passes from the air, through the glass and back into the air), a distortion effect will also occur.
An important parameter for different objects
The main indicator in this situation is the ratio of the sine of the angle of incidence to a similar parameter, but for distortion. As follows from the law described above, this indicator is a constant value.
Moreover, when the value of the decline slope changes, the same situation will be typical for a similar indicator. This parameter is of great importance because it is an integral characteristic of transparent substances.
Indicators for different objects
Thanks to this parameter, you can quite effectively distinguish between types of glass, as well as various precious stones. It is also important for determining the speed of light in various environments.
Note! The highest speed of light flow is in a vacuum.
When moving from one substance to another, its speed will decrease. For example, in diamond, which has the highest refractive index, the speed of photon propagation will be 2.42 times higher than that of air. In water, they will spread 1.33 times slower. For different types of glass, this parameter ranges from 1.4 to 2.2.
Note! Some glasses have a refractive index of 2.2, which is very close to diamond (2.4). Therefore, it is not always possible to distinguish a piece of glass from a real diamond.
Optical density of substances
Light can penetrate through different substances, which are characterized by different optical densities. As we said earlier, using this law you can determine the density characteristic of the medium (structure). The denser it is, the slower the speed at which light will propagate through it. For example, glass or water will be more optically dense than air.
In addition to the fact that this parameter is a constant value, it also reflects the ratio of the speed of light in two substances. The physical meaning can be displayed as the following formula:
This indicator tells how the speed of propagation of photons changes when moving from one substance to another.
Another important indicator
When a light flux moves through transparent objects, its polarization is possible. It is observed during the passage of a light flux from dielectric isotropic media. Polarization occurs when photons pass through glass.
Polarization effect
Partial polarization is observed when the angle of incidence of the light flux at the boundary of two dielectrics differs from zero. The degree of polarization depends on what the angles of incidence were (Brewster's law).
Full internal reflection
Concluding our short excursion, it is still necessary to consider such an effect as full internal reflection.
The phenomenon of full display
For this effect to appear, it is necessary to increase the angle of incidence of the light flux at the moment of its transition from a more dense to a less dense medium at the interface between substances. In a situation where this parameter exceeds a certain limiting value, then photons incident on the boundary of this section will be completely reflected. Actually, this will be our desired phenomenon. Without it, it was impossible to make fiber optics.
Conclusion
The practical application of the behavior of light flux has given a lot, creating a variety of technical devices to improve our lives. At the same time, light has not yet revealed all its possibilities to humanity and its practical potential has not yet been fully realized.
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REFRACTION INDEX(refractive index) - optical. characteristic of the environment associated with refraction of light at the interface between two transparent optically homogeneous and isotropic media during its transition from one medium to another and due to the difference in the phase velocities of light propagation in the media. The value of P. p. is equal to the ratio of these speeds. relative
P. p. of these environments. If light falls on the second or first medium from (where is the speed of light With), then the quantities absolute pp of these averages. In this case, a the law of refraction can be written in the form where and are the angles of incidence and refraction.
The magnitude of the absolute power factor depends on the nature and structure of the substance, its state of aggregation, temperature, pressure, etc. At high intensities, the power factor depends on the intensity of light (see. Nonlinear optics). In a number of substances, P. changes under the influence of external influences. electric fields ( Kerr effect- in liquids and gases; electro-optical Pockels effect- in crystals).
For a given medium, the absorption band depends on the light wavelength l, and in the region of absorption bands this dependence is anomalous (see Fig. Light dispersion).In X-ray. region, the power factor for almost all media is close to 1, in the visible region for liquids and solids it is about 1.5; in the IR region for a number of transparent media 4.0 (for Ge).
Lit.: Landsberg G.S., Optics, 5th ed., M., 1976; Sivukhin D.V., General course, 2nd ed., [vol. 4] - Optics, M., 1985. V. I. Malyshev,