Methods for determining the speed of light. Speed ​​of light and methods of measuring it. The first proposals were put forward by Galileo: a lantern and a mirror were installed on the tops. The speed of light and methods for determining it The speed of light test was the first to measure the speed of light using a laboratory method

Really, how? How to measure the highest speed in Universe in our modest, Earthly conditions? We no longer need to rack our brains over this - after all, over several centuries, so many people have worked on this issue, developing methods for measuring the speed of light. Let's start the story in order.

Speed ​​of light– propagation speed electromagnetic waves in a vacuum. It is denoted by the Latin letter c. The speed of light is approximately 300,000,000 m/s.

At first, no one thought about the issue of measuring the speed of light. There is light - that’s great. Then, in the era of antiquity, the prevailing opinion among scientific philosophers was that the speed of light is infinite, that is, instantaneous. Then it happened Middle Ages with the Inquisition, when the main question of thinking and progressive people was “How to avoid getting caught in the fire?” And only in epochs Renaissance And Enlightenment The opinions of scientists multiplied and, of course, were divided.

So, Descartes, Kepler And Farm were of the same opinion as the scientists of antiquity. But he believed that the speed of light is finite, although very high. In fact, he made the first measurement of the speed of light. More precisely, he made the first attempt to measure it.

Galileo's experiment

Experience Galileo Galilei was brilliant in its simplicity. The scientist conducted an experiment to measure the speed of light, armed with simple improvised means. At a large and well-known distance from each other, on different hills, Galileo and his assistant stood with lit lanterns. One of them opened the shutter on the lantern, and the second had to do the same when he saw the light of the first lantern. Knowing the distance and time (the delay before the assistant opens the lantern), Galileo expected to calculate the speed of light. Unfortunately, for this experiment to succeed, Galileo and his assistant had to choose hills that were several million kilometers apart. I would like to remind you that you can order an essay by filling out an application on the website.

Roemer and Bradley experiments

The first successful and surprisingly accurate experiment in determining the speed of light was that of a Danish astronomer Olaf Roemer. Roemer used the astronomical method of measuring the speed of light. In 1676, he observed Jupiter's satellite Io through a telescope, and discovered that the time of the eclipse of the satellite changes as the Earth moves away from Jupiter. The maximum delay time was 22 minutes. Calculating that the Earth is moving away from Jupiter at a distance of the diameter of the Earth's orbit, Roemer divided the approximate value of the diameter by the delay time, and received a value of 214,000 kilometers per second. Of course, such a calculation was very rough, the distances between the planets were known only approximately, but the result turned out to be relatively close to the truth.

Bradley's experience. In 1728 James Bradley estimated the speed of light by observing the aberration of stars. Abberation is a change in the apparent position of a star caused by the movement of the earth in its orbit. Knowing the speed of the Earth and measuring the aberration angle, Bradley obtained a value of 301,000 kilometers per second.

Fizeau's experience

The scientific world of that time reacted with distrust to the result of the experiment of Roemer and Bradley. However, Bradley's result was the most accurate for over a hundred years, right up to 1849. That year, a French scientist Armand Fizeau measured the speed of light using the rotating shutter method, without observing celestial bodies, but here on Earth. In fact, this was the first laboratory method for measuring the speed of light since Galileo. Below is a diagram of its laboratory setup.

The light, reflected from the mirror, passed through the teeth of the wheel and was reflected from another mirror, 8.6 kilometers away. The speed of the wheel was increased until the light became visible in the next gap. Fizeau's calculations gave a result of 313,000 kilometers per second. A year later, a similar experiment with a rotating mirror was carried out by Leon Foucault, who obtained a result of 298,000 kilometers per second.

With the advent of masers and lasers, people have new opportunities and ways to measure the speed of light, and the development of the theory also made it possible to calculate the speed of light indirectly, without making direct measurements.

The most accurate value of the speed of light

Humanity has accumulated vast experience in measuring the speed of light. Today, the most accurate value for the speed of light is considered to be 299,792,458 meters per second, received in 1983. It is interesting that further, more accurate measurement of the speed of light turned out to be impossible due to errors in the measurement meters. Currently, the value of a meter is tied to the speed of light and is equal to the distance that light travels in 1/299,792,458 of a second.

Finally, as always, we suggest watching an educational video. Friends, even if you are faced with such a task as independently measuring the speed of light using improvised means, you can safely turn to our authors for help. You can order a test paper online by filling out an application on the Correspondence Student website. We wish you a pleasant and easy study!

Laboratory methods for determining the speed of light are essentially improvements on Galileo's method.

a) Interrupt method.

Fizeau (1849) was the first to determine the speed of light in laboratory conditions. Characteristic feature His method is the automatic registration of the start and return moments of the signal, carried out by regularly interrupting the light flux (gear wheel). The scheme of Fizeau's experiment is shown in Fig. 9.3. Light from source S goes between the teeth of a rotating wheel W to the mirror M and, having been reflected back, must again pass between the teeth to the observer. For convenience, eyepiece E, serving for observation, is placed opposite A, and the light turns from S To W using a translucent mirror N. If the wheel rotates, and at such an angular speed that during the movement of light from A To M and back in place of the teeth there will be slits, and vice versa, then the returning light will not be transmitted to the eyepiece and the observer will not see the light (the first eclipse). As the angular velocity increases, the light will partially reach the observer. If the width of the teeth and gaps are the same, then at double speed there will be a maximum of light, at triple speed there will be a second eclipse, etc. Knowing the distance aM=D, number of teeth z, angular speed of rotation (number of revolutions per second) n, you can calculate the speed of light.

Or With=2Dzn.

The main difficulty of determination lies in the exact moment of the eclipse. Accuracy increases with increasing distance D and at interruption speeds that allow observation of higher order eclipses. Thus, Perrotin made his observations at D=46 km and observed a 32nd order eclipse. Under these conditions, high-aperture installations, clean air (observations in the mountains), good optics, and a strong light source are required.

IN Lately Instead of a rotating wheel, other, more advanced methods of interrupting light are successfully used.

b) Rotating mirror method.

Foucault (1862) successfully implemented the second method, the principle of which had been proposed even earlier (1838) by Arago for the purpose of comparing the speed of light in air with the speed of light in other media (water). The method is based on very careful measurements of short periods of time using a rotating mirror. The experimental design is clear from Fig. 9.4. Light from source S guided by the lens L on a rotating mirror R, is reflected from it in the direction of the second mirror WITH and goes back, passing path 2 CR=2D during t. This time is estimated by the angle of rotation of the mirror R, the rotation speed of which is precisely known; the angle of rotation is determined from the measurement of the displacement of the bunny given by the returning light. Measurements are made using an eyepiece E and translucent plate M, playing the same role as in the previous method; S 1 – position of the bunny with a stationary mirror R, S" 1 – when the mirror rotates. An important feature of Foucault’s installation was its use as a mirror WITH concave spherical mirror, with the center of curvature lying on the axis of rotation R. Due to this, the light reflected from R To WITH, always ended up back on R; in the case of using a flat mirror WITH this would happen only with a certain mutual orientation R And WITH, when the axis of the reflected cone of rays is located normal to WITH.

Foucault, in accordance with Arago's original plan, also used his device to determine the speed of light in water, because he managed to reduce the distance RС up to 4 m, giving the mirror 800 revolutions per second. Foucault's measurements showed that the speed of light in water is less than in air, in accordance with the ideas wave theory Sveta.

Michelson's last (1926) installation was made between two mountain peaks, so the resulting distance D» 35.4 km (more precisely, 35,373.21 m). The mirror was an octagonal steel prism rotating at a speed of 528 rps.

The time it took for the light to travel the full way was 0.00023 s, so the mirror had time to rotate 1/8 of a revolution and the light fell on the edge of the prism. Thus, the bunny’s displacement was relatively insignificant, and the determination of its position played the role of a correction, and not the main measured value, as in Foucault’s first experiments, where the entire displacement reached only 0.7 mm.

Very accurate measurements of the speed of propagation of radio waves were also made. In this case, radiogeodetic measurements were used, i.e. determining the distance between two points using radio signals in parallel with precise triangulation measurements. The best value obtained by this method, reduced to vacuum, is c = 299,792 ± 2.4 km/s. Finally, the speed of radio waves was determined using the method of standing waves generated in a cylindrical resonator. The theory allows us to relate data on the dimensions of the resonator and its resonant frequency with the speed of the waves. The experiments were done with an evacuated resonator, so reduction to a vacuum was not required. The best value obtained by this method is c = 299,792.5 ± 3.4 km/s.

c) Phase and group speeds of light.

Laboratory methods for determining the speed of light, which allow these measurements to be made on a short basis, make it possible to determine the speed of light in various media and, therefore, test the relationships of the theory of light refraction. As has already been mentioned several times, the refractive index of light in Newton’s theory is equal to n=sin i/sin r=υ 2 /υ 1, and in the wave theory n=sin i/sin r=υ 1 /υ 2 where υ 1 is the speed of light in the first medium, and υ 2 – speed of light in the second medium. Arago also saw in this difference the possibility of an experimentum crucis and proposed the idea of ​​an experiment, which was carried out later by Foucault, who found for the ratio of the speeds of light in air and water a value close to , as follows from Huygens’ theory, and not, as follows from Newton’s theory.

Conventional determination of refractive index n=sin i/sin r=υ 1 /υ 2 from the change in the direction of the wave normal at the boundary of two media gives the ratio of the phase velocities of the wave in these two media. However, the concept of phase velocity is applicable only to strictly monochromatic waves, which are not realistically feasible, since they would have to exist indefinitely in time and howl infinitely extended in space.

In reality, we always have a more or less complex impulse, limited in time and space. When observing such a pulse, we can highlight some specific place, for example, the place of maximum extension of that electrical or magnetic field, which is an electromagnetic pulse. The speed of the pulse can be identified with the speed of propagation of any point, for example, the point of maximum field strength.

However, the medium (with the exception of vacuum) is usually characterized by dispersion, i.e. monochromatic waves propagate with different phase velocities depending on their length, and the pulse begins to deform. In this case, the question of the speed of the impulse becomes more complex. If the dispersion is not very large, then the pulse deformation occurs slowly and we can monitor the movement of a certain field amplitude in the wave pulse, for example, the maximum field amplitude. However, the speed of movement of the pulse, called by Rayleigh group velocity, will differ from the phase velocity of any of its constituent monochromatic waves.

For simplicity of calculations, we will imagine a pulse as a set of two sinusoids of equal amplitude that are close in frequency, and not as a set of an infinite number of close sinusoids. With this simplification, the main features of the phenomenon are preserved. So, our impulse, or, as they say, a group of waves, is composed of two waves.

where the amplitudes are assumed to be equal, and the frequencies and wavelengths differ little from each other, i.e.

where and are small quantities. Impulse (wave group) at there is a sum at 1 and at 2, i.e.

Introducing the notation, let us represent our momentum in the form where A not constantly, but changes in time and space, but changes slowly, because δω And δk– small (compared to ω 0 and κ 0) quantities. Therefore, allowing for a certain carelessness in speech, we can consider our impulse to be a sinusoid with a slowly changing amplitude.

Thus, the speed of the impulse (group), which, according to Rayleigh, is called group velocity, is the speed of movement amplitudes, and, consequently, energy, carried by a moving impulse.

So, a monochromatic wave is characterized by a phase velocity υ=ω /κ , indicating the speed of movement phases, and the impulse is characterized by the group velocity u=dω/dκ, corresponding to the speed of propagation of the field energy of this pulse.

It is not difficult to find a connection between u And υ . Indeed,

or, since and therefore,

those. finally

(Rayleigh formula).

Difference between u And υ the more significant the greater the dispersion dυ/dλ. In the absence of dispersion ( dυ/dλ=0) we have u=υ. This case, as already said, occurs only for vacuum.

Rayleigh showed that in the known methods for determining the speed of light, by the very essence of the method, we are not dealing with a continuously lasting wave, but breaking it into small segments. The gear wheel and other interrupters in the interruption method provide weakening and increasing light excitation, i.e. group of waves. The same thing happens in Roemer's method, where the light is interrupted by periodic darkening. In the rotating mirror method, light also stops reaching the observer when the mirror is rotated sufficiently. In all these cases, we measure the group velocity in a dispersive medium, not the phase velocity.

Rayleigh believed that in the light aberration method we measure the direct phase velocity, because there the light is not interrupted artificially. However, Ehrenfest (1910) showed that the observation of light aberration is in principle indistinguishable from Fizeau’s method, i.e. also gives group velocity. Indeed, the aberration experience can be reduced to the following. Two disks with holes are rigidly fixed on a common axis. Light is sent along a line connecting these holes and reaches the observer. Let's put the whole apparatus into rapid rotation. Since the speed of light is finite, light will not pass through the second hole. To transmit light, it is necessary to rotate one disk relative to the other by an angle determined by the ratio of the speeds of the disks and light. This is a typical aberration experience; however, it is no different from Fizeau’s experiment, in which instead of two rotating disks with holes, there is one disk and a mirror for turning the rays, i.e. essentially two disks: the real one and its reflection in a fixed mirror. So, the aberration method gives the same as the interruption method, i.e. group speed.

Thus, in Michelson's experiments with both water and carbon disulfide, the ratio of group rather than phase velocities was measured.

In 1676, Danish astronomer Ole Römer made the first rough estimate of the speed of light. Roemer noticed a slight discrepancy in the duration of the eclipses of Jupiter's moons and concluded that the movement of the Earth, either approaching or moving away from Jupiter, changed the distance that the light reflected from the satellites had to travel.

By measuring the magnitude of this discrepancy, Roemer calculated that the speed of light is 219,911 kilometers per second. In a later experiment in 1849, French physicist Armand Fizeau found the speed of light to be 312,873 kilometers per second.

As shown in the figure above, Fizeau's experimental setup consisted of a light source, a translucent mirror that reflects only half of the light falling on it, allowing the rest to pass through a rotating gear and a stationary mirror. When light hit the translucent mirror, it was reflected onto a gear wheel, which split the light into beams. After passing through a system of focusing lenses, each light beam was reflected from a stationary mirror and returned back to the gear wheel. By making precise measurements of the speed at which the gear wheel blocked the reflected beams, Fizeau was able to calculate the speed of light. His colleague Jean Foucault improved this method a year later and found that the speed of light is 297,878 kilometers per second. This value differs little from the modern value of 299,792 kilometers per second, which is calculated by multiplying the wavelength and frequency of laser radiation.

Fizeau's experiment

As shown in the pictures above, light travels forward and returns back through the same gap between the teeth of the wheel when the wheel rotates slowly (bottom picture). If the wheel spins quickly (top picture), an adjacent cog blocks the returning light.

Fizeau's results

By placing the mirror 8.64 kilometers from the gear, Fizeau determined that the speed of rotation of the gear required to block the returning light beam was 12.6 revolutions per second. Knowing these figures, as well as the distance traveled by the light, and the distance the gear had to travel to block the light beam (equal to the width of the gap between the teeth of the wheel), he calculated that the light beam took 0.000055 seconds to travel distance from the gear to the mirror and back. Dividing by this time the total distance of 17.28 kilometers traveled by the light, Fizeau obtained a value for its speed of 312873 kilometers per second.

Foucault's experiment

In 1850, French physicist Jean Foucault improved Fizeau's technique by replacing the gear wheel with a rotating mirror. Light from the source reached the observer only when the mirror completed a full 360° rotation during the time interval between the departure and return of the light beam. Using this method, Foucault obtained a value for the speed of light of 297878 kilometers per second.

The final chord in measuring the speed of light.

The invention of lasers has enabled physicists to measure the speed of light with much greater accuracy than ever before. In 1972, scientists at the National Institute of Standards and Technology carefully measured the wavelength and frequency of a laser beam and recorded the speed of light, the product of these two variables, to be 299,792,458 meters per second (186,282 miles per second). One of the consequences of this new measurement was the decision of the General Conference of Weights and Measures to adopt as the standard meter (3.3 feet) the distance that light travels in 1/299,792,458 of a second. Thus / the speed of light, the most important fundamental constant in physics, is now calculated with very high confidence, and the reference meter can be determined much more accurately than ever before.

There are various methods for measuring the speed of light, including astronomical ones and using various experimental techniques. Quantity measurement accuracy WITH is constantly increasing. The table provides an incomplete list of experimental works to determine the speed of light.

The pictures show a reproduction of the drawing of the Roemer, as well as a schematic interpretation.

Römer's astronomical method is based on the measurement speed light from observations from Earth of eclipses of Jupiter's satellites. Jupiter has severalabout satellites that are either visible from Earth near Jupiter, or

Let at a certain moment in time the EarthZ1 and Jupiter J1 are in opposition, and at this moment in time one of Jupiter’s satellites, observed from Earth, disappears in the shadow of Jupiter (the satellite is not shown in the figure). Then, if we denote by R and r the radii of the orbits of Jupiter and Earth and by c the speed of light eta in in the coordinate system associated with the Sun C, on Earth, the departure of the satellite into the shadow of Jupiter will be recorded (R-r)/s seconds later than it occurs in the time reporting system associated with Jupiter.

After 0.545 years, Earth Z2 and Jupiter J2 are in conjunction. If at this time the nth eclipse of the same satellite of Jupiter occurs, then on Earth it will be registered with a delay of (R+r)/s seconds. Therefore, if the period of revolution of the satellite around Jupiter is t, then the time interval T1 occurring between the first and nth eclipses observed from the Earth is equal to

After another 0.545 year, Earth 33 and Jupiter 3 will again be in opposition. During this time, (n-1) revolutions of the satellite around Jupiter and (n-1) eclipses occurred, the first of which took place when the Earth and Jupiter occupied positions Z2 and Yu2, and the last when they occupied the positions Z3 and Yu3. The first eclipse was observed on Earth with a delay (R+r)/s, and the last with a delay (R-r)/s in relation to the moments of the satellite leaving the shadow of the planet Jupiter. Therefore, in this case we have

Roemer measured the time intervals T1 and T2 and found that T1-T2 = 1980 s. But from the formulas written above it follows that T1-T2 = 4r/s, therefore c = 4r/1980 m/s. Taking r, the average distance from the Earth to the Sun, equal to 1500000000 km, we find the value for the speed of light to be 3.01 * 10 6 m/s.

This result was the first measurement of the speed of light.

In 1725 James Bradley discovered that the star Draco, located at the zenith (i.e. directly overhead), makes an apparent motion with a period of one year in an almost circular orbit with a diameter equal to 40.5 arc seconds. For stars visible elsewhere in the sky, Bradley also observed a similar apparent motion - generally elliptical.

The phenomenon observed by Bradley is called aberration. It has nothing to do with the star's own motion. The reason for the aberration is that the speed of light is finite, and observation is carried out from the Earth moving in orbit at a certain speed v.

The angle of the cone at which the apparent trajectory of the star is visible from the Earth is determined by the expression: tgα=ν/c

Knowing the angle α and the speed of the Earth's orbit v, we can determine the speed of light c.

He obtained a value for the speed of light equal to 308,000 km/s.

In 1849, you were the first to determine the speed of light in laboratory conditions. A. Fizeau. His method was called the cogwheel method. A characteristic feature of his method is the automatic recording of the start and return moments of the signal, carried out by regularly interrupting the light flux (gear wheel).

Figure shows a diagram of an experiment to determine the speed of light using the gear wheel method.

The light from the source passed through the chopper (the teeth of the rotating wheel) and, reflected from the mirror, returned again to the gear wheel. Knowing the distance between the wheel and the mirror, the number of teeth of the wheel, and the speed of rotation, you can calculate the speed of light.

Knowing the distance D, the number of teeth z, the angular speed of rotation (rpm) v, you can determine the speed of light. He got it to be equal to 313,000 km/s.

Throughout his life, the American physicist Albert Abraham Michelson(1852–1931) improved the technique for measuring the speed of light. Creating increasingly complex installations, he tried to obtain results with minimal error. In 1924–1927, Michelson developed an experimental design in which a beam of light was sent from the top of Mount Wilson to the top of San Antonio (a distance of about 35 km). The rotating shutter was a rotating mirror, manufactured with extreme precision and driven by a specially designed high-speed rotor that makes up to 528 revolutions per second.

By changing the rotation frequency of the rotor, the observer achieved the appearance of a stable image of the light source in the eyepiece. Knowing the distance between the installations and the frequency of rotation of the mirror made it possible to calculate the speed of light.

From 1924 to the beginning of 1927, five independent series of observations were carried out, increasing the accuracy of measuring distance and rotor speed. The average measurement result was 299,798 km per second.

The results of all Michelson’s measurements can be written as c = (299796 ± 4) km/s.

The top figure shows a diagram of Michelson's experiment. The figure below shows a simplified diagram of the experiment. The user can change the rotation frequency of the octagonal prism, observing the movement of the light pulse and ensuring that it hits the observer's eyepiece.

The frequency can be changed from 0 to 1100 rpm in steps of 2 s –1. To make it easier to set the frequency in the experiment, a coarse speed control knob has been made; more precise settings can be set using additional keys to the right of the frequency window. The optimal result is achieved at 528 and 1056 rpm. At 0 revolutions, a static beam of light is drawn from the source to the observer.

An example of calculating the speed of light for an experiment in which the observer detects the appearance of light at a mirror rotation frequency of 528 s –1.

Here ν and T are the frequency and period of rotation of the octagonal prism, τ 1 is the time during which the light beam manages to travel the distance L from one installation to another and return back, it is also the time of rotation of one face of the mirror.

Based on materials from www.school-collection.edu.ru

One of the important properties is the speed of light propagation in vacuum and other optical media. The enormous value of the speed of light compared to the speed of propagation of various moving objects observed by humans in practical life posed many difficulties both in explaining many optical phenomena and in the practical determination of the speed of light. To show how difficult it was for a person to perceive the possibility of moving matter, in this case light, at enormous speeds, we can give an example of determining the speed of light undertaken by the Italian scientist Galileo Galilei, who, together with his collaborator, positioned themselves on two neighboring mountain peaks and signaled each other with the light of lanterns . One participant in this experiment opened the lid of the lantern and turned on the clock at the same time. The second participant, having received a light signal, also opened the lantern and sent light in the direction of the first experimenter, who, having received a response signal, stopped the clock. Knowing the distance between the tops of the mountains and the time it takes light to travel this distance back and forth, you can get the speed of light. It is, of course, clear to us why this attempt to determine the speed of light did not give the desired results.

It soon became clear that in order to measure the speed of propagation of light with the required accuracy, it was necessary to have large distances for the light to travel, firstly, and it was necessary to measure time with very high accuracy, secondly.

To obtain accurate time readings, light modulation is used, and three main modulation methods are used:

• Gear method,
• Rotating mirror method
• Electric shutter method.

In all these methods, the propagation time is determined from a measurement of the modulation frequency.

Let us briefly consider these three options for light modulation using examples.

Fizeau's method. Figure 1.3.1 shows circuit diagram installations used in the Fizeau method, where the light flux is modulated by a rotating gear wheel. Light from a light source 1 condenser system is directed to a translucent mirror 2 , reflected from which passes between the teeth of a rotating gear wheel 5 . Next, the collimator system 3 directs a beam of rays onto a concave mirror 4 , reflected from which the light travels back along the same path to the translucent mirror 2 . Observation is made by the human eye through an eyepiece 6 .

If the gear wheel is stationary, then the light will pass through the gap between the teeth and return back through the same gap. By setting the gear wheel in rotation and increasing the rotation speed, it is possible to achieve that during the time the light comes from the wheel 5 to the mirror 4 and back the wheel will turn the width of the tooth and the tooth will take the place of the gap. In this case, the light will not enter the eyepiece 6 . By further increasing the speed of rotation of the wheel, you can obtain the passage of light back through the adjacent gap, etc.

Fizeau had a wheel with 720 teeth and a double path length of the light beam of about 17 km. From his experiments, the speed of light turned out to be 3.15. 10 10 cm/With. The main mistake here is related to the difficulty of recording the moment of darkening. Further improvements to this method led to more accurate measurements of the speed of light.

Rotating mirror method. This method, proposed by Wheatstone, was used by Foucault in 1960. The installation diagram is shown in Fig. 1.3.2. From the radiation source 1 light passing through a translucent mirror 2 and lens 3 guided by a rotating mirror 4 to a spherical mirror 5 . Reflected from the mirror 5 , the light flux went back and was focused by the observation system, including A(with a fixed mirror 4 ). With a rotating mirror, during the time the light travels twice the path L, the mirror had time to rotate through a certain angle and the light flux reflected from it in the reverse direction was focused at a point B. Measuring the distance between A And B, we get the angle at which the mirror rotates 4 and, therefore, knowing the speed of rotation of the mirror, the time it takes light to travel the distance. At , the found value of the speed of light propagation turned out to be equal to 2.98. 10 10 cm/With. Distance between A And B was equal to only 0.7 mm, and the main source of errors lay in the inaccuracy of measuring this distance.

Kerr electric shutter method. In this method, a Kerr cell acts as a modulating device (a Kerr cell filled with a polar liquid and placed between crossed nicols transmits light only when an electric field is applied). The installation diagram is shown in Fig. 1.3.3. Light from a mercury lamp 1 passes through a Kerr gate onto a translucent mirror 2 , is reflected from it to the right and hits the mirror 3 . After reflection from mirror 3, the light in the reverse path of the rays hits the energy receiver 8 .

Part of the light energy passes through a translucent mirror and overcomes the path determined by the mirrors 4 , 5 , 6 , 7 and back, also hits the receiver 8 .

The accuracy of this method is determined high frequency modulation of the light flux created by a Kerr cell exposed to a high-frequency electric field, and the ability to accurately measure the phase shift of two light streams coming from the mirror 3 and from the mirror 7 .

The value obtained for the speed of light is . The modern generally accepted value for the speed of light in a vacuum.

For optical media with a refractive index, the speed of light is determined by the expression: .