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Despite the fact that in ordinary life we ​​do not have to calculate the speed of light, many have been interested in this quantity since childhood.

Watching lightning during a thunderstorm, every child probably tried to understand what caused the delay between its flash and thunderclaps. Obviously, light and sound have different speeds. Why is this happening? What is the speed of light and how can it be measured?

In science, the speed of light is the speed at which rays move in air or vacuum. Light is electromagnetic radiation that is perceived by the human eye. He is able to move in any environment, which has a direct impact on his speed.

Attempts to measure this quantity have been made since ancient times. Scientists of ancient times believed that the speed of light was infinite. The same opinion was expressed by physicists of the 16th–17th centuries, although even then some researchers, such as Robert Hooke and Galileo Galilei, assumed finitude.

A major breakthrough in the study of the speed of light occurred thanks to the Danish astronomer Olaf Roemer, who was the first to draw attention to the delay in the eclipse of Jupiter's moon Io compared to initial calculations.

Then the scientist determined the approximate speed value to be 220 thousand meters per second. British astronomer James Bradley was able to calculate this value more accurately, although he was slightly mistaken in his calculations.


Subsequently, attempts to calculate the real speed of light were made by scientists from different countries. However, only in the early 1970s, with the advent of lasers and masers that had a stable radiation frequency, were researchers able to make an accurate calculation, and in 1983 it was taken as a basis modern meaning with correlation to relative error.

If we talk in simple language, the speed of light is the time it takes a sunbeam to travel a certain distance. It is customary to use the second as the unit of time, and the meter as the distance unit. From the point of view of physics, light is a unique phenomenon that has a constant speed in a specific environment.

Suppose a person is running at a speed of 25 km/h and is trying to catch up with a car that is traveling at a speed of 26 km/h. It turns out that the car is moving at 1 km/h faster than a runner. With light everything is different. Regardless of the speed of movement of the car and the person, the beam will always move relative to them at a constant speed.

The speed of light largely depends on the substance in which the rays propagate. In a vacuum it has a constant value, but in a transparent environment it can have different indicators.

In air or water its value is always less than in vacuum. For example, in rivers and oceans the speed of light is about ¾ of the speed in space, and in air at a pressure of 1 atmosphere it is 2% less than in vacuum.


This phenomenon is explained by the absorption of rays in transparent space and their re-emission by charged particles. The effect is called refraction and is actively used in the manufacture of telescopes, binoculars and other optical equipment.

If we consider specific substances, then in distilled water the speed of light is 226 thousand kilometers per second, in optical glass - about 196 thousand kilometers per second.

In a vacuum, the speed of light per second has a constant value of 299,792,458 meters, that is, a little more than 299 thousand kilometers. In the modern view, it is the ultimate. In other words, no particle, no celestial body is capable of reaching the speed that light develops in outer space.

Even if we assume that Superman will appear and fly at great speed, the beam will still run away from him with greater speed.

Although the speed of light is the maximum achievable in vacuum space, it is believed that there are objects that move faster.

For example, sunbeams, shadows, or phases of oscillation in waves are capable of this, but with one caveat - even if they develop superspeed, energy and information will be transmitted in a direction that does not coincide with the direction of their movement.


As for the transparent medium, there are objects on Earth that are quite capable of moving faster than light. For example, if a beam passing through glass slows down its speed, then electrons are not limited in the speed of movement, so when passing through glass surfaces they can move faster than light.

This phenomenon is called the Vavilov–Cherenkov effect and is most often observed in nuclear reactors or in the depths of the oceans.

The speed of light in different media varies significantly. The difficulty is that the human eye does not see it in the entire spectral range. The nature of the origin of light rays has interested scientists since ancient times. The first attempts to calculate the speed of light were made as early as 300 BC. At that time, scientists determined that the wave propagated in a straight line.

Quick response

They managed to describe with mathematical formulas the properties of light and the trajectory of its movement.

became known 2 thousand years after the first research.

What is luminous flux?

A light beam is an electromagnetic wave combined with photons. Photons are understood as the simplest elements, which are also called quanta of electromagnetic radiation. The luminous flux in all spectra is invisible. It does not move in space in the traditional sense of the word. To describe the state of an electromagnetic wave with quantum particles, the concept of the refractive index of an optical medium is introduced. The luminous flux is transferred in space in the form of a beam with a small cross section

. The method of movement in space is derived by geometric methods. This is a rectilinear beam, which, at the border with various media, begins to refract, forming a curvilinear trajectory. Scientists have proven that the maximum speed is created in a vacuum; in other environments, the speed of movement can vary significantly. Scientists have developed a system in which a light beam and a derived value are the basis for the derivation and reading of certain SI units.

Some historical facts

Galileo Galilei noticed that Jupiter had an interval between eclipses of its four satellites of 1320 seconds. Based on these discoveries, in 1676, Danish astronomer Ole Roemer calculated the speed of propagation of a light beam as 222 thousand km/sec. At that time, this measurement was the most accurate, but it could not be verified by earthly standards.

After 200 years, Louise Fizeau was able to calculate the speed of a light beam experimentally. He created a special installation with a mirror and a gear mechanism that rotated at high speed. The light flux was reflected from the mirror and returned back after 8 km. As the wheel speed increased, a moment arose when the gear mechanism blocked the beam. Thus, the speed of the beam was set at 312 thousand kilometers per second.

Foucault improved this equipment, reducing the parameters by replacing the gear mechanism with a flat mirror. His measurement accuracy turned out to be closest to the modern standard and amounted to 288 thousand meters per second. Foucault made attempts to calculate the speed of light in a foreign medium, using water as a basis. The physicist was able to conclude that this value is not constant and depends on the characteristics of refraction in a given medium.

A vacuum is a space free of matter. The speed of light in vacuum in the C system is designated Latin letter C. It is unattainable. No item can be overclocked to such a value. Physicists can only imagine what might happen to objects if they accelerate to such an extent. The speed of propagation of a light beam has constant characteristics, it is:

• constant and final;
• unattainable and unchangeable.

Knowing this constant allows you to calculate with what maximum speed objects can move in space. The amount of propagation of a light beam is recognized as a fundamental constant. It is used to characterize space-time. This is the maximum permissible value for moving particles. What is the speed of light in a vacuum? The current value was obtained through laboratory measurements and mathematical calculations. She equal to 299.792.458 meters per second with an accuracy of ± 1.2 m/s. In many disciplines, including school ones, approximate calculations are used to solve problems. An indicator equal to 3,108 m/s is taken.

Light waves visible to man spectrum and X-ray waves can be accelerated to readings approaching the speed of light. They cannot equal this constant, nor exceed its value. The constant was derived based on tracking the behavior of cosmic rays at the moment of their acceleration in special accelerators. It depends on the inertial medium in which the beam propagates. In water, the transmission of light is 25% lower, and in air it will depend on temperature and pressure at the time of calculations.

All calculations were carried out using the theory of relativity and the law of causality derived by Einstein. The physicist believes that if objects reach a speed of 1,079,252,848.8 kilometers/hour and exceed it, then irreversible changes will occur in the structure of our world and the system will break down. Time will begin to count down, disrupting the order of events.

The definition of meter is derived from the speed of a light ray. It is understood as the area that a light beam manages to travel in 1/299792458 of a second. This concept should not be confused with the standard. The meter standard is a special technical device cadmium-based with crosshatching, allowing you to see given distance physically.

Speed ​​of light - absolute value speed of propagation of electromagnetic waves in vacuum. In physics, it is traditionally denoted by the Latin letter “c” (pronounced [tse]). The speed of light in a vacuum is a fundamental constant that does not depend on the choice of inertial reference frame (IFR). It refers to the fundamental physical constants that characterize not just individual bodies, but the properties of space-time as a whole. According to modern concepts, the speed of light in a vacuum is the maximum speed of particle movement and the propagation of interactions. Also important is the fact that this value is absolute. This is one of the postulates of SRT.

In a vacuum (emptiness)

In 1977, it was possible to calculate the approximate speed of light equal to 299,792,458 ± 1.2 m/s, calculated based on the 1960 standard meter. It is currently believed that the speed of light in a vacuum is a fundamental physical constant, by definition exactly equal to 299,792,458 m/s, or approximately 1,079,252,848.8 km/h. The exact value is due to the fact that since 1983, the standard meter has been taken to be the distance that light travels in a vacuum in a period of time equal to 1/299,792,458 seconds. The speed of light is symbolized by the letter c.

Michelson's experiment, fundamental to SRT, showed that the speed of light in a vacuum does not depend either on the speed of the light source or on the speed of the observer. In nature, the following propagate at the speed of light:

actual visible light

other types of electromagnetic radiation (radio waves, x-rays, etc.)

From the special theory of relativity it follows that the acceleration of particles with rest mass to the speed of light is impossible, since this event would violate the fundamental principle of causality. That is, it is excluded that the signal exceeds the speed of light, or the movement of mass at such a speed. However, the theory does not exclude the movement of particles in space-time at superluminal speeds. Hypothetical particles moving at superluminal speeds are called tachyons. Mathematically, tachyons easily fit into the Lorentz transformation - they are particles with imaginary mass. The higher the speed of these particles, the less energy they carry, and vice versa, the closer their speed is to the speed of light, the greater their energy - just like the energy of ordinary particles, the energy of tachyons tends to infinity as they approach the speed of light. This is the most obvious consequence of the Lorentz transformation, which does not allow a particle to accelerate to the speed of light - it is simply impossible to impart an infinite amount of energy to a particle. It should be understood that, firstly, tachyons are a class of particles, and not just one type of particle, and, secondly, no physical interaction can propagate faster than the speed of light. It follows from this that tachyons do not violate the principle of causality - they do not interact in any way with ordinary particles, and the difference in their speeds between themselves is also not equal to the speed of light.

Ordinary particles that move slower than light are called tardyons. Tardions cannot reach the speed of light, but only approach it arbitrarily close, since in this case their energy becomes unlimitedly large. All tardyons have rest mass, unlike massless photons and gravitons, which always move at the speed of light.

In Planck units, the speed of light in a vacuum is 1, that is, light travels 1 unit of Planck length per unit of Planck time.

In a transparent environment

The speed of light in a transparent medium is the speed at which light travels in a medium other than a vacuum. In a medium with dispersion, phase and group velocities are distinguished.

Phase velocity relates the frequency and wavelength of monochromatic light in a medium (λ=c/ν). This speed is usually (but not necessarily) less than c. The ratio of the phase speed of light in a vacuum to the speed of light in a medium is called the refractive index of the medium. The group speed of light in an equilibrium medium is always less than c. However, in nonequilibrium media it can exceed c. In this case, however, the leading edge of the pulse still moves at a speed not exceeding the speed of light in vacuum.

Armand Hippolyte Louis Fizeau experimentally proved that the movement of a medium relative to a light beam is also capable of influencing the speed of propagation of light in this medium.

Negation of the postulate about the maximum speed of light

IN last years There are often reports that in so-called quantum teleportation the interaction propagates faster than the speed of light. For example, on August 15, 2008, the research group of Dr. Nicolas Gisin from the University of Geneva, studying bound photon states separated by 18 km in space, allegedly showed that “interactions between particles occur at a speed of approximately one hundred thousand times higher speed Sveta". Previously, the so-called Hartmann paradox - superluminal speed with the tunnel effect - was also discussed.

A scientific analysis of the significance of these and similar results shows that they fundamentally cannot be used for superluminal transmission of any signal or movement of matter.

History of light speed measurements

Ancient scientists, with rare exceptions, considered the speed of light to be infinite. In modern times this issue became the subject of debate. Galileo and Hooke admitted that it was finite, although very large, while Kepler, Descartes and Fermat still defended the infinity of the speed of light.

The first estimate of the speed of light was given by Olaf Roemer (1676). He noticed that when the Earth and Jupiter are on opposite sides of the Sun, eclipses of Jupiter's satellite Io are delayed by 22 minutes compared to calculations. From this he obtained a value for the speed of light of about 220,000 km/sec - inaccurate, but close to the true one. Half a century later, the discovery of aberration made it possible to confirm the finiteness of the speed of light and refine its assessment.

Really, how? How to measure the most high 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– speed of propagation of electromagnetic waves in 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, among learned philosophers The prevailing opinion 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 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 until 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. By far the most exact value 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 fill out an application on the Correspondence Student website. We wish you a pleasant and easy study!

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 moons 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.