Many people have known about the existence of such a concept as the “speed of light” from early childhood. But not everyone knows in detail about the phenomenon.

Many drew attention to the fact that during a thunderstorm there is a delay between a flash of lightning and the sound of thunder. The outbreak, as a rule, reaches us faster. This means that it has greater speed than sound. What is the reason for this? What is the speed of light and how is it measured?

What is the speed of light?

Let's first understand what the speed of light is. Scientifically, this is such a quantity that shows how quickly the rays move in a vacuum or in air. You also need to know what light is. This is the radiation that is perceived by the human eye. Speed ​​depends on environmental conditions, as well as other properties, for example, refraction.

Interesting fact: It takes 1.25 seconds for light to travel from Earth to a satellite, the moon.

What is the speed of light in your own words?

To put it simply, the speed of light is the time span over which a light beam travels any distance. Time is usually measured in seconds. However, some scientists use different units. Distance is also measured in different ways. Basically - this is a meter. That is, this value is considered in m / s. Physics explains this as follows: a phenomenon that moves with a certain speed (constant).

To make it easier to understand, let's look at the following example. The cyclist moves at a speed of 20 km / h. He wants to catch up with the driver of the car, whose speed is 25 km / h. If you count, then the car goes 5 km / h faster than a cyclist. With rays of light, things are different. No matter how fast the first and second people move, the light, with respect to them, moves with constant speed.

What is the speed of light?

When not in a vacuum, various conditions affect light. The substance through which the rays pass, including. If the number of meters per second does not change without oxygen access, then in an environment with air access, the value changes.

Light travels more slowly through various materials such as glass, water and air. This phenomenon is given a refractive index to describe how much they slow down the movement of light. Glass has a refractive index of 1.5, which means that light passes through it at a speed of about 200 thousand kilometers per second. The refractive index of water is 1.3, and the refractive index of air is slightly more than 1, which means that air only slightly slows down the light.

Therefore, after passing through air or liquid, the speed slows down, becoming less than in vacuum. For example, in various reservoirs, the speed of movement of the rays is 0.75 of the speed in space. Also, with a standard pressure of 1.01 bar, the rate slows down by 1.5-2%. That is, under terrestrial conditions, the speed of light varies depending on environmental conditions.

For such a phenomenon, they came up with a special concept - refraction. That is, the refraction of light. It is widely used in various inventions. For example, a refractor is a telescope with an optical system. Also, with the help of this, binoculars and other equipment are also created, the essence of which is the use of optics.

In general, the smallest ray can be refracted through ordinary air. When passing through a specially created optical glass, the speed is approximately 195 thousand kilometers per second. This is almost 105 thousand km / s less than the constant.

The most accurate value of the speed of light

Physicists over the years have gained experience in researching the speed of light rays. At the moment, the most accurate value of the speed of light is 299,792 kilometers per second. The constant was established in 1933. The number is still relevant.

However, further difficulties arose with the determination of the indicator.This was due to meter error. Now the meter itself directly depends on the speed of light. It is equal to the distance that the rays travel in a certain number of seconds - 1 / speed of light.

What is the speed of light in vacuum?

Since light is not affected by various conditions in a vacuum, its speed does not change as it does on Earth. The speed of light in a vacuum is 299,792 kilometers per second. This indicator is the limit. It is believed that nothing in the world can move faster, even cosmic bodies that move pretty fast.

For example, a fighter, a Boeing X-43, which exceeds the speed of sound by almost 10 times (more than 11 thousand km / h), flies slower than a beam. The latter moves more than 96 thousand kilometers per hour faster.

How was the speed of light measured?

The very first scientists tried to measure this value. Different methods were used. In the period of antiquity, people of science believed that it is infinite, therefore it is impossible to measure it. This opinion remained for a long time, up to the 16-17th century. In those days, other scientists appeared who suggested that the beam has an end, and that speed can be measured.

The famous astronomer from Denmark Olaf Roemer brought knowledge of the speed of light to a new level. He noticed that the eclipse of Jupiter’s moon is late. Previously, no one paid attention to this. Consequently, he decided to calculate the speed.

He put forward an approximate speed, which was equal to about 220 thousand kilometers per second. Later, a scientist from England James Bradley took up the study. Although he was not completely right, he slightly approached the current research results.

After some time, most scientists became interested in this quantity. The research involved people from different countries. However, until the 70s of the 20th century there were no grandiose discoveries. Since the 1970s, when lasers and masers (quantum generators) were invented, scientists conducted research and obtained the exact speed. The current value has been relevant since 1983. Corrected only small errors.

Galileo's Experience

A scientist from Italy surprised all the researchers of those years with the simplicity and genius of his experience. He managed to measure the speed of light using ordinary tools that were at his fingertips.

He and his assistant climbed the neighboring hills, having previously calculated the distance between them. They took the lit lanterns, equipped them with dampers that open and close the lights. In turn, opening and closing the light, they tried to calculate the speed of light. Galileo and the assistant knew in advance with what delay they would open and close the light. When one has opened, the other does the same.

However, the experiment was a failure. To make it work, scientists would have to stand at a distance of millions of kilometers from each other.

The Experience of Römer and Bradley

This study has already been briefly written above. This is one of the most progressive experiences of the time. Römer used knowledge in astronomy to measure the speed of the rays. It happened in the year 76 of the 17th century.

The researcher observed Io (the satellite of Jupiter) through a telescope. He discovered the following pattern: the more our planet moves away from Jupiter, the greater the delay in the eclipse of Io. The biggest delay was 21-22 minutes.

Assuming that the satellite is moving away at a distance equal to the length of the diameter of the orbit, the scientist divided the distance by time. As a result, he received 214 thousand kilometers per second. Although this study is considered very approximate, because the distance was approximate, it approached the current indicator.

In the 18th century, James Bradley supplemented the study. To do this, he used aberration - a change in the position of the cosmic body due to the motion of the Earth around the sun. James measured the angle of aberration, and, knowing the speed of our planet, he got a value of 301 thousand kilometers per second.

Fizeau Experience

Researchers and ordinary people were skeptical of the experiences of Römer and James Bradley. Despite this, the results were closest to the truth and relevant for over a century. In the 19th century, Arman Fizeau, a scientist from the capital of France, Paris, contributed to the measurement of this quantity. He used the rotary shutter method. Also, like Galileo Galilei with his assistant, Fizeau did not observe celestial bodies, but investigated in the laboratory.

The principle of experience is simple. A ray of light was aimed at the mirror. Reflecting from it, light passed through the teeth of the wheel. Then it hit another reflective surface, which was located at a distance of 8.6 km. The wheel was rotated, increasing speed, until the beam was visible in the next gap. After calculations, the scientist received a result of 313 thousand km / s.

Later, the study was repeated by the French physicist and astronomer Leon Foucault, receiving a result of 298 thousand km / s. The most accurate result at that time. Later measurements were carried out using lasers and masers.

Is superluminal speed possible?

There are objects faster than the speed of light. For example, sunbeams, shadow, wave vibrations. Although theoretically they can develop superluminal speed, the energy they emit will not coincide with their motion vector.

If a light beam passes, for example, through glass or water, then electrons can overtake it. They are not limited in speed of movement. Therefore, under such conditions, light does not move faster than anyone.

This phenomenon is called Vavilov-Cherenkov effect. Most often found in deep reservoirs and reactors.