About the speed of light, physics has always been controversial. Both Kepler and Descartes believe that the propagation of light takes no time and is instantaneous. But Galileo believed that although the speed of light was fast, it could be measured. 1607, Galileo made the earliest experiment to measure the speed of light.
Galileo's method is to let two people stand on two mountains a mile apart, each with a lamp. The first man raised his lamp first, and the second man saw the first man's lamp and immediately raised his own lamp. The time interval from the first person holding up the lamp to the second person's lamp is two miles. But because the speed of light is so fast, this method simply doesn't work. But Galileo's experiment opened the prelude to the study of the speed of light in human history.
1676, Danish astronomer Luo Mai first proposed an effective method to measure the speed of light. When observing the eclipse cycle of Jupiter's satellites, he found that they are different at different time periods of the year; The period of the earth between the sun and Jupiter is fourteen or fifteen days different from that of the sun between the earth and Jupiter. He thinks this phenomenon is caused by the speed of light, and he also infers that it takes 22 minutes for light to pass through the earth's orbit. 1In September, 676, Luo Mai predicted that the Europa eclipse at 5: 25: 45 am on October 9th would be delayed by 10 minute. Scientists at the Paris Observatory observed with suspicion and finally confirmed Romai's prediction.
Luo Mai's theory was not immediately accepted by the French Academy of Sciences, but it was recognized by the famous scientist Huygens. Huygens first calculated the speed of light: 2 14000 km/s according to his data and the radius of the earth. Although this value is far from the most accurate data measured so far, it inspired Huygens' research on fluctuation theory. More importantly, the error of this result lies not in the error of the method, but in Luo Mai's wrong speculation about the time when light travels through the earth. The result of modern Luo Mai method after various modifications is 298,000 km/s, which is very close to the accurate value measured in modern laboratory.
1725, British astronomer bradley discovered the phenomenon of "aberration" of stars, and confirmed Luo Mai's theory in an unexpected way. At first, he couldn't explain this phenomenon until 1728, when he was on a ship, inspired by the relative relationship between the wind direction and the ship's course, he realized that the propagation of light speed and the revolution of the earth caused the "aberration" phenomenon. He estimated that it takes 8 minutes 13 seconds for sunlight to reach the earth by using the ratio of the revolution speed of the earth to the speed of light. This value is more accurate than that determined by Lomax method. The measured value of Caidre proves Romai's statement about the limit of light speed.
Since the 17th century, the measurement of the speed of light has become an important basis for the debate on the nature of light. However, due to the limitation of the experimental environment at that time, scientists could only measure the propagation speed of light in vacuum by astronomical methods, but could not solve the problem that light was affected by the propagation medium, so the debate on this issue has been unresolved.
In the eighteenth century, the scientific community was unremarkable, and the development of optics almost stagnated. After Bradley, after more than a century of brewing, it was not until the middle of19th century that new scientists and new methods appeared to measure the speed of light.
1849, Frenchman free soul first designed an experimental device for measuring the speed of light on the ground. The principle of his method is similar to Galileo's. He placed a point light source at the focus of the lens, a gear between the lens and the light source, and another lens and a plane mirror at another distance of the lens in turn. The plane mirror is located at the focus of the second lens. The light emitted by the point light source becomes parallel light after passing through the gear and the lens, and the parallel light converges at a point on the plane mirror after passing through the second lens, and returns by the original way after being reflected by the plane mirror. Because the gear has backlash and gear teeth, when the light passes through the backlash, the observer can see the returning light, and when the light just touches the gear teeth, it will be blocked. The time from the beginning to the first disappearance of the returning light is the time required for the light to go back and forth once. According to the speed of the gear, it is not difficult to find this time. In this way, the speed of light measured by the free soul is 315000 km/s. Because the gear has a certain width, it is difficult to accurately measure the speed of light by this method.
1850, the French physicist Foucault improved the method of free soul. He only used a lens, a rotating flat mirror and a concave mirror. The parallel light converges to the center of concave mirror through the rotating plane mirror, and the time can also be calculated by using the rotating speed of the plane mirror. The speed of light measured by Foucault in this way is 298,000 km/s. In addition, Foucault also measured the propagation speed of light in water. By comparing the propagation speed of light in air, he measured the refractive index of light emitted from air into water. After the particle theory was overthrown by the wave theory, this experiment made a judgment on the particle theory again, which brought the final impact to the particle theory of light.
1928, Carolas and Mittstad put forward the Kerr box method to measure the speed of light for the first time. 195 1 year, and the speed of light measured by Burke in this way is 299,793 km/s.
Light wave is a small part of the electromagnetic spectrum, and every electromagnetic wave in the electromagnetic spectrum has been accurately measured by contemporary people. 1950, Eisen proposed the cavity vibration method for measuring the speed of light. The principle of this method is that when the microwave passes through the cavity, it will vibrate when its frequency is a certain value. According to the length of the cavity, the wavelength of the cavity can be calculated, and then the wavelength of the cavity can be converted into the wavelength of light in vacuum, and the speed of light can be calculated from the wavelength and frequency.
At present, the most accurate speed of light is obtained by wavelength and frequency. In 1958, frum calculated the exact value of light speed: 299792.5 0. 1 km/s, and in 1972, Evans measured the optimal value of light speed in vacuum at present: 299792457.40.1m/.
The measurement of light speed is of great significance in the process of optical research. Although it has been more than 300 years since people tried to measure the speed of light to measure it more accurately, every progress during this period has promoted the development of geometric optics and physical optics, especially in the debate between particle theory and wave theory. The measurement of the speed of light provides a very important basis for this famous scientific debate.