How is the speed of light calculated?

1607, Galileo made his first attempt to measure the speed of light: two experimenters each took a shielding lamp at night and stood on two hills about 1.6km apart. The first experimenter turned on the light first and recorded the time when the light was turned on. The second experimenter turned on his own light immediately after seeing the light. When the first experimenter saw the light of the second experimenter, he immediately wrote down the time. Then the propagation speed of light is calculated according to the recorded time interval and the distance between the two peaks. Although the principle of this method for measuring the speed of light is correct, it cannot measure the speed of light. This is because the speed of light is very high, and it only takes about one thousandth of a second to go back and forth between two peaks which are about 1.6km apart. Such a short time is much shorter than the reaction time of the experimenter, even with the sophisticated timing instrument.

To measure the speed of light, you need to use a long distance, or use clever methods to accurately measure a short time interval. Scholars after Galileo measured the speed of light in these two directions.

1676, Danish astronomer Romer (1644 ~ 17 10) discovered through astronomical observation that light travels at a limited speed. Using the data observed by Romer, the speed of light can be calculated. This method belongs to the method of using large distance.

In order to measure the speed of light in a short distance on the ground, scientists have designed various ingenious experimental methods to accurately measure short time intervals. 1849, French physicist Fesso (18 19 ~ 1896) first measured the speed of light on the ground. Later, many scientists adopted more accurate methods to measure the speed of light. Below.

Michelson chose two peaks and measured the distance between them. On the first peak, a strong light source S and an octahedral prism A are installed (see the figure below). The light emitted by the light source S passes through the slit, hits the surface 1 of the octahedral mirror A, and then is reflected by the concave mirror B placed on another peak, and then by the plane mirror M, and then is reflected by B back to the first peak.

If the octahedron is rotated, when the light is reflected back, the octahedron's face 3 has deviated from the original direction, and the light reflected by the face 3 no longer enters the telescope, so that the observer cannot observe the image of the light source S. Adjust the rotation speed of the octahedron appropriately, so that when the reflected light reaches the octahedron, the octahedron just rotates, and the face 2 just turns to the original position of the face 3, and the light reflected by the face 2 enters the telescope. You can see the image of s again. According to the time taken for octahedron to turn to 1/8 and the distance between two peaks, the speed of light in air can be calculated. After Michelson correction, it is concluded that the propagation speed of light in vacuum is C = (299796 4) km/s. 。

The speed of light is a basic constant in physics. Scientists have been trying to measure the speed of light more accurately. 1970, they began to measure the speed of light with lasers. Laser velocimetry greatly improves the measurement accuracy. According to the resolution of the15th international metrology conference, the most reliable value of the speed of light in vacuum is defined as

C=(299792458 1) m/s

Simple calculation, 3.0× 108 m/s can be taken.

There is an octahedral mirror Tu Tu in the following address.

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