What Is a Beam Spread?
The premise of light traveling in a straight line is in the same homogeneous medium. The linear propagation of light must not only be in a homogeneous medium, but it must also be the same medium. It can be referred to as the straight line of light, but not the straight line of light. Light is refracted on the contact surface of two homogeneous media, and then the light does not propagate in a straight line [1] .
- Light, light source
- Law of light propagation
- Light travels in a straight line in the same homogeneous medium.
- Speed of light
- The speed at which light (electromagnetic waves) travel in a vacuum. The 2013 accepted value is C = 299 792 458 m / s (exact value) [4] .
- Generally rounded to 3x10 8 m / s, which is one of the most important physical constants.
- Before the 17th century, astronomers and physicists believed that the speed of light was infinite, and the light emitted by cosmic stars reached the earth instantaneously. Galileo first questioned this. In 1607, he conducted an experimental measurement of the speed of light between the tops of the two mountains. The speed of the light was too large and the experimental device was too crude to succeed. In 1676, Danish astronomer Romer, using astronomical observations, successfully measured the speed of light for the first time. In 1849, French scientist Fissau successfully measured the speed of light on the ground for the first time in the laboratory with a clever device. In 1973, Evanson of the US Bureau of Standards adopted the laser method to use frequency and wave sum to determine the speed of light (299792 485 + 1.2) m / s. It was confirmed by the 15th International Conference on Metrology in 1975 that the above speed of light was used as an international recommended value. The new definition of the meter was adopted at the 17th International Conference on Metrology in 1983 as the length of the "vacuum" stroke of light in the 1/299 792 458 second time interval.
- In this way, the speed of light has become a defined value, and its accuracy is zero. There is no need to make precise measurements in the future. The unit of length in meters and time in seconds are directly related through this defined value.
- One of the basic principles of the special theory of relativity is the principle of constant speed of light. This is consistent with the definition of the speed of light as a fixed value. But so far, some people are still testing whether the speed of light is constant under higher accuracy.
- Except for vacuum, the material through which light energy passes is called a (light) medium. The speed of light in the medium is less than the speed of light in the vacuum. The speed of light in water is 2.25 × 10 8 m / s. Speed of light in glass: 2.0 × 10 8 m / s. Speed of light in ice: 2.30 × 10 8 m / s. Speed of light in air: 3.0 × 10 8 m / s (approximate number, it should actually be less than 299792458 m / s). Speed of light in alcohol: 2.2 × 10 8 m / s.