1972, K.M. evenson and others in the United States directly measured the laser frequency ν and wavelength λ in vacuum, and calculated C = (299792458) according to the formula V =1√ μ0 ε0 (where V is the speed of electromagnetic wave in vacuum, μ 0 is the magnetic permeability of vacuum and ε 0 is the vacuum dielectric constant). 1975 15 the international metrology conference confirmed that the above-mentioned light speed value is internationally recommended. 1983 17 the international metrology conference adopted a new definition of meter, in which the speed of light c=299792458 m/s is the specified value and the length unit meter is defined by the specified value. Since the speed of light in vacuum has become a defined value, there is no need to measure the speed of light in the future.

In any transparent or translucent medium (such as glass and water), the speed of light will decrease; The specific speed of light in a medium is the refractive index of this medium. The change of gravity can bend the space where light propagates, making light bend like a convex lens, which seems to bypass the massive celestial body. The bending phenomenon of light is called gravitational lens effect. According to the irregularity of light in the out-of-spectrum band, the age and distance of luminous galaxies can be calculated.

I always thought it was the refraction of light. I remember there will be such a math problem in junior high school. The speed of a person on land is v 1, and the speed of swimming in water is v2. He wants to save a man in the water. What is the best route? It's like the refractive index. Light will choose the shortest route. Refractive index n=v 1/v2.

There is also the curve propagation of light. What I think is that in a sphere with high density and mass, its gravitational field is so strong that its time field is distorted, and light will choose the nearest route, while it chooses the shortest route in time. Only this straight line in time may be a curve in space.