Ever since school days, we know that the speed of light, according to Einstein's laws, is an insurmountable maximum in the Universe. Light travels from the Sun to the Earth in 8 minutes, which is approximately 150,000,000 km. It takes only 6 hours to reach Neptune, but it takes decades for spacecraft to overcome such distances. But not everyone knows that the value of the speed can vary significantly depending on the medium in which the light passes.
Formula for the speed of light
Knowing the speed of light in vacuum (c ≈ 3108 m/s), we can determine it in other media based on their refractive index n. The very formula for the speed of light resembles the laws of mechanics from physics, or rather, the definition of distance using time and the speed of an object.
For example, we take glass, the refractive index of which is 1.5. According to the formula for the speed of light, v=c / n, we get that the speed in this medium is approximately 200,000 km / s. If we take a liquid, such as water, then the speed of propagation of photons (particles of light) in it is 226,000 km / s with a refractive index of 1.33.
Formula for the speed of light in air
Air is also a medium. Therefore, it has the so-called optical density. If in a vacuum photons do not encounter obstacles on their way, then in a medium they spend some time on excitation of atomic particles. The denser the environment, the more time it takes for this very excitement. The refractive index (n) in air is 1.000292. And that doesn't deviate much from the limit of 299,792,458 m/s.
American scientists have managed to slow down the speed of light to almost zero. Greater than 1/299,792,458 sec. light speed can not overcome. The thing is that light is the same electromagnetic wave as x-rays, radio waves or heat. The only difference is the difference between wavelength and frequency.
An interesting fact is the absence of mass in a photon, and this indicates the absence of time for this particle. Simply put, for a photon that was born several million, or even billions of years ago, not a second of time has passed.
