Question

why speed of light is constant? explain please

Answer 1

Since I'm too lazy to explain, check this site out:

https://van.physics.illinois.edu/qa/listing.php?id=2605

Answer 2

Well, there is no "reason" for why the speed of light is constant. We don't have reasons for why constants are what they are. The best answer is that the conditions were just right to produce a constant speed of light when physical laws became what they are shortly after the Big Bang. Several of my professors throughout undergraduate and graduate school have said that the reason why physical constants are what they are is, "Just because they are."

The speed of light as a constant gained theoretical substance when James Clerk Maxwell in the process of formulating his famous equations (Maxwell's Equations) hypothesized that light was an electromagnetic wave and traveled at the speed of a constant expressed in his equations. That constant was c, and in his equations was defined as:\[c=\frac{ 1 }{ \sqrt{\epsilon _{0}\mu _{0}}}\]where ε0 is a constant known as the permitivity of free space; and μ0 is known as the permeability of free space. They are constants that show up in Maxwell's equations, which all together are the basis of our theory of electromagnetism.

Note that the speed of light is only constant in a given medium, which is to say that it is constant in a vacuum, in air, and in water (for example), but the actual speed of light in each of those mediums is different. The speed of light in air is about 1.000293 times slower than it is in a vacuum. The speed of light in water is about 4/3 slower than it is in a vacuum.