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It is an empirical fact
In you opinion, which factors can reduce the speed?
speed of light can be altered by depends upon many factors... first of all photon is not light.. light is a electromagnetic wave produced by a particle (has mass) called photon. so, when a photon is emitted from its source its moving at speed of light by emitting its energy as light. once it reached its minimum energy levels i.e it has lost its energy by emitting light, it will loose its speed and slows down and as it slows down gradually it will come to a point where it has very limited energy and unable to make any motion.. then it will be pulled back by the gravity of its source or any near by mass... and another situation is like when it hits any atom. it will hit an electron and loses its energy by increasing electron state of level and get diverted to another direction..in this context, the photon looses its energy and slows down.. the reason we see different colors in a prism... when it looses energy, the electromagnetic waves will convert from different forms of lights with different wavelengths... True Story.. I know its kinda hard to believe but trust me it makes a lot of sense then "Black Holes" and "Worm Holes"
You can think of this in two ways: either look at Maxwell's Equations, which are covered in fundamentals of physics II, or think about special relativity. I don't know Maxwell's equations and it's fairly advanced stuff so I usually think about relativity. Special relativity tells us that as we accelerate something more and more it gains mass relative to a stationary observer, which makes it harder to accelerate more. Eventually this becomes so great that no matter how much energy we put in, the object's speed will increase by a tiny negligible amount only and never reach or exceed a certain speed. however, some particles have no mass (photons) so they can travel at this certain speed. This 'certain speed' is what we call the speed of light, c. However, the speed of light isn't constant: it changes in different mediums such as the interstellar medium, the atmosphere or a block of glass. Check out this video: https://www.youtube.com/watch?v=CiHN0ZWE5bk
Well.. Simple answer to the question is that light is an Electromagnetic wave and they travel with the same velocity. That's constant 3*10^8 m/s.