Unlike sound, the speed of whch differs depending on the air density (which is why people qualify the speed as being at sea-level and at zero degrees Celsius), the speed of light is not subject to atmospheric conditions, and thus never varies..
The speed of light is not only the fastest speed that anything in the Universe can travel, it's regarded as a universal constant.
Whether we shine a flashlight, look at the Moon or Sun, or measure a galaxy from billions of light years away, the speed of light is the one thing that never changes..
Nothing Light is a "universal speed limit" and, according to Einstein's theory of relativity, is the fastest speed in the universe: 300,000 kilometers per second (186,000 miles per second)..
Nothing you do to your own motion or to the light's motion will change that speed.
By passing that light into a non-vacuum medium, you can change its speed so long as it's in that medium.
Light of different energy will change its speed by slightly different amounts, depending on the properties of that medium..
The speed of light is a universal constant in a vacuum, like the vacuum of space.
However, light *can* slow down slightly when it passes through an absorbing medium, like water (225,000 kilometers per second = 140,000 miles per second) or glass (200,000 kilometers per second = 124,000 miles per second)..
A variable speed of light (VSL) is a feature of a family of hypotheses stating that the speed of light may in some way not be constant, for example, that it varies in space or time, or depending on frequency..
The problem of understanding relativity, and especially that of the principle of the constancy of the speed of light, has led to the formulation of a new theory concerning the nature of light.
According to the new theory, light has a wave-like nature and a variable speed, which decreases near the gravitational bodies..
