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# Test yourself now

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One of the most exciting discoveries in physics during the last century, and the cornerstone of Einstein's Theory of Relativity, is that light travels at a constant speed in a given medium. Light also has a maximum speed at which it can propagate, and nothing can move faster than the speed of light. The maximum speed at which light can travel is when it propagates through free space (a vacuum) at $$\text{299 792 485}\text{ m·s^{-1}}$$. A vacuum is a region with no matter in it, not even air. However, the speed of light in air is very close to that in a vacuum.
We use the symbol $$c$$ to represent the speed of light in a vacuum and approximate it as $$c = \text{3} \times \text{10}^{\text{8}}\text{ m·s^{-1}}$$
The speed of light, $$c$$, is constant in a given medium and has a maximum speed in vacuum of
$c = \text{3} \times \text{10}^{\text{8}}\text{ m·s^{-1}}$