The speed of light

The speed of light in the vacuum of free space is an important physical constant usually denoted by the symbol c ₀ or simply c. The metre is defined such that the speed of light in a vacuum is exactly 299,792,458 metres per second.

The speed of light is of fundamental importance in physics. It is the speed of not just visible light, but of all electromagnetic radiation, as well as gravitation waves and anything having zero rest mass. In Einstein’s theory of relativity the speed of light plays the crucial role of a conversion factor between space and time within space time. This theory together with the principle of causality requires that no matter or information can travel faster than the speed of light.

The speed of light is so great that for many purposes it can be regarded as instantaneous. However, where long distances or accurate time measurements are involved, the finite speed of light can be important. For example, in the Global Positioning System (GPS), a GPS receiver measures its distance to satellites based on how long it takes for a radio signal to arrive from the satellite. In astronomy, distances are often measured in light-years, the distance light travels in a year (around ten trillion kilometers).

The speed of light when it passes through a transparent or translucent material medium, like glass or air, is less than its speed in a vacuum. The speed is described by the refractive index of the medium. In specially-prepared media, the speed can be tiny or even zero.

Задание2. Ответьте на вопросы:

1. What role does the speed of light play in Einstein’s theory of relativity?

2. Can it be regarded as instantaneous?

3. How are distances measured in astronomy?

4. Can the speed be tiny, or even zero?

5. Can the finite speed of light be important?

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