# What is the Speed of Light?

Level 1 (suitable for all)

Speed of light specifies how fast the light propagates in empty space (vacuum). It is usually abbreviated with the letter $$c$$ and has the following exact value in vacuum:

Value of the speed of light
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When calculating with the speed of light, its value is mostly rounded up and compactly noted: $$c = 3\cdot 10^8 \, \frac{\mathrm{m}}{\mathrm{s}}$$.

• The speed of light in vacuum is a physical constant (i.e. fixed) and is the maximum speed in our universe. Only the light can reach the speed value given in equation 1.

• Light is an electromagnetic wave, with a specific wavelength. All other electromagnetic waves, such as radio waves, infrared radiation, gamma radiation, also propagate at the speed of light.

• The speed of light is used to define how large "1 meter" is. In fact, one meter is the distance that light travels within approximately 3,333... nanoseconds:

Definition of one meter
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• The constancy of the speed of light is used in the special theory of relativity to derive that the time can be slowed down and the space can be contracted. The two effects are respectively called time dilation and length contraction. 