定义：

某一粒子的速度是很简单明确的概念，但是光的速度（以及其其它波的现象）则是非常复杂的问题。有几种不同概念的速度（尤其是当光在介质中传播时），并且它们具有不同的数值： 

• 相速度是相位波前传播的速度。 
• 群速度是光强最大值传播的速度（例如，脉冲峰值）。 
• 信息传播的相速度和群速度不同，参阅词条因果关系。 

图1给出了不同的速度。这个例子中，不同频率成分的相速度随频率线性变化：高频成分波前传播更慢。脉冲最大值与波前重叠，并且以群速度传播。更多细节在群速度词条中给出。 

图1：光脉冲在色散介质中的传播。不同频率成分的相位波前以不同的速度传播，并且脉冲以群速度传播，群速度小于相速度。 

当光在非各向同性介质中传播时，情况更加复杂，尤其是在波导中。 
真空中的相速度等于群速度（平面波）为c = 299 792 458 m/s。在国际单位制中（SI），真空光速的定义与实际值完全符合。并且与秒的定义结合（采用铯原子超精细跃迁定义），可以确定米的长度。 

在有些情况下，通常存在吸收或增益共振时，相速度甚至群速度会大于真空光速（参阅超光速传输，“快光”），但是这并不违反因果关系。还有一些情况下，在一个很窄的光谱区域光速会变慢（慢光）。窄带共振时，能够观察到速度减小很多，例如，在超冷气体中会发生。 
更奇特的效应是在有些情况下存在负群速度和负的dn / dω。 

在基础物理中，真空光速非常重要。爱因斯坦相对论理论的一个基石就是真空光速是恒定的，即在所有惯性系统中都是相等的，并且与传播方向无关。不存在光以太，在其中光具有默认速度。爱因斯坦根据这一假设得到了关于时间和空间具有深远意义的结论。例如，实验上证实的时间延缓效应，移动物体的长度收缩以及任何物体都无法达到或者超过光速。光在理论中的核心作用表明电磁场是直接与时空有关的，尽管这一关系目前还不能完全理解。

Definition: the velocity with which light propagates in a medium

More specific terms: phase velocity, group velocity, velocity of information transport

Units: m/s

Whereas the velocity of some particle is a quantity which is based on a fairly simple and unambiguous concept, the velocity of light (as of other wave phenomena) is a much more sophisticated matter. There are different kinds of velocities, which are different conceptually and can (particularly for light propagation in media) have substantially different values:

• The phase velocity is the velocity with which wavefronts propagate.
• The group velocity determines the speed with which intensity maxima propagate (e.g. the peaks of pulses).
• The velocity of information transport can differ from both phase and group velocity, see the article on causality.

Figure 1 illustrates the different velocities. In that example, the phase velocities of different frequency components vary linearly with frequency: the wavefronts of the higher-frequency components (drawn at higher positions) travel more slowly. The pulse maximum forms where the wavefronts coincide, and propagates with the (lower) group velocity. More details are given in the article on group velocity.

Figure 1: Propagation of a light pulse in a dispersive medium. Note that the phase fronts of different frequency components (shown as straight lines) propagate with different velocities, and the pulse propagates with the group velocity, which is lower than all the phase velocities.

Further complications can arise from light propagation in inhomogeneous media, particularly in waveguides.

In vacuum, phase and group velocity (defined for plane waves) are identical at c = 299 792 458 m/s. Within the International System of Units (SI), the vacuum velocity of light has been defined to match this value exactly. Together with the definition of the second (via a hyperfine transition of cesium atoms), this determines the length of a meter.

In some situations, often associated with absorption or gain resonances, the phase velocity or even the group velocity of light can exceed the vacuum velocity of light (→ superluminal transmission, “fast light”), although this is not associated with a violation of causality. There are other situations where the group velocity of light at least within a narrow spectral region is reduced (slow light). Enormous velocity reductions can be observed for narrowband resonances, as occur e.g. in ultracold gases.

The perhaps most peculiar effect is the occurrence of negative group velocities in situations with strongly negative dn / dω.

Velocity of Light and Theory of Relativity

The vacuum velocity of light plays a very important role in fundamental physics. One of the cornerstones of Einstein's theory of relativity is that the vacuum velocity of light is constant, i.e., it is the same in all inertial systems and does never depend on the propagation direction. In other words, there is not such a thing as a light aether, defining a single system where light has its “default” velocity. This seemingly innocent assumption lead Einstein to very far-reaching conclusions concerning the nature of space and time. Examples are the experimentally well confirmed phenomena of time dilatation, length contraction of moving objects, and the impossibility for any massive objects to reach or exceed the vacuum velocity of light.

The central role of light in this theory indicates that electromagnetism is intimately related to the nature of spacetime, although that relation is still not entirely understood.