Visible light is the portion of the electromagnetic spectrum between wavelengths of about 400 nm and 800 nm (in air). The different wavelength mixtures are interpreted by our brain as colors, ranging from red at the lower frequency to violet at the highest.
Speed of light
According to standard modern physical theory, the speed of light propagation (and all other electromagnetic radiation) in vacuum is a physical constant (notated as c). Regardless of the reference frame of an observer or the velocity of the object emitting the light, every observer will obtain the same value for the speed of light upon measurement.
c (exactly 299,792,458 metres per second, or about thirty centimetres in a nanosecond) is the maximal speed of any particle or information. This has been confirmed to a high degree of accuracy by experiment and observation in our "neighbourhood" (on a universal scale) in space and time .
Albert Einstein developed the theory of relativity by applying the consequences of the above to classical mechanics. Experimental confirmations of the theory of relativity indirectly confirm that the velocity of light has a constant magnitude.
Since the speed of light in vacuum is constant, one may measure time and distance in terms of c. Both the SI unit of length and SI unit of time have been defined in terms of wavelengths and cycles of light; currently, the meter is defined as the distance travelled by light in a certain amount of time: this relies on the constancy of the velocity of light for all observers. Distances in physical experiment or astronomy are commonly measured in in light seconds, minutes, or years.
In passing through materials, light is slowed to less than c, by the ratio called the refractive index of the material. On the microscopic scale this is caused by continual absorption and re-emission of the photons that compose the light by the atoms or molecules through which it is passing.
Recent experimental evidence shows that is is possible for the group velocity of light to exceed c. One experiment made the group velocity of laser beams travel for extremely short distances through caesium atoms at 300 times c. However, it is not possible to use this technique to transfer information faster than c; the product of the group velocity and the velocity of information transfer is equal to the normal speed of light in the material squared.
The speed of light may also appear to be exceeded in some phenomena involving evanescent waves. Again, it is not possible that information is transmited faster than c.
Help, real physicists, please give a better explanation!