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Speed of light

Jul 20, 1996 08:06 AM
by Bjorn Roxendal

>3. The velocity of light different outside our atmosphere. Again this was a
>big 19th century debate, put to rest by Einstein's relativity based on
>observations that the speed of light is a constant.


actually the speed of light is constant within a specific medium, but varies between different media:


To be precise, what we usually call the "speed of light" is
really the speed of light in a vacuum (the absence of matter).
In reality, the speed of light depends on the material that
light moves through. Thus, for example, light moves slower
in glass than in air, and in both cases the speed is less than in
a vacuum. However, the density of matter between the stars
is sufficiently low that the actual speed of light through most
of interstellar space is essentially the speed it would have
through a vacuum, so we don't make much error by ignoring
the difference. 

The preceding statements about the constant speed of light
refer to the speed of light in a particular medium, such as a
vacuum. Within such a medium, the speed is constant, but
light changes its speed when it moves from one medium (say
air) to another (say glass). This change of speed at the
boundary between two different media is the principle that
causes a lens in a telescope or eyeglasses to work. 


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