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What really cause light to appear slower in media?

This came to me as a question from one of my friend while I was convinced that the answer is NO. But I have read in Special Theory of relativity that we CAN make light take more time to travel a distance.
And then there's also one more thing which is kind of misleading:

Light speed is slow in optically dense media

So is it possible to slow it down?

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marked as duplicate by Qmechanic, David Z Oct 2 '12 at 1:02

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

Related: – Qmechanic Sep 29 '12 at 20:18
up vote 4 down vote accepted

Not only is it possible to reduce the speed of light, but it is impossible to measure the speed of light unless it has been reduced because light moving though a medium other than space will not be traveling at C - but will be traveling slower because of interference with matter (not necessarily molecules).

However your particular question might be rooted in the fact that light will always be measured as moving at V (reduced speed of light) no matter the velocity of the observer -- even if the observer is moving at .9V. So in a sense light cannot be measured at moving slower that V in any given medium -- but that velocity is most likely not as fast as C.

There are equations for calculating the speed of light in different mediums (such as air). V=C/N where N is the refractive index of the medium. So, in air (which has a refractive index very close to 1), light is nearly moving at 3.00x10^8 m/s. However, in water (which has a refractive index of 1.33) the speed of light would be closer to 2.25x10^8 m/s.

There is a famous experiment where physicists measured the speed of light in Bose-Einstein condensate. Light travels at 38 MPH in Bose-Einstein condensate, slower than a vehicle on the freeway.

You say:

But I have read in Special Theory of relativity that we CAN make light take more time to travel a distance.

However, as far as a photon is concerned it does not experience time. A photon can travel anywhere in the universe in an instant. I recommend you look at the relativistic velocity equations. You should find, if you solve the equation for time, with velocity=c, that the time will be undefined.

The trick with relativity is understanding that 1-you will never be able to understand it completely, and 2-the frame of reference can make all the difference, understanding a problem and all the frames involved can be the hardest part of solving it.

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The last para is the best part. Thanks for the neat explanation. – Atif Imran Sep 30 '12 at 5:17

When one talks about changes of the speed of light, they are referring to changes of refractive index, which is the square root of the product of permeability and permittivity of a substance. The permeability is generally defined so that it varies little, so what really controls the speed of light is the permittivity, which is also known as the dielectric constant. Vacuum permittivity and vacuum permeability and the speed of light are thus directly related in classical theories of light.

One needs to understand that classically, the vacuum was still seen as an idealized abstraction where there was absolutely no matter present. In this regime certain other properties were idealized, such as setting the magnetization density to zero, and setting the susceptibility to zero.

Classically one can see that asking whether light could travel faster than it could in a vacuum was akin to whether there were densities less than zero. This question does however raise itself again as it relates to question of cosmological inflation.

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