# Speed of light - permeability?

• lntz
In summary, light travels at a different speed in different media due to interactions with the particles in the medium. In a vacuum, there are no particles to slow down the light, but in air, the particles cause a slower speed of light. However, in solid media like glass, the collective response of atoms also plays a significant role in determining the speed of light.
lntz
so what i am about to write is quite possibly wrong, but this is just how i was thinking about why the speed of light is different in a vacuum as it is in glass or water etc. i am writing this to see whether my train of thought is on the right lines, and if it isn't to hopefully get an insight into the way i should think about it.

if the speed of light, c, is 3x10^8 ms-1 approx. in a vacuum (free space), because there is nothing there for the light to 'knock into' or interact, and therefore nothing to slow it down, i think of it as a kind of 'resistance' (i know this isn't a correct term to use here, but they're similar ideas). and in air the speed of light is somewhat slower due to the fact that there are particles moving about in this free space, i might assume, that it is these particles that slow the light down.

however what i was thinking is that air is mostly free space anyway, so does a photon actually travel at speed c, for most of the time, but it is 'held up' by some kind of interaction with an electron? so it's actual speed doesn't really change, but it's trip time increases due to these interactions... so when a photon travels from A-D, it must 'stop' at B and C along the way, but it does travel from A-B at speed c and the same for B-D. it is just the fact that whatever happens at those points takes some time to occur

please let me know if that's a sensible idea or not.

thanks, lntz.

lntz said:
so what i am about to write is quite possibly wrong, but this is just how i was thinking about why the speed of light is different in a vacuum as it is in glass or water etc. i am writing this to see whether my train of thought is on the right lines, and if it isn't to hopefully get an insight into the way i should think about it.

if the speed of light, c, is 3x10^8 ms-1 approx. in a vacuum (free space), because there is nothing there for the light to 'knock into' or interact, and therefore nothing to slow it down, i think of it as a kind of 'resistance' (i know this isn't a correct term to use here, but they're similar ideas). and in air the speed of light is somewhat slower due to the fact that there are particles moving about in this free space, i might assume, that it is these particles that slow the light down.

however what i was thinking is that air is mostly free space anyway, so does a photon actually travel at speed c, for most of the time, but it is 'held up' by some kind of interaction with an electron? so it's actual speed doesn't really change, but it's trip time increases due to these interactions... so when a photon travels from A-D, it must 'stop' at B and C along the way, but it does travel from A-B at speed c and the same for B-D. it is just the fact that whatever happens at those points takes some time to occur

please let me know if that's a sensible idea or not.

thanks, lntz.

In some senses you are on the right track. It is the interactions between light and the media which cause the effective speed of light in that medium to slow down. Thus, the effects of these interactions, averaged over the whole medium, manifest in the permittivity, permeability, index, etc.

However, for solid media, like glass, it is not really correct that atomic level interactions like you have described solely determine the optical properties of the media. In solids the collective response of the constituent atoms becomes very important.

Zapperz has a great post on this topic in the FAQ:

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## What is the speed of light?

The speed of light is a fundamental constant in physics, denoted by the letter "c". It is approximately 299,792,458 meters per second (m/s) in a vacuum.

## How is the speed of light measured?

The speed of light is typically measured using a variety of methods, including the use of lasers and mirrors, interferometry, and the observation of celestial events such as supernovae.

## Is the speed of light constant?

According to Einstein's theory of relativity, the speed of light is constant and unchanging in all reference frames. This means that no matter how fast an observer is moving, they will always measure the speed of light to be the same value.

## What are the implications of the speed of light?

The constant speed of light has many implications in physics, including the concept of time dilation and the relationship between energy and mass in Einstein's famous equation E=mc^2.

## What is the permeability of the speed of light?

The permeability of the speed of light refers to the ability of light to travel through different mediums. In a vacuum, the speed of light is constant, but in other materials, such as air or water, it may be slowed down due to interactions with particles in the medium.

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