Transmission of light and velocity of light

[gemini23]

Greetings,

When I think of light being "transmitted" through air, I have always used the idea that the light is not interacting... it is just passing through. However, I had a student ask me, if transmitted light is not interacting with air molecules, then why is the speed of light slower in air, relative to a vacuum. Being a chemist, I had to think about this...

Clearly, light must be interacting with air, else the velocity would be the same as in vacuo. So, two questions:

1) What is the nature of the interactions of visible light with say, O2 or N2, as it is being propagated along and

2) Given this, is it even correct to say that light is being "transmitted" if there are these interactions?

Thanks!

 

[rmfw]

Hi !

I think part of the wave energy is transferred to the air particles and as Compton showed the wave length of the wave is reduced after that energy is transferred, and by the formula v= λ * ƒ it's clear why the velocity is lower than in vacuum.

I'm not sure if this is the right explanation, just my guess.

 

[UltrafastPED]

For any transparent material the relevant process is known as "Forward Coherent Scattering":

1. "Forward" because that is the direction in which the image is transmitted
2. "Coherent" because otherwise the image would be degraded
3. "Scattering" because any absorption/emission process would remove certain colors

So what is the theory? It looks just like the Huyghens wavelet mechanism for "wave transmission":
http://en.wikipedia.org/wiki/Huygens–Fresnel_principle

The scattering events result in a phase delay of the wavefront; this is why light speed is "slower" in air/glass/water than in a vacuum - and it also explains why light resumes its previous velocity when it exits: the scattering ends, so there is no more phase delay.

In addition there is no change in the light spectrum before and after; the energy of the individual photons is unchanged by the elastic scattering process. And no, it is not Compton scattering - that is a high energy process.

Richard Feynman discusses this and many other things having to do with light in his 4-part lecture:
"QED: The strange theory of light and matter" - available in book form, or you can see the original lectures if you have about five hours to spend on this.
http://vega.org.uk/video/subseries/8

 

[gemini23]

Thanks Ultrafast - that is very helpful.

 

[sardar]

Greetings,

I can provide some insight into the transmission of light and its velocity. Light is a form of electromagnetic radiation, and its speed is determined by the properties of the medium it travels through. In a vacuum, light travels at a constant speed of approximately 3 x 10^8 meters per second. However, when light travels through a medium such as air, it interacts with the molecules present which causes a decrease in its speed. This is due to the fact that the molecules in the medium absorb and re-emit the light, causing it to travel a slightly longer distance and thus slowing it down. This interaction is what causes the speed of light to be slower in air compared to a vacuum.

To answer your first question, the nature of the interactions between light and air molecules is primarily through absorption and re-emission. The molecules absorb the energy of the light and then release it as heat or re-emit it as light. This process happens continuously as the light travels through the medium, resulting in a slower velocity.

As for your second question, it is correct to say that light is being "transmitted" through air, as it is still able to pass through the medium, even though it is being slightly slowed down. It is important to note that the interactions between light and air molecules do not significantly affect the overall transmission of light, as the majority of the light still passes through the medium. Therefore, we can still consider light to be transmitted through air, even though there are interactions occurring.

I hope this helps to clarify the nature of light transmission and velocity in different mediums. Please let me know if you have any further questions or if I can provide any additional information. Thank you.