Phase and signal velocity of light inside a medium

[lightarrow]

Don't know if this is the correct forum.

While we know that light's phase velocity in a linear medium with refration index n is c/n, what can we say about signal's velocity? If I switch on a laser inside the medium (let's say glass) at t = 0, when will the first photon arrive to a detector placed L metres away? At t = L/(c/n) or at t = L/c or else?

 

[Meir Achuz]

If n does not vary too rapidly with wavelength, a pulse of light will travel with the group velocity $$v_g=\frac{\partial\omega}{\partial k}$$.

 

[lightarrow]

If n does not vary too rapidly with wavelength, a pulse of light will travel with the group velocity $$v_g=\frac{\partial\omega}{\partial k}$$.

Do you know the dispersion relation $$\omega(k)$$ or $$n(\lambda)$$ in the visible range for a common glass?

 

[Cthugha]

This depends a bit on the kind of glass you have.

Have a look at the download page of Schott glasses (a rather large company dealing with glasses and optical filters):

http://www.schott.com/advanced_optics/english/tools_downloads/download/index.html#Catalogues and Datasheets

Techical information 29 has some basic definitions and shows refraction index versus wavelength for fused silica glass. In the top of the page there is also a link to a glass map leading to an overview of some commonly used glasses, where n versus wavelength is given for 20 or so standard wavelengths for each filter. Maybe look for BK7. This is also a very common glass.

 

[lightarrow]

This depends a bit on the kind of glass you have.

Have a look at the download page of Schott glasses (a rather large company dealing with glasses and optical filters):

http://www.schott.com/advanced_optics/english/tools_downloads/download/index.html#Catalogues and Datasheets

Techical information 29 has some basic definitions and shows refraction index versus wavelength for fused silica glass. In the top of the page there is also a link to a glass map leading to an overview of some commonly used glasses, where n versus wavelength is given for 20 or so standard wavelengths for each filter. Maybe look for BK7. This is also a very common glass.

Thanks for the interesting link.
So, you agree that signal velocity = group velocity for common glasses in the visible range?

 

[Meir Achuz]

So, you agree that signal velocity = group velocity for common glasses in the visible range?

Yes.

 

[lightarrow]

Yes.

Thanks to both (but you are the same person?)