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Phase and signal velocity of light inside a medium

  1. Jan 1, 2009 #1
    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?
  2. jcsd
  3. Jan 1, 2009 #2


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    If n does not vary too rapidly with wavelength, a pulse of light will travel with the group velocity [tex]v_g=\frac{\partial\omega}{\partial k}[/tex].
  4. Jan 3, 2009 #3
    Do you know the dispersion relation [tex]\omega(k)[/tex] or [tex]n(\lambda)[/tex] in the visible range for a common glass?
  5. Jan 3, 2009 #4


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    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_opti...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.
  6. Jan 3, 2009 #5
    Thanks for the interesting link.
    So, you agree that signal velocity = group velocity for common glasses in the visible range?
  7. Jan 3, 2009 #6


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  8. Jan 5, 2009 #7
    Thanks to both (but you are the same person?)
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