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Group velocity information

  1. Jan 5, 2012 #1
    Infrared radiation, of wavelength λair = 1um in air, travels through a
    dispersive medium with refractive index n = 1.4505 and with
    dn/dλair = -0.01 per um at this wavelength. Calculate the speed at which the
    radiation carries information.

    So know that

    c/n = λf

    radiation carries information at group velocity dw/dk

    can also show that dw/dk = c/(n + w(dw/dn))

    but don't know where to go from here

    feel http://en.wikipedia.org/wiki/Group_velocity has some useful expression in alternative expressions but need to be able to show, and can't
     
  2. jcsd
  3. Jan 5, 2012 #2
    w is angular frequency. In the other expression are formula needed to calculate w. The convert dn/d(lambda) into dw/dn.
     
  4. Jan 5, 2012 #3
    how?
     
  5. Jan 5, 2012 #4
    Take at look at the link you posted in the other expressions sections. It all there.
     
  6. Jan 5, 2012 #5
    I know the expressions are there, I just can't fiddle around with the differetiation
     
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