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Dielectric constant and index of refraction

  1. Feb 24, 2005 #1

    StatusX

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    I'm a little confused. My source says the dielectric constant for water is 80. Then I have the equation that gives the index of refraction as [itex]n=\sqrt{\epsilon_r}[/itex] (since it isn't very magnetic). But the index of refraction for water is 1.33. What am I missing?
     
  2. jcsd
  3. Feb 24, 2005 #2
    They are both frequency dependent terms. However:

    [tex] n(\omega) \equiv \sqrt{\epsilon(\omega)}[/tex]

    So your value are obviously not for the same frequency, you need to figure out which one is correct for the given frequency - I would guess refractive index implies the frequency is for visible light.

    And your dielectric constant = 80 is relevant for a completeley different frequency ie not visible light.
     
  4. Feb 24, 2005 #3

    Claude Bile

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    Most quoted values for dielectric constant are for frequencies in the kHz, whereas visible light is hundreds of THz, so don't be surprised if there is a disrepancy. Refractive index values are measured at a standard optical frequency (Sodium D line perhaps, I'm not too sure).

    What value you use obviously depends on what you are trying to solve.

    Claude.
     
  5. Feb 24, 2005 #4
    Also varies with temperature.
    DC = 80 corresponds to 80*F.(using radio frequency, Mhz). At 212*F, dielectric constant is ~53 for water.
    :wink:
     
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