Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Dielectric constant and index of refraction

  1. Feb 24, 2005 #1


    User Avatar
    Homework Helper

    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

    User Avatar
    Science Advisor

    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.

  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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook