0
$\begingroup$

I know that the refractive index of water increases with increased concentration of salt.

But I also read that $$n = \sqrt{\varepsilon_r\mu_r}$$ And that the relative dielectric constant decreases with increased concentration of salt in water.

There seems to be a contradiction as if the dielectric constant decreases then the refractive index must decrease too.

Can someone please tell me where I'm going wrong in understanding this concept?

$\endgroup$
1
  • 1
    $\begingroup$ You may be confusing low frequency dielectric constant with the value of $\epsilon_r$ at optical frequencies; they are very different. IIRC, the MHz and less value of $\epsilon_r$ for water is about 80. $\endgroup$ Feb 1, 2018 at 8:30

1 Answer 1

1
$\begingroup$

The 'dielectric constat', or 'relative permittivity' of water varies with frequency. At 1kHz electromagnetic frequency, it is around 80, and adding salt (relative permittivity of about 3-15 (Ref)) to water decreases water's relative permittivity (have a look at Ref). In the visible light domain for visible light (about 600THz, 500nm), the relative permittivity of water is only 1.77. Adding salt to water increases its relative permittivity.

There is another good explanation here.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.