Why Do Lightwaves Penetrate Salt Water While Radiowaves Cannot?

[Thierry12]

Can someone explain to me the equation on complex conductivity found here :
http://farside.ph.utexas.edu/teaching/jk1/lectures/node58.html"

I am trying to understand why lightwaves penetrates salt water (conductor) but radiowaves can't.

 

[Xezlec]

Good observation. The fact that light waves have a skin depth much deeper than what that formula seems to say they should is interesting, isn't it? But let's look at our assumptions. What numbers did you plug into the formula? The frequency of light is easy to find out. The magnetic permeability is a constant. But where did you get your value of conductivity of salt water? Is conductivity of salt water a constant?

I would think the answer to this question lies in the fact that salt water conducts by big, heavy ions (from the salt) moving around. And big, heavy ions don't like to move around as fast as TRILLIONS of Hertz (the frequency of light). This means that the resistance to extremely high frequencies would be extremely high. So the conductivity at a hundred trillion Hertz isn't the same number as the conductivity at DC (0 Hz), which is probably what you looked up.

The moral is: don't assume a number like conductivity of a substance is a constant! It may depend on other circumstances, like frequency.

 

[charng]

The equation for complex conductivity is a mathematical representation of the electrical properties of a material, specifically its ability to conduct electricity. In simpler terms, it describes how easily an electric current can flow through the material. This concept is important in understanding why some materials, like salt water, can conduct electricity while others, like air, cannot.

In the case of salt water, the presence of salt ions allows for the movement of electric charges, making it a good conductor of electricity. This means that when an electric field, such as that of a lightwave, is applied to salt water, the charges can easily move and conduct the electricity. This is why lightwaves can penetrate salt water.

On the other hand, radio waves have much lower frequencies and longer wavelengths compared to lightwaves. This means that they do not have enough energy to cause the movement of charges in salt water, making it a poor conductor for radio waves. In other words, the complex conductivity of salt water is not high enough to allow for the efficient flow of electric current at the frequency and wavelength of radio waves.

I hope this explanation helps you understand the relationship between complex conductivity and the ability of different materials to conduct electricity at different frequencies.