Why Do Lightwaves Penetrate Salt Water While Radiowaves Cannot?

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    Complex Conductivity
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SUMMARY

Lightwaves penetrate salt water due to their high frequency, which allows for deeper skin depth compared to radiowaves. The discussion highlights the importance of understanding complex conductivity, particularly how the conductivity of salt water varies with frequency. While light operates at trillions of Hertz, the conductivity of salt water, influenced by heavy ions, significantly increases resistance at such high frequencies. Therefore, conductivity is not a constant and must be evaluated in the context of the frequency being considered.

PREREQUISITES
  • Understanding of complex conductivity and its equations
  • Knowledge of electromagnetic wave behavior in conductive mediums
  • Familiarity with the concept of skin depth in wave propagation
  • Basic principles of ion movement in electrolytes
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  • Research the equation for complex conductivity in detail
  • Study the effects of frequency on the conductivity of various materials
  • Learn about skin depth calculations for different electromagnetic waves
  • Explore the behavior of ions in salt water under varying frequencies
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Physicists, electrical engineers, and anyone interested in the interaction of electromagnetic waves with conductive materials, particularly in marine environments.

Thierry12
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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.
 
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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.
 

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