Remember your physics classes? Remember Snell's Law? When light (and radio waves) cross the interface from one medium to another (e.g. their path crosses from the air into the ocean), the light or electromagnetic radiation can either be reflected or refracted depending on the refractive index of the two mediums.

In accordance with Snell's Law, it's possible that electromagnetic radiation would be reflected back by the interface of the two mediums. This occurs when the radiation propagates from one medium into another medium that has a higher refractive index. Any radiation that strikes the interface of two such media with an angle of incidence at least as large as the critical angle would be totally reflected back into the source medium. This is called total internal reflection.

A common example of how this is useful for radio communications is the use of the ionosphere to propagate HF radio signals. Ham Radio operators can bounce their signals relatively far around the world by reflecting them off of the ionosphere with its higher refractive index than lower layers of the atmosphere.

Anyhow, on a related note (and getting to the point of this node), the ionosphere causes problems for scientists wishing to communicate via radio in the arctic. The Earth's rotation causes a high concentration of free electrons in the ionosphere to be channeled up to the arctic region. This is why the Aurora Borealis can be seen up in Alaska but not from my home in California. The same high concentration of free electrons that causes the beautiful auroras also wreaks havoc upon the radio communications in the region.

To deal with this problem, scientists living and working atop the arctic ice have learned of an alternative way to propagate radio waves towards one another. The arctic ice is about 2-5 meters deep. And the upper regions are made of purer water than the lower regions which are much more salty. It turns out that salty ice has a higher refractive index than pure ice. Thus by sinking their antennae into the pure ice, scientists can bounce radio waves off the interface between the pure ice and the salty ice to communicate for long distances free of the interference of the ionospheric conditions prevalent in that region. This is because towards the bottom of the ice there exists an interface which affords their radio waves total internal reflection.

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