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If glass and similar materials refract visible light effectively, what materials would be best for focusing lower frequencies of EM radiation, if any? If not, what other methods exist for focusing these ranges?

The thought was inspired by wondering how you might build a camera that captures 'light' given off by Wifi routers

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Anything that 2.4Ghz EM waves (microwaves) will pass through that has a different index of refraction than air can be used as as a lens or prism. A really common material for this is wax.

Also, you can reflect microwaves using metal surfaces. This is how a satellite dish works.

For a simplified model of your wifi router, you can think about the microwaves as a bright light bulb flashing on and off very fast. An antenna is mostly designed to record the signal ("pattern of flashes") rather than the distribution of microwaves in space. To get an idea of how the signal propagates you'd need to take intensity samples in the space around the signal and generate a volumetric map.

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Actually, air itself with different water content and density forms refracting layers that can focus microwaves (and other electromagnetic radiation). This is called "ducting"/anomalous propagation.

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