Why radio waves and light are fundamentally the same phenomenon of electromagnetic radiation, but you can produce and receive one with a simple metal rod (antenna) and only the other can be focused with glass lens?

Question

Also, why you can produce radio waves with a simple amplifier circuit and a piece of wire, but need special devices like an LED or a discharge tube to produce light?

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Answer 1

Radio and light are electromagnetic light. That means that they can influence the electrons in something conductive. However, in order for the effect to be very strong, the conductive thing needs to be on a similar scale to the wavelength of the waves. The shortest radio waves start at a few millimeters and go up to kilometers. Their scale is in the range of things we can see and hold.

Visible light has wavelengths on a scale similar to largish molecules. Molecules with a lot of double bonds can act like wires or antennas. Look up the structure of carotenoids, and you’ll see why the absorb light and give vegetables distinct colors. The length of the molecule determines the wavelengths that it can absorb.

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Carrots have color because the molecules are acting like antennas at the right wavelength of light. Different length molecules will give different colors.

So TLDR, it’s because the wavelength needs to be close to the size of the antenna and visible light is way too small.