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

The electromagnetic wave goes up and down, with peaks and troughs. The peaks and troughs of the electromagnetic waves are closer together the higher the frequency. The distance between the peaks and troughs is called “wavelength”. Different wavelengths makes waves behave differently for different materials.

A radio wave can be produced from an antenna because the peaks and troughs of a wave at that frequency are on a human scale. In order to produce one, you push and pull the electric field on a piece of wire that’s on the scale of the wavelength you’re producing, and you get a transmission. 300 Megahertz, for example, has a wavelength of 1 meter. FM Radio, which is on the order of ~ 100 MHz, has a wavelength of about 3 meters. WIFi has a wavelength of about 12 cm.

Light, on the other hand, has a wavelength on the order of hundreds of nanometers, a billionth of a meter. At this scale of wavelength, the wave behaves totally differently, and you can’t use the “push-pull” trick to produce a signal as easily anymore, because your antenna would have to be very small.