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?

In: 0

Answer 1

We could/can receive light with a simple metal rod, the issue is it has to be much much smaller than the antennas we use for radio.

The range of wavelengths an antenna can receive is related to its overall length. Make it too long or too short and you won’t be able to detect the signal. That length depends on the wavelength of the signals being received, but the optimum length is 1/2 or 1/4 of the wavelength of your signal. Well visible light has a wavelength between 380 – 750 nanometers. Meaning your 1/4 length optimum antenna is gonna ned to be between 95-185 nanometers. Thats….tiny. If you make it bigger, particularly much bigger you run into the problem of interference from signals with higher wavelength.

But there is a different problem. The information we get via radio and the information we get via light. With radio we just need to know the data hidden inside the transmission. We use known information about how the waves are transmitted and then vary that information (amplitude for AM, frequency for FM) to encode the data. You can encode the same information in different radio bands, the same song, the same words, whatever. You don’t have to tune to different radio bands to hear each individual note. We can even transmit pictures using radio waves because we encode them.

But when we LOOK at something we are using EM radiation in a different way. We aren’t encoding information in the frequency or amplitude like we do with radio. We are, instead, directly measuring the properties of the radiation and assigning values (colors, brightness, etc.) based on that. AND, unlike AM/FM radio, we also care about WHERE the signal is coming from. If I want to hear a song on my radio I don’t need to know or care which direction the signal is coming from. But when I am looking at something I need to look in the direction its coming from. The direction is important. The location of the signal in 3D spaces is important.

In the end, yes, its all EM radiation, but how we use it and what information we get from it is very different for radio vs visible light. Because of that how we receive and interpret it matters and what works for one type of information transmission doesn’t work in the other situation and vice versa.