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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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.
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.
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.
I see good info in here but they don’t really seem to answer the questions properly.
>but you can produce one with a simple metal rod
As others have mentioned an antenna needs to be about the same length as the wavelength you’re trying to transmit. But any AC circuit will generate an electromagnetic field, you can usually detect some at 50 or 60 Hz from the power grid without having a proper antenna.
The reason we cant (read don’t) Is because its really hard and not very effective. Its not easy to get a an electrical circuit to run at the frequencies required to generate light.
>And the other can be focused with a glass lens
Radio frequencies can also be focused by a lens but not a glass one. But anything of a reasonable size will only be able to be used on very high frequencies in the microwave category due to the wavelength of the frequency. Lower frequencies will need a larger lens to actually bend the radio waves. [stack exchange post](https://electronics.stackexchange.com/questions/507895/why-dont-we-use-lenses-for-rf).
From Pepsi to chocolate, from pasta to kebab, there are many types of food.
You can’t eat pasta with a straw, but would work with Pepsi.
Electromagnetic waves interact with the environment based on their wavelenght, and can be very different one another like foods can be.
For each type, there’s a specific behavior, therefore a specific set of tools that work with it.
You can easily make a long wave with a “slow”instrument like an electric circuit, like radio, tv, and so on. It’s not a difficult frequency to artificially make with electricity, then you transmit the wave by sending the electric signal to the antenna. The antenna ups and downs in electric charge will induce a wave in the electromagnetic field.
Light is in a wavelenght so small that bounces on most things surface, but big enough to not penetrate. Can be detected by special means. For example our eyes can electrochemically sense it. Cameras can electrically sense it. Photo film does chemically sense it. You can make light by glowing stuff, hence you don’t directly make light, you excite materials that emit light when excited. As the wave is too short to artificially make it with a circuit and antenna. Nice that light bounces on stuff because it allows mirrors and lenses to work.
Even tighter waves like X-ray are so tight they can go through materials. You can’t pick ‘em up with human body sensing, they go through it. However they behave like light in glass, with some metals. So we can use very strange detectors or emitters, connected to metal plates/wires that capture and transmit the wave like a mirror or optical fiber would with light.
It’s all about the wavelenght, and what that wavelenght is able to interact with. I overly simplified three spectrums, but there are so many with so many properties.
I see good info in here but they don’t really seem to answer the questions properly.
>but you can produce one with a simple metal rod
As others have mentioned an antenna needs to be about the same length as the wavelength you’re trying to transmit. But any AC circuit will generate an electromagnetic field, you can usually detect some at 50 or 60 Hz from the power grid without having a proper antenna.
The reason we cant (read don’t) Is because its really hard and not very effective. Its not easy to get a an electrical circuit to run at the frequencies required to generate light.
>And the other can be focused with a glass lens
Radio frequencies can also be focused by a lens but not a glass one. But anything of a reasonable size will only be able to be used on very high frequencies in the microwave category due to the wavelength of the frequency. Lower frequencies will need a larger lens to actually bend the radio waves. [stack exchange post](https://electronics.stackexchange.com/questions/507895/why-dont-we-use-lenses-for-rf).
I see good info in here but they don’t really seem to answer the questions properly.
>but you can produce one with a simple metal rod
As others have mentioned an antenna needs to be about the same length as the wavelength you’re trying to transmit. But any AC circuit will generate an electromagnetic field, you can usually detect some at 50 or 60 Hz from the power grid without having a proper antenna.
The reason we cant (read don’t) Is because its really hard and not very effective. Its not easy to get a an electrical circuit to run at the frequencies required to generate light.
>And the other can be focused with a glass lens
Radio frequencies can also be focused by a lens but not a glass one. But anything of a reasonable size will only be able to be used on very high frequencies in the microwave category due to the wavelength of the frequency. Lower frequencies will need a larger lens to actually bend the radio waves. [stack exchange post](https://electronics.stackexchange.com/questions/507895/why-dont-we-use-lenses-for-rf).
From Pepsi to chocolate, from pasta to kebab, there are many types of food.
You can’t eat pasta with a straw, but would work with Pepsi.
Electromagnetic waves interact with the environment based on their wavelenght, and can be very different one another like foods can be.
For each type, there’s a specific behavior, therefore a specific set of tools that work with it.
You can easily make a long wave with a “slow”instrument like an electric circuit, like radio, tv, and so on. It’s not a difficult frequency to artificially make with electricity, then you transmit the wave by sending the electric signal to the antenna. The antenna ups and downs in electric charge will induce a wave in the electromagnetic field.
Light is in a wavelenght so small that bounces on most things surface, but big enough to not penetrate. Can be detected by special means. For example our eyes can electrochemically sense it. Cameras can electrically sense it. Photo film does chemically sense it. You can make light by glowing stuff, hence you don’t directly make light, you excite materials that emit light when excited. As the wave is too short to artificially make it with a circuit and antenna. Nice that light bounces on stuff because it allows mirrors and lenses to work.
Even tighter waves like X-ray are so tight they can go through materials. You can’t pick ‘em up with human body sensing, they go through it. However they behave like light in glass, with some metals. So we can use very strange detectors or emitters, connected to metal plates/wires that capture and transmit the wave like a mirror or optical fiber would with light.
It’s all about the wavelenght, and what that wavelenght is able to interact with. I overly simplified three spectrums, but there are so many with so many properties.
From Pepsi to chocolate, from pasta to kebab, there are many types of food.
You can’t eat pasta with a straw, but would work with Pepsi.
Electromagnetic waves interact with the environment based on their wavelenght, and can be very different one another like foods can be.
For each type, there’s a specific behavior, therefore a specific set of tools that work with it.
You can easily make a long wave with a “slow”instrument like an electric circuit, like radio, tv, and so on. It’s not a difficult frequency to artificially make with electricity, then you transmit the wave by sending the electric signal to the antenna. The antenna ups and downs in electric charge will induce a wave in the electromagnetic field.
Light is in a wavelenght so small that bounces on most things surface, but big enough to not penetrate. Can be detected by special means. For example our eyes can electrochemically sense it. Cameras can electrically sense it. Photo film does chemically sense it. You can make light by glowing stuff, hence you don’t directly make light, you excite materials that emit light when excited. As the wave is too short to artificially make it with a circuit and antenna. Nice that light bounces on stuff because it allows mirrors and lenses to work.
Even tighter waves like X-ray are so tight they can go through materials. You can’t pick ‘em up with human body sensing, they go through it. However they behave like light in glass, with some metals. So we can use very strange detectors or emitters, connected to metal plates/wires that capture and transmit the wave like a mirror or optical fiber would with light.
It’s all about the wavelenght, and what that wavelenght is able to interact with. I overly simplified three spectrums, but there are so many with so many properties.
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.
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.
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