Such as a flash light .im sure some kid has done this 4 a science fair. Basically stepping down a flashlight into radio waves. Beause its all about controlling wavelength right?

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... or frequency, being the same.

How would one convert a signal of 50hz into one of 1Khz?

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This is being done every day using well known technology. Depending on what you are really trying to do, it can be done pretty cheap, too. But, you basically get what you pay for.

Light (i.e. light waves) are electromagnetic and can be converted to normal, everyday electricity with a solar cell. You can get a cheap one or an expensive one. How much you get is up to you.

Radio transmitters come in various price ranges but if you don't care what type of radio frequency you create you can build your own with on a small circuit board for a small cash outlay. Be careful you don't get in trouble with the laws that regulate radio frequency emissions.

So, take it from there. There are plenty of people who know the basics and many engineers that can help you convert light to radio waves. And, radio waves are also electromagnetic waves.

Maybe you have an idea for going directly from light to radio? You will be doing a lot of experiments and learning a lot as you go along. You may even become an engineer along the way. Good luck in your studies. There is much to learn.

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There is a reason why science has been successful and technology is widespread. Don't be afraid to do the math and apply the laws of physics.

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It could be a way to convert radio waves into the amplified power to heat or light or provide mechanical power for human machines easily and cheaply without consuming fossil fuels

and the solution is very very near

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The radio spectrum is considered 10kHz to 10GHz
Light begins at Infra red spectrum about 700 nm or 300 GHz

The down converting light technology has had limited success in splitting
photons in two it is not readily available or practical.
It is not known to have a simple method.

Since the energy goes up as the frequency increases,
and the energy is directly proportional to the frequency.
Ultra fast semiconductors are slow in comparison and cannot
handle much energy.

Some are working on it but no cost effective solution yet.

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I may try to build that one myself but how can this be used to convert light to radio waves? Don't you think this is unhelpful? Please explain how you think this is helpful.

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There is a reason why science has been successful and technology is widespread. Don't be afraid to do the math and apply the laws of physics.

If one can work out how to change frequency at one part of the spectrum, it might work in another part of it

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