How can we make invisible lights visible?

[Catch22meifucan]

Is there a way to combine lights that are not visible to our eyes and make them part of the visible spectrum?

 

[Simon Bridge]

If the lights are in the visible spectrum but are too weak for our eyes to detect, then you can combine them to make them visible - but you are asking about light that is not part of the visible spectrum ... so no: there is no way to combine light sources, all of which glow off the visible spectrum, so that our eyes can see them.

The usual approach is to make a detector that gives off light in the visible spectrum when light not in the visible spectrum is detected.

 

[e.bar.goum]

Every green laser you see is a result of something called "frequency doubling" or "second harmonic generation (SHG)", which is a process that does exactly what you ask.

SHG occurs when you pass photons of a single frequency through a non-linear material. The photons effectively "combine" to give photons of twice the energy of the original. In the case of a green laser, you start with an infrared laser of wavelength 1064 nm, and double it through a potassium dihydrogen phosphate crystal, giving a green laser with wavelength 532 nm.

But this is sort of a special case. You need special crystals, and you can't do it in general, as Simon rightly says.

 

[sophiecentaur]

Is there a way to combine lights that are not visible to our eyes and make them part of the visible spectrum?

I think this is a question about 'Mixing'. It is possible to beat two waves together and get products at the Sum and Difference frequencies. It works fine at Radio Frequencies (in nearly all radio receivers) but it isn't so easy at light frequencies. All mixing uses a Non Linearity and RF Diodes are two a penny (literally). There are not so many devices that will work so easily at optical frequencies but there has been work done.
See this wiki link.