Plants only absorb very particular aspects of light. As far as I remember, it's only visible light, yes.

More information here: http://www.webexhibits.org/causesofcolor/7A.html

Hydroponic lighting is optimized for those particular wavelengths in the ~450 nm and ~650 nm peeks. Some plants absorb additional wavelengths, and could likely be targeted with other lights. The key though is that chlorophyll are like mitochondria -- highly preserved between species -- and they evolved to only absorb particular frequencies.

That’s really interesting, thanks for the explanation. Makes me think of tuning LED lighting the same way a mechanic might set spark timing. Obviously in reality the right lighting for a plant is known but it’s neat to be able to see the effect.

It may be well known but there are many practical considerations. It certainly requires a lot of tuning. For example, if you want to target all four peaks of absorption by cholorphyll a and b, that’s four wavelengths, but many people settle for just two due to cost. It’s all a giant optimization problem!

Research is inconclusive, but there are indications that UV light (although not required for photosynthesis) can have effects on yields and either on resistance against pests (insects/molds/fungi) or directly attack/kill/weaken those pests. So basically 'it depends', and people do their own tests and then determine if, in their specific circumstances, it is a net positive (economically) to add UV lights to their setups.

I think this is the main message. People in this thread are talking in absolutes about 'agriculture', but the reality is that there are many, many things that we don't understand well enough at the above-micro-level scale to be able to say 'you need to measure x, y and z and then we can calculate/model the optimal setup to grow something'. People are basically relying on (informed) trial and error, with feedback cycles measures in months or years, so it just takes a long long time for progress to be made - and even then, we usually don't know exactly why something works in a specific case. It's not like 'oh let's run an A/B test for the best color of our 'buy' button for a few days'.