An Answermug friend mentions "the photoelectric effect" which I know nothing about so I Googled it. What do you DO with it?

Solar panels for electricity.

I would also Google it.

I didn't have time to make this look pretty, but here's an additional slant on how to look at it.  The info is from 2 sites---the Khan academy on the photoelectric effect and this site:  

https://www.quickanddirtytips.com/education/science/einsteins-legacy-the-photoelectric-effect?page=1 


Introduction: What is the photoelectric effect?

When light shines on metal, electrons can be ejected from the surface of the metal in a phenomenon known as the photoelectric effect. This process is also often referred to as photoemission, and the electrons that are ejected from the metal are called photoelectrons. In terms of their behavior and their properties, photoelectrons are no different from other electrons. The prefix, photo-, simply tells us that the electrons have been ejected from a metal surface by incident light.

But perhaps the most important application of the photoelectric effect was setting off the quantum revolution, according to Scientific American---It led physicists to think about the nature of light and the structure of atoms in an entirely new way.

The photoelectric effect has direct applications in the use of photocells and solar cells where energy is produced due to incident photons.

 More importantly, however, the photoelectric effect set off the quantum revolution---. Experimental physicists began to think about the nature of light and the structure of atoms, the foundation of the world around us, in an entirely new way.

Perhaps the biggest lesson to be learned from Einstein’s work on the photoelectric effect is to always remember to think outside the box. If our normal theories aren’t working, sometimes the answer is to make new ones. Einstein himself said, “We cannot solve our problems with the same thinking we used when we created them.”

Element's example - the solar panels on a house roof - are just one of very many applications.

I had a small calculator powered by a miniature version placed just above the display. Similar versions are used for recharging camping-lanterns, decorative path marker-lamps in gardens, and so on.

Photo-electric cells and their opposites, the light-emitting diode (l.e.d.) which emits light when an electric current passes through it are also used a lot hidden inside electronic equipment. E.g. in process-control, counting objects on a conveyor-belt by passing them though a light-beam from an l.e.d. facing a photo-electric cell. While the object eclipses the l.e.d., the photo-electric cell "switches off", and that is detected as one count by the associated electronic circuit.

You might also have noticed the centre-margins of newspapers printed with small coloured squares and other symbols. These are part of the automatic press-control system, which I think uses special l.e.ds and photo-cells to detect the symbols so the system can keep the machine in precise adjustment as it runs. 

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And for the extreme perhaps... telecommunications.

You are in the Western USA, I am in Southern England. The signals carrying our conversation run for most of their journeys as digital pulses of light along glass fibres in cables buried on land and laying on the Atlantic sea-floor. An l.e.d. changes each electrical digit into a light pulse and feeds it into the sending end; a photo-electric cell translates it back to an electrical pulse at the receiving end.

(The practice is rather more complicated that that, of course. "Repeaters" (special amplifiers) at intervals keep the signal strength up on its long journey, and the signal also passes through networks of telephone-exchanges and Internet servers in both countries, but that is the basic principle.)  

Oddly, it's almost a circle.

The very first telecommunications were  by telegraph, which could only use Morse Code - a system of dots, dashes and spaces. If we had been alive in 1920 rather than 2020, and I sent you a telegram, a human operator with a Morse key would have turned the letters manually into electrical Morse Code. That would have been switched by manually-operated telephone-exchanges to a cable-station at a beach called Porthcurno*, in Cornwall, in England's far South-West. There it would have entered an electrical cable running right across the ocean to (I think) New York. This system also needed repeaters, and I believe on land some repeaters were people reading the messages on a special receiver, and re-typing them into the next system of cables. Eventually your local operator would read the Morse dashes and dots and type them in words onto the paper form for delivery to you.

The differences really are that the modern system is fully-automatic, the message travels as light rather than electricity for most of its journey, and the code is not Morse dashes, dots and spaces; but computer-digital, i.e. binary dots and spaces. And of course, it can also carry speech, video and music - also in digital form.

Porthcurno  cable-station is still a major submarine-cable terminal; with its modern, automatic, l.e.d. and photo-cell equipment in a different but adjacent building. The old manually-operated telegraph-repeater station has been preserved as a museum, with its manual telegraph equipment still in working order but internally only, for demonstrating to visitors while the present communications literally flash through the fibre-optical cables emerging from the sea only yards away. Including perhaps, this one.

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*{Porthcurno - That name might ring a non-telephony bell if you are a theatre-lover. It is also the location of the outdoor Minack Theatre.}