By that I mean a switch that will toggle when the dielectic constant near it changes.
I hope dielectic constant is the correct term. Do such switches exist? Is it more complex than that?

https://en.wikipedia.org/wiki/Touch_switch

Cool beans,now I know what to look for on the Net.

I took my electronics class in french. No choice,it was only available in french.
For security reasons I wasn't allowed a french/english dictionary or to record what was being said.
That means I was often playing catch-up. Even reading the text books only helped so much.
I was very proud to get 98% on one exam.

That was thirty five years ago.
If you don't use it you lose it.
It's still a lot of fun to tinker.

Touch switches are common for home use overseas but seem to be rare in the US. I found them online a few years ago and nearly all of the switches in my home are touch switches. Just wave your hand past the switch plate and the lights go on. It's a direct replacement for any one-way switch (1-3 pole). You can do it for two-way switches but that's a bit more complicated. If you are into DIY and low voltage DC, check these out: https://www.banggood.com/search/capa...ch-switch.html If you want one (5-12V DC) pre-wired with CCTV power cord ends this: https://www.banggood.com/LED-1-Key-B...r_warehouse=CN It's a reputable Chinese company where I ordered most of the things to convert my home to all LED.

I’m sure I could design one. What is your application?

This is just an idea.
I was thinking of making a simple white L.E.D. light for the end of my handle bars.
Something that turned on automatically when a hand approached the grip.
There is no way I want to throw good hard cash for a manufactured light and
then have to take it off/out of the handle bars every time I lock up the bike.
I am already going to have to do that for the front and maybe the rear light.
If some idiot wants to steal an inconspicuous L.E.D. there is nothing I can do.

the general touch switch relies on the user's body picking up the nearby AC waveform from adjacent mains power. I'm not sure that this would be sufficient on a bicycle used outdoors.

There could be ways to use a variable capacitance, though. You could set up an oscillator that would change frequency as the capacitance changes. Another circuit could watch the oscillator frequency and turn the light on or off when the frequency changes.
The downside of this is that the circuit will be using power all of the time, albeit not a lot of power.

Maybe you could just use a little reed switch to control power to the light? This just requires placing a magnet nearby the reed switch to turn it on. One of the SON lights uses this idea... is the Edeluxe?
The nice thing is that if you've got an old bike speedometer laying around, it has a reed switch that you can use! Most of them use reed switches as the sensor.

Steve in Peoria

Interesting on the reed switches being used in older speedometers.
They are also used in intrusion alarms. The fact is I may have a door contact lying around.
I will have to give the light switch idea more thought.
I do think that handlebar end lights are a good idea. It gives motorists a better heads up.