LEDs Demonstrate Quantum Mechanics

This is a strip of battery-powered LED Christmas Lights which are usually available at Walgreens at Christmas. I got them out to work on a project, turned them on, and was struck by something odd. I used to buy these by the bucket at the low price of three packs for $10 because it was a cheap way to get fairly bright LEDs that were already shrinkwrapped and easy to use. They've changed the lens design to spread out the light more, but they can still be useful. However, that's not what I'm interested in now.

Notice that the red LEDs are really bright, the yellows are dim and the rest are off. This suprised me and made be take a moment to consider what was going on and come up with an explanation.

A clue is that the batteries are mostly used up and aren't able to provide as much voltage as when they were fresh.

LEDs are weird beasts. Like filament light bulbs, if electrons get pushed through them, they start to glow, but unlike filament light bulbs, each color takes a minimum voltage before it will glow. Less than that voltage, you get nothing.

You might wonder why that is. Alhtough the story is really more complicated than this, essentially the LED turns the energy in one of those moving electrons into a bit of light. The energy that an electron has is related to its voltage.

And the energy of light is related to its frequency. The red part of the spectrum has the lowest frequencies and the violet part of the spectrum has the highest frequencies.

So low voltage electrons have low energy and can only make low energy light which is light with low frequencies which is red light. It takes higher energy electrons to make green and blue light. Since there aren't any high energy electrons, the green and blue LEDs can't make any light.

Perhaps you could put two electrons together to make blue light? That doesn't work in most cases, though. Quantum means more or less "the smallest possible amount."

With fresh batteries, everything started to work as you'd expect.

 

Communicating with Light

Connect an LED to the output of an audio source in such a way that the LED flashes with the source. Then attach an applified speaker to a solar cell and presto you are communicating via light.

<http://www.exploratorium.edu/square_wheels/modulated_led.pdf>

The Exploratorium has a series of books called "Snacks" which explain how to make cool science experiments. Unlike some other books, though, the experiments actually work, and also unlike other some other books their books come with correct explanations of the science behind them. Check out a selection at

http://www.exploratorium.edu/snacks/

You can find the books at most big public libraries. See a list at Amazon here.

Oh, full disclosure, I work/volunteer with the Exploratorium, but I've never worked on a Snack book.

Conducting Glass

Tap water conducts electricity, which seems kind of weird since water isn't a metal and has a covalent bond. Of course it conducts because it has ions dissolved in it that can move around and act like the electrons (in a way) as those in a wire. Pure water without ions is a lousy conductor.

Glass is the prototypical example of an insulator. It too is composed mostly of non-metals and has a covalent bond. And like tap water it too has ions. Why doesn't it conduct?

The ions are frozen in place, of course. Let's not argue whether or not room temperature glass is a solid -- suffice it to say that the particles aren't going anywhere fast. On the other hand, if you melt the glass, then the ions can move and then it will conduct electricity.

How to do this? Connect two lamp bases in series and then connect them to a wall plug protected by a GFCI. Carefully break the glass globe off a bulb. You will see a plug of glass with two wires coming out of it. The wires are in turn connected to the filament. Clip the wires to the plug of glass. Put an intact bulb in one socket and a the broken bulb in the other socket. Plug them into the wall. Careful, careful. You've got live current here. The GFCI will help but it's not perfect.

Take a torch and heat the glass plug until it glows red/orange. The other bulb will then light up. Notice that the plug will stay molten from the heat running through the wire. Unplug the apparatus. The bulb will go out and the molten glass will cool. If you plug it back in before the glass freezes, the bulb will light up again. If you unplug and wait until the glass is cooler (although still hot enough to burn you) before plugging it back in, then the bulb will not go back on.

Electric Pickle

The electric pickle doesn't really show anything useful, but it's fun. The pickle always glows on only one side, but not always the same one. The glow is clearly from sodium ionization like a sodium lamp. Presumeably the current heats up the pickle and boils water away and at some point ionizes the sodium. It will go for some time but once you unplug the pickle it won't glow again.

This gives a starting explanation, and I always like a link to Hyperphysics <http://hyperphysics.phy-astr.gsu.edu/Hbase/electric/pickle.html>

Some plans here.