Category Archives: Physics
Look at Me
Invisible Numbers
I’ve posted before on how liquid-crystal display (LCD) monitors emit polarized light (and can be birefringent), and some of the fun you can have with this, and everyone is probably familiar with other uses as well.
LCD’s don’t emit light by themselves; they rely on backlighting or on reflecting ambient light, which passes through a polarizer behind the display. In this short video we can see what happens with a calculator display when you take the top polarizer off:
We see nothing at all. The effect of the display is not visible to us, because we are not (very) sensitive to polarized light. When we put the polarizing screen in place, then we can see what’s happening — the display has a “zero” energized, which has a different polarization than the rest of the display and blocks the light that has passed through the display and been reflected and polarized. When we rotate the screen, the light is blocked from the rest of the display, and the light from the zero passes through. At an angle, light of each polarization makes it through, so you can’t see the digit at about 45º.
Violating Betteridge's Law
Does a Magnet Gun Conserve Momentum?
Betteridge’s Law of Headlines: “Any headline which ends in a question mark can be answered by the word ‘no'”. But this is a physics topic, and conservation of momentum is a pretty well-established law if there is no net external force on the system. So we expect the answer to be “yes”. Thus you can tell Rhett is not a headline-writer looking to stir up controversy, else he would have written something like Does a Magnet Gun Violate Conservation of Momentum?
Looks like a fun toy, and I’m a sucker for fun toys. To the bat cave lab!
Burning for You
Christmas with Faraday: The Chemical History of a Candle
Faraday gave a series of famous Christmas lectures each year at the Royal Institution — a tradition that continues today. One of the earliest, on the chemistry and physics of flames, became a popular book: The Chemical History of a Candle.
These lectures were a gift that Faraday gave year after year to those who showed up to receive it: the gift of wonder at the natural world that continues to surprise us, even today, with its mysterious workings.
The Nose Knows Physics
According to the song, Rudolph’s nose is shiny, which means it reflects rather than emits light. Useless for navigating fog.
To which I responded
Nose also glows & bright. Since it’s red we could determine temperature if a thermal source & estimate Rudolph’s calorie needs
If Rudolph’s nose is a thermal source it will follow the Stefan-Boltzmann power law, which tells us the radiated power depends on the fourth power of temperature. Something red-hot will have a temperature of about 1000 K. Now this is an estimate and since it’s raised to the fourth power, will give us a large error bar on our answer. But let’s go with that because I don’t have a calculator handy. For the emitted power we multiply by the area, a few square centimeters (converted to square meters) and Stefan’s constant. Assuming I did the math correctly, we get about 10 Watts. The temperature should not be as large as 2000 K, which would give us and answer 16 times as large. (I am ignoring the “power absorbed” term in the equation, because at these temperatures it’s going to be small — 300K or less)
There’s also the emissivity. The nose is shiny, meaning the emissivity is not close to 1. So perhaps we double our guesstimate. Tens of Watts, maybe as large as 100 Watts as a probable value.
A thermal source has a maximum luminous efficacy of 95 lumens/Watt, at a temperature of around 6600 K but actual bulb filaments that give us white(ish) light are a lot closer to 10 lumens/Watt. So the nose probably emits around 1000 lumens at best — this is not even as bright as a traditional 100W light bulb, but is around what low-beam halogen headlights emit. However, those have reflectors on them to direct most of the light into a beam. Rudolph’s nose emits into a much larger area.
So perhaps the nose is not a thermal source (unless it’s much larger than I estimated) — the radiation is not because it is hot. We could check this if we knew the spectrum of the light being emitted. Perhaps it is some other type — does Rudolph have an LED nose?
Baby, it's Cold Inside
How cold is cold enough? Eliminating entropy picokelvins from absolute zero
The team demonstrated their technique with two experiments, using a gas of rubidium-87 atoms in a square optical lattice. In the first, they started with a known number of atoms at each site (between one and four) all at the ground energy level. Then, by modulating the frequency, they gradually removed all the extra atoms, finishing with only one in each lattice site—a minimal entropy configuration.
In the second experiment, instead of starting with a known number of atoms all at the ground level, they loaded the lattice with a random number per site, with some excited and some at the ground level. As before, by sweeping the frequency, they removed all the extra atoms.
Casual Physics Friday
hyper-efficient solar cells that aren’t actually efficient! or, giving good science a bad title
It’s ironic (or perhaps siliconic since this is about solar cells) to find a takedown article (which is otherwise OK) that says things like “relaxed momentum conservation” and explain it like this:
These strange little beasties are tiny bits of semiconductor material — tiny enough that lots of strict physical laws (like conservation of momentum) get to relax in some ways, making all sorts of fantastic things possible.
The phrase show up in a few places on the net, but the context there is not that the law is relaxed, but that the conditions are. Silicon is an indirect-bandgap material, meaning that the bands on either side of the bandgap do not line up if you lot energy vs. momentum, as you can see here. This requires a “phonon assist” for an excitation — you need the right vibration, with the right momentum at the time the photon arrives in order to promote the election. In other words, it’s harder to meet the conditions of momentum conservation.
But that’s in a crystal. So what I suspect is that in a quantum dot, the conditions are easier to meet, because it looks more like a direct bandgap material. Not that the law itself is more relaxed.
Now You're Cooking With Gas
“Pyro board” is unfortunately as detailed as it gets; it appears to be a 2-D version of a Rubens tube. The sound waves give you high- and low-pressure regions, which show up as little/no flame or big flame (lower pressure = more flow, from Bernoulli’s principle).
The video of a one-dimensional system I linked to a while ago has some explanation to go along with it.
Is it Still Right Twice a Day?
Astrophile: Stopped clocks deepen pulsar enigmas
Some pulsars go dark, though, and Camilo’s was not the first. In the 1970s, some regular pulsars were spotted switching off for a few seconds to a few minutes, a phenomenon known as “nulling”. And in the past decade, a new class of pulsars has been found , in which the silences can range from minutes to a few hours. They were dubbed rotating radio transients, or RRATs. Around the same time, a pulsar was found that pulsed for about a week and then switched off for about a month before repeating the cycle.
There are papers discussing the possibility that precession could cause this, i.e. the pulsar is still “on” but not pointing at us during the nulling interval, but I didn’t see that brought up in the article.