[jai]

ahhh! isnt it that glow in the dark paint that they put on it?

or alternatively, for those clocks that do not contain such advanced technology, is it that the hands are just above the surface of the clock face? so that in low light, even when there are no visible shadows there is an almost impercepatable diffence in the shade of colour caused by the shadow of the hands. this difference in shade, though not always noticed  tricks the eye into thinking that the hands are lighter in shade - or glowing. much in the same way that some of those cool sixties paintings make your eyes wobble and the colours jump out and move (though that has to do with the colour perception and the tone value or something like that).

[jai]

also just fournd out, courtesy of my dad, that the old clock and watch hands used to contain radium to make them glow. the new glow in the dark hands are just a flourescing paint rather than a paint with a half life of it's own....

yes, but.........

[Furwa]

Its flourescent paint/sticker =D
And so um it glows...Like all other flourscent stuff.

[gsmollin]

There are several right answers to this one.
1) Years ago the answer was radium-laced paint. I had one of those watches, and I could see the scintillation discharges in the clock face, in the dark, if I used an eye loupe. Legend has it that the unfortunate women who painted the watch faces by hand used to point their brushes by twirling them in their mouths. If this is true, the radium would have caused their jaws to fall off.
2) Today, watches use a phos-phorescent paint that stores light energy, then releases it in the dark. These are not nearly as bright as the radium dials were.
3) There was an expensive watch, advertised some years ago, that claimed to be filled with tritium, which made its dial glow brightly in the dark.
4) Lately, electro-flourescent screens have become common on electronic digital watches. Although I haven't personally seen these used on a watch with hands, it is probably possible to engineer one.

[gsmollin]

As an aside to last week's lightning question (great essay, NS), I was unfortunate to be within a few feet of a lightning discharge once, and my salient memory is the nature of the sound it produced. We are used to rumbles, booms, and bangs from thunder. When one is close enough, the sound is the most intense SNAP one has ever heard. The electrical nature is un-mistakable.

[NakedScientist (OP)]

ANSWER TO "HOW DOES GLOW IN THE DARK PLASTIC WORK AND WHAT MAKES THE HANDS ON WATCHES GLOW"

The answers given above are pretty much correct.

Things that glow in the dark are referred to as 'phosphors' and are materials which can soak up energy and then re-radiate it as visible light. Put simply, when these substances absorb energy (in the form of light, heat or radiation) some of their electrons become excited and are catapulted up to a higher energy state. Light is emitted (and the substance glows) when the excited electrons fall back to their 'ground state', releasing the extra energy that they picked up previously.

Television screens (the non-LCD / Plasma screen variety) and fluorescent tubes (strip lights) rely on precisely this effect. In a TV the screen is coated with a phosphor which is excited by a stream of electrons produced by a cathode ray gun at the back of the set. In a strip light the electricity excites electrons in the atoms of the metallic element mercury. The excited mercury atoms emit ultraviolet light which hits the phosphor coating on the glass of the tube, which in turn then emits visible (white) light.

The phosphors used in glow in the dark stickers and badges, clock and watch faces commonly contain the compounds zinc sulphide (often with some copper mixed in too) or strontium aluminate. These substances are added to the polymer used to make the plastic. They produce a soft green glow which can, with the correct engineering, persist for minutes to hours.

Another way to make things glow in the dark, but without them needing to be 'charged up' by prior exposure to light, is to use a long-lived radioactive substance, such as radium. The radioactive material can be combined with an appropriate phosphor which is excited by the radioactivity and converts the energy of the radiation into visible light - making the hands of the clock or watch glow.

So, in summary, cheaper clocks and watches use phosphors which soak up light and then release it very slowly to make their hands glow for several hours afterwards. More expensive (and military) timepieces rely on a radioactive substance to energise the phosphor so that they can glow continuously.

[roberth]

So, TNS, are you saying that my watch (Rolex Submariner) contains a radioactive substance? I thought that they stopped using that stuff a while ago.

[NakedScientist (OP)]

The substance used (as correctly stated above) is a paint containing tritium (an isotope of hydrogen which decays (breaks down) emitting beta particles (fast moving electrons)). These beta particles excite a phosphor (also in the paint) which converts the energy from the beta particle into visible light.

The half life of tritium is 12.3 years. In other words, every 12.3 years the number of radioactive nuclei has declined by half. So you might need to get the paint touched up to keep the glow as bright.

I think Rolex, certainly in the past, use(d) precisely this technique to keep their watches glowing.

TNS

[NakedScientist (OP)]

Here's this week's QOTW :

WHAT IS A SHOOTING STAR ? IS IT REALLY A STAR ?

[roberth]

I think a shooting star is a small asteroid or space rock burning up as it enters the Earth's atmosphere. It's not a star.

[OmnipotentOne]

yeah he's pretty much got it, a meteor that hits the earths atmosphere and burns up on the way in, causing that streak of light.

To see a world in a grain of sand.

[gsmollin]

Yea, come on, NS, you can do better than that. Why don't you ask what shooting stars are composed of?

[NakedScientist (OP)]

Better still, why don't you tell us ! []

[NakedScientist (OP)]

HERE IS THIS WEEK'S QOTW :

WHAT IS THE 'FOG' THAT APPEARS FROM THE TOP OF A BOTTLE OF FIZZY DRINK OR CHAMPAGNE AFTER YOU OPEN IT ?

(Hint - we're not talking about the drink spraying up when you shake the bottle !)

TNS

[gsmollin]

Meteorites: There are two basic kinds, stoney and iron. Stoney meteororites have been hard to find, since they look like any other stone, but they have been collected from Antarctic glaciers in substantial numbers. The other type is the iron meteorite, containing up to 15% nickel. These are the more famous type, and some large ones have been found. Their crystal structure is consistent with having been cooled at a depth of a hundred kilometers or so inside of an asteroid, then having been released by a collision.

Pop-bottle fog: The fog is composed of very small dilute carbonic acid droplets. The fog is produced by the sudden pressure drop inside the bottle when you open it. A bottle of "fizzy drink or champagne" is carbonated- it contains carbon dioxide gas dissolved in water, aka carbonic acid. The high pressure inside the bottle maintains the vapor pressure of the carbonic acid in equilibrium. When you open it, the carbon dioxide comes out of solution and forms the bubbles we see in fizzy drinks. At the same time, the pressure in the gas over the liquid in the bottle decreases, and the gas becomes supersaturated with the water vapor and CO2, and forms a fog.

[NakedScientist (OP)]

An interesting addendum about meteorites :

Scientists at the University of Arizona announced recently that they think life may have started with help from a meteorite. Their argument centres on the element phosphorus (P), which plays a pivotal role in life's biology. It forms the backbone of our genetic material (DNA and RNA), stabilises our cell membranes (as phospholipids) and provides cells with the molecular equivalent of a rechargeable battery (as the ubiquitous energy molecule ATP).

But in the early earth 4000 million years ago, when life began, phosphorus was relatively scarce because it was locked up in stable, unreactive, minerals like apatite (calcium phosphate).

So it seems strange that such a rare chemical (in terms of its chemical availability) should be given such a central role in developing life. Unless, of course, life began in a place richly endowed with chemically reactive phosphorus...around a meteorite impact site, for instance.

Meteorites fall into 2 broad categories (as gsmollin has clearly explained above) - petrous (stony) and ferrous (irony!). Iron meteorites contain iron and another mineral, schreibersite, which is iron nickel phosphide and very rare on earth.

But unlike Earth's unreactive forms of phosphorus (apatite), schreibersite eagerly participates in reactions with fresh water to produce phosphorus compounds very similar to those found in life today, elading scientists to speculate that a meteorite impact might have provided the catalyst that got life started on the early earth.

The Arizona scientists argue that if an iron-rich meteorite (containing schreibersite) crash landed in a pool of fresh water the  area would become enriched with biologically useful forms of phosphorus, perhaps explaining how "The Devils Element" landed the lead role in the story of life.

[neilep]

...and I always thought Meteorites was a religious or other solemn ceremony conducted seconds before spacey rocky slammed into the planet ...oh well...guess I got that wrong.

'Men are the same as women...just inside out !'

[NakedScientist (OP)]

ANSWER TO LAST WEEK'S QOTW "WHAT IS THE 'FOG' THAT APPEARS FROM THE TOP OF A BOTTLE OF FIZZY DRINK OR CHAMPAGNE AFTER YOU OPEN IT ?"

Fizzy drinks are saturated with carbon dioxide. When you open the bottle the escaping gas bubbles carry tiny droplets of water into the neck of the bottle. At the same time, the pressure above the liquid (keeping the carbon dioxide in solution) suddenly drops, which causes the temperature to fall.

The principle is the same as a fridge which cools the interior by rapidly expanding a compressed gas. The huge increase in entropy associated with the expansion of the gas more than accommodates a small enthalpy (temperature) decrease.

In the bottle neck the lowered temperature allows the water droplets carried out by the escaping gas to cling together by a process called hydrogen bonding. Water molecules resemble tiny boomerangs with an oxygen atom at the centre and a hydrogen atom forming each 'arm'. Because the oxygen attracts electrons more strongly than hydrogen the oxygen is slightly negatively charged and the hydrogens are slightly positive. These charge differences can weakly glue different water molecules together so they hang in a fog. Because the effect is very weak, the fog rapidly disappears when the temperature rises and the molecules start to move about too quickly to hang together.

So there you have it, the origin of fizz bottle fog.

TNS

[NakedScientist (OP)]

HERE IS THIS WEEK'S QOTW :

"HOW DO MATCHES WORK ? WHAT MAKES A SAFETY MATCH 'SAFE' ?"

TNS

[nilmot]

Matches have sulphur and other chemical which I don't know yet but I will find out because i saw it on a book before, when scrapped against a rough surface that provides the energy needed for the reaction.

When matches were invented it was originally found by a scientist John Walker where he was stiring potassium cabonate and antimony with a stick. He scraped the stck on the floor with purpose of getting rid of the chemical but the mixture caugth fire.

They use to self combust because the mixture of chemical was reactive enough when it have contact with air and light. 'Something' which I will also find out were added to it to prevent it from happening.

Tom