Question of the Week - Old Version

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Offline ukmicky

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Re: Question of the Week - Old Version
« Reply #300 on: 17/01/2006 04:25:14 »
Are but what do you define as visible,rats have ultraviolet vision so is that visible.just because we cant see it dosent mean its not visible[:D]

Michael                 HAPPY NEW YEAR                    
« Last Edit: 17/01/2006 04:32:59 by ukmicky »

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Offline DoctorBeaver

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Re: Question of the Week - Old Version
« Reply #301 on: 17/01/2006 04:33:15 »
Absolutely, Michael. I would say something is visible if it can be detected by any creature's optical sensing mechanism or an enhancement therefore e.g. a microscope or telescope.
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Online chris

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Re: Question of the Week - Old Version
« Reply #302 on: 17/01/2006 12:57:00 »
Why is it visible on a cold day, but not on a hot day ?

Chris

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Offline ariel

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Re: Question of the Week - Old Version
« Reply #303 on: 17/01/2006 19:15:03 »
The moisture in warm air you exhale is chilled and condensed when it reaches cold air- so its visible like fog!
when its hot outside...it isn't chilled, and doesn't condense- so its not visible not like fog!
ariel

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Offline DoctorBeaver

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Re: Question of the Week - Old Version
« Reply #304 on: 17/01/2006 19:40:51 »
I refer the honourable gentleman to the answer I gave some moments ago [:D]
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Offline ariel

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Re: Question of the Week - Old Version
« Reply #305 on: 18/01/2006 21:40:30 »
psh, if you're referring to me..im not a gentleman :-)
« Last Edit: 19/01/2006 00:11:08 by ariel »
ariel

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Offline rosy

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Re: Question of the Week - Old Version
« Reply #306 on: 18/01/2006 21:45:44 »
The bit everyone's already said:
Air cools, the amount of water it will hold decreases, some of the water condenses out and forms droplets.
Now, the bit that actually answers the question:
The droplets of water have a different refractive index to air, so light is bent through the droplets and what you see through them is not the same as you'd see through the patch of air they've replaced.

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Offline DoctorBeaver

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Re: Question of the Week - Old Version
« Reply #307 on: 19/01/2006 11:05:41 »
Ariel - It was to Chris, not you
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Offline ariel

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Re: Question of the Week - Old Version
« Reply #308 on: 20/01/2006 00:20:51 »
yay
ariel

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Re: Question of the Week - Old Version
« Reply #309 on: 20/01/2006 16:52:24 »
quote:
Originally posted by rosy

The bit everyone's already said:
Air cools, the amount of water it will hold decreases, some of the water condenses out and forms droplets.
Now, the bit that actually answers the question:
The droplets of water have a different refractive index to air, so light is bent through the droplets and what you see through them is not the same as you'd see through the patch of air they've replaced.



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Re: Question of the Week - Old Version
« Reply #310 on: 29/01/2006 16:20:18 »
Here's the answer to last week's QOTW :

"WHY DO WE SEE OUR BREATH ON COLD DAYS ?"

This was a trick because there were several parts to the answer. Rosy was the closest. No one, for instance, mentioned why our breathe was damper than the air we inhaled...

Here's the answer:

We use our lungs to pick up oxygen from the air we breathe in, and to expel carbon dioxide, a waste product. This process occurs in a system of tiny air sacs called "alveoli" which give the lungs, when viewed up close, the appearance of a piece of sponge. The surfaces of the alveoli are kept moist to facilitate the exchange of gases between the blood and the airspaces.

Because the alveolar surfaces and the trachea, mouth and nasal passages are moist the air we breathe out picks up water and leaves the body at 37 degrees C (body temperature) and 100% humidity. If you add up how much water a person breathes out in a day it's almost half a litre (400 ml). This is referred to as "insensible losses".

When you breathe out this water-laden air, it immediately begins to cool and as it does so it becomes easier for molecules of water vapour (H2O) to cling together. If the air around you is sufficiently cold your breath cools very rapidly, and the water molecules quickly glue themselves together to form tiny water droplets. These droplets behave like miniature lenses, bending the light passing through them and thereby making them visible.

This doesn't happen on a hot day because the ambient air is warm enough to keep the water molecules zipping around freely - in other words they can't cling to each other to form droplets - and so your water-soaked breath remains invisible...

Chris



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Re: Question of the Week - Old Version
« Reply #311 on: 29/01/2006 16:21:48 »
Right, since I'm on a roll, here's this week's QOTW:

"WHAT ARE RADIO WAVES, AND HOW ARE THEY PRODUCED AND RECEIVED?"

Have a go, below.

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Offline Soul Surfer

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Re: Question of the Week - Old Version
« Reply #312 on: 29/01/2006 19:37:59 »
Radio waves are part of the lower frequency end of the electromagnetic spectrum which includes microwaves,infra red,light ultra violet, X rays and gamma rays as well as long medium and short radio waves.

The simple way to understand them is by remembering that a changing electrical field or cuurent flowing will produce a magnetic field and a changing magnetic field will produce an electric field and this can go on even through a vacuum with no other physical structures involved.  they travel as waves at the velocity of light in a vacuum but in solids liquids and gases that are non conducting (they travel a bit slower.  conducting surfaces short out the induced currents and cause the waves to be reflected.  Light travels at 300,000 Km/sec so a low frequency radio wave with about 300 Khz is about 1km long.  (remember for any wave motion frequency times wavelenth = the velocity of the wave) The most familiar radio waves are the medium frequency waves at about 1Mhz (wavelength abouut 300 meteres) and Ultra high frequency television waves at about 500MHz  (wavelength 3/5 metre or 60cm.

Now as these waves propagate through space the cause voltages to appear across and currents to flow between  beteween points seperated by half a wavelength and a good way of detecting and launcing them is to have electrical conductors in pairs a quarter of a wavelength long to detect or launch the waves  (a half wave dipole) The process of detecting or launcing these waves is essentially symmetrical in that good structures to detect them are good structures to launch them.

To launch them you just need to generate an alternating current at the required frequency and apply it to the half wave dipole.  At very low frequencies it is possible to do this with an electrical generator but most RF generators require the use of an electromagnetic tuned cucuit coupled to an amplifier to create an oscillator.

An electromagnetic tuned ciruit consists of a capacitor that will store electrical charge when a voltage is applied (a bit like a battery) an inductor is a bit like an electromagnet which generates a magnetic field when a current flows through it.  If you connect them together in series or in parallel and the  the charge in the capacitor chn discharge through the coil and create a magnetic field and when the charge runs out the collapsing magnetic field can recharge the capacitor so they resonate a bit tlike a weight dangling from a spring after you pull it down.

coupling a resonant circuit to an amplifier using an electrobic device like a transisto alolows you to amlify any signals induced by radio waves in your half wave dipole or alternatively if you connect the output of the amplifier back to its input through the tuned circuit so that the sigtnal is fed back in phase with the  input it will osscillate at the resonant frequency of the circuit.

Now all this is going on at millions of cycles per second and you want to see something tthat you can understand to kniow that you have detected some radio waves so you need to turn the AC signal into DC which you can detect usinf a meter.  To do this you put a diode which is a device that allows electrical current to flow one way but not the other and thus gives you a signal when some radio waves are detected

In its simplest form you can transmit and detect you radio waves by switcing your transmitter on and off and detecting the level chage using your diode receiver but you can do it much faster than this using electronics.  Sensdin amplitude modulated audio sinmals or coded amplidude modulated signals to create pictures when they are arranged in lines to form television.

Unfortunately amplitude modulated signals are very subject to interference because any other signals at a similar frequency will mess them up so techniques to frequency or even phase modulate the signals are now more standard.

Learn, create, test and tell
evolution rules in all things
God says so!
Learn, create, test and tell
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Offline ukmicky

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Re: Question of the Week - Old Version
« Reply #313 on: 29/01/2006 21:10:49 »
Hi Ian or anyone

How wide is the gamma frequency part of the EM spectrum,does it go on for ever .
I know its the shortest wavelenght but could there be undiscovered frequencies beyond gamma with properties we cant detect yet and therefore if found would be clasified as something diferent to GAMMA[:)]

Michael                 HAPPY NEW YEAR                    
« Last Edit: 29/01/2006 21:33:35 by ukmicky »

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Offline Soul Surfer

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Re: Question of the Week - Old Version
« Reply #314 on: 29/01/2006 23:34:23 »
In general gamma rays go right up to the limit which is when the photon gets so energetic it turns into a black hole!

Not quite sure where the limit between xrays and gamma rays is  probably somewhere around 1Mev.

Just checked one rather old reference and it suggesed that X-rays and gamma rays overlapped a bit and that X-rays were produced by violently stopping electrons as in an X-ray tube but gamma rays were photons originating from nuclear reactions ie coming from an excited nucleus.

Another more modern reference suggested gamma rays tarted at wavelengths less than 0.1nm  and frequencies in excess of 10^18 Hz

There is really not much need for any distinction above the threshold for gamma rays because except for the fact that the photons get more and more energetic their general properties remain much the same and there are no gaps in knowledge right up to the limit of the highes energy cosmic rays.

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« Last Edit: 29/01/2006 23:48:19 by Soul Surfer »
Learn, create, test and tell
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Offline ukmicky

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Re: Question of the Week - Old Version
« Reply #315 on: 30/01/2006 01:23:12 »
Cheers ian. It was something that i've always wondered.

I couldnt see any reason why the EM spectrum couldnt go on forever and into wavelenths  which we couldnt detect as yet

Michael                 HAPPY NEW YEAR                    
« Last Edit: 30/01/2006 01:24:12 by ukmicky »

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Offline ScooterTrash

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Re: Question of the Week - Old Version
« Reply #316 on: 05/02/2006 03:37:01 »
quote:
Originally posted by NakedScientist

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.

 

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Offline ScooterTrash

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Re: Question of the Week - Old Version
« Reply #317 on: 05/02/2006 04:15:33 »
Whoops!
Sorry for the double-post! I'm new...
(It sucks havin a small brain)
Anyhow, I found this post, an was hoping someone here could shed some light on what might be sheddin light on the phosphourescent buttons of my remote control.
There was a storm here last night and the power went out. I was in the dark for several hours and noticed that the glow-in-the-dark buttons were flashing. The flashing was somewhat random, and continued all night. The plastic glow-in-the-dark stars on my bedroom celing were also brightly flashing. Normally the photon energy has been released from the phosphor after a few minutes, but this went on for a couple hours.
There was a distant lightning storm and I was wondering if that might have been the source of the radiation?
What frequencies of light are absorbed by this stuff?
I know that radium will cause this phosphor to glow, and I was wondering if possibly Radon or some other element may have been the source.
I thought that since I live near the water that perhaps a marine or aircraft radar might have been the source so I put one of the glow-in-the-dark stars in my microwave for a few seconds. (thinking that my microwave was close to the same frequency as Radar.)...
No significant results.
Next I used an infra-red LED; It DID in fact light the star.
Could ionizing radiation frequencies cause the phosphor to blink?
I'm interested in this because I live very close to a nuclear submarine base.
Thanks in advance.


quote:
Originally posted by NakedScientist

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.

 

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Online chris

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Re: Question of the Week - Old Version
« Reply #318 on: 07/02/2006 17:17:45 »
There's not much I can add to the highly comprehensive answer already supplied for "What are radio waves etc" by soul surfer. Thank you for saving me a job!

So here's this week's QOTW:

"IF A PERSON SWINGS BACK AND FORTH ON A ROPE, AT WHAT POINT IN THEIR TRAVEL IS THE ROPE SUPPORTING THEM MOST LIKELY TO SNAP?"

Have a go, below...

Chris

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Offline ukmicky

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Re: Question of the Week - Old Version
« Reply #319 on: 07/02/2006 19:37:36 »
At the bottom of the arc i would have thought.

Michael

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Offline neilep

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Re: Question of the Week - Old Version
« Reply #320 on: 07/02/2006 19:52:47 »
Yes I agree with the maestro because I hear he's a swinger and a part time Tarzan !!....at the bottom of the arc....and if it's not...then it should be !!...

HANG ON !!....just before I press ' submit ' I may feel inclined to change my mind and say it's when the rope is half way between the bottom of the swing and the end of the swing because at that point the person will want to continue going on in a straight line .....and I'm just picturing in my head where I have seen people fall of swinging ropes !!...it's usually half way up the arc..because of Momentum...or is it Inertia ?...it's one of those two...Yes..Momentum will try to yank the person off the rope and snap it at that point....I think.  Say the bottom of the swing is 6 'o' clock and the end of the swing is 3 and 9 'o' clock then the rope will break between 7 and 8 or between 4 and 5.

Men are the same as women.... just inside out !!
« Last Edit: 07/02/2006 19:57:08 by neilep »
Men are the same as women, just inside out !

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Offline ukmicky

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Re: Question of the Week - Old Version
« Reply #321 on: 07/02/2006 20:01:25 »
No its at the bottom point where the downward motion of the swing suddenly changes to upwards movement creating positive g'forces

Michael

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Offline neilep

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Re: Question of the Week - Old Version
« Reply #322 on: 07/02/2006 21:14:52 »
quote:
Originally posted by ukmicky

No its at the bottom point where the downward motion of the swing suddenly changes to upwards movement creating positive g'forces

Michael




[:D]OK OK Keep your hair on !![:D]...actually I'm about to get my hair shaver and go down Bald Avenue again  !!..

HMmm it seems the rope fairies have misinformed me then....well, i'm guessing anyway but you sound so sure..so..i'm convinced too .....[:D]

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Offline ukmicky

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Re: Question of the Week - Old Version
« Reply #323 on: 07/02/2006 21:19:18 »
I could be wrong, it has been known

Michael

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Offline DoctorBeaver

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Re: Question of the Week - Old Version
« Reply #324 on: 12/02/2006 16:46:19 »
Most likely at the point where I get out my machete because my friend is filming it for "You've Been Framed" & I want the £100 or however much it is these days. [:D]
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Offline ukmicky

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Re: Question of the Week - Old Version
« Reply #325 on: 12/02/2006 16:53:54 »
Doc your back, nice to see

Michael

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Re: Question of the Week - Old Version
« Reply #326 on: 13/02/2006 20:04:08 »
i have broken many many tarzan swings and 90 percent of the time the rope breaks not at the top of the ark but just as you start to swing back some thing do do with the change in centrifugal force perhaps?

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Re: Question of the Week - Old Version
« Reply #327 on: 27/02/2006 17:15:48 »
Ok, this was a hard one to explain, but here's the solution to:

"AT WHAT POINT IN ITS ARC IS A ROPE SWING MOST LIKELY TO BREAK?"

The simple answer is at bottom dead centre. At this point the kinetic energy of the swing and passenger is at its greatest, and the effect of gravity is acting straight down through the rider on all of his weight. So the swing is most likely to break at the mid point of its travel.

Now for the more complicated proof of concept.

If we consider a swing (and rider) which together have a mass of m suspended on a cord of length L (with negligible mass), swinging through an angle of x degrees (where 0 degrees is the horizontal).

Lets assume that the swing begins held out at 90 degrees i.e. horizontal (admittedly a scary ride!). Starting from this point, the speed of the swing at any point in its subsequent arc of travel will be such that the kinetic energy (0.5 * m * velocity(v) ^ 2) is equal to the change in potential energy from its starting point. The change in potential energy will be:

the mass of the swing and rider(m) * gravity * L * sin(x)

So 0.5 * m * v^2 = m * g * L * sin(x)

hence v^2= m * g * L * sin(x)/0.5 * m

hence v^2= 2 * g * L * sin(x)

Now the pull exerted by the swing on the rope is given by the formula speed squared / radius of arc, or v^2 / L (length of swing rope).

So the pull here is v^2 / L which equals 2 * g * sin(x).

The contribution due to gravity at any point along the travel will be g * sin(x).

Since F (tension in the string)= m * a

then the tension (T) must be m * ((2 * g * sin(x) + g * sin(x))

which equals 3mg sin(x).

So, at any point the tension in the rope supporting the swing will be 3 * mass of swing and rider * sin(angle travelled).

If we substitute into this formula, sin(0) - horizontal - is 0, so at thte start point, when the speed is zero, there is no tension on the rope.

One third of the way to the bottom dead centre (30 degrees), sin(30) = 0.5 so the tension is 3mg * 0.5

Two thirds of the way to the bottom (60 degrees), sin (60) = 0.666667 so the tension is 3mg * 0.666667

And at the bottom (90 degrees), sin (90) = 1 so the tension is 3mg. This is the maximum. So the rope has to be capable of holding three times the rider's weight.

I hope that helps to lay that one to rest.

Chris
« Last Edit: 27/02/2006 17:17:48 by chris »
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Re: Question of the Week - Old Version
« Reply #328 on: 27/02/2006 17:18:38 »
Okay, here's this week's QOTW:

"WHAT IS A HOLOGRAM, AND HOW ARE THEY MADE?"

Have a go, below.

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Offline neilep

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Re: Question of the Week - Old Version
« Reply #329 on: 27/02/2006 17:54:04 »
It's a telegram with no substance whatsoever [:)]



ps: Well done Michael..I reckon you got the ropey question right...with no strings attached of course !! [:)]

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Re: Question of the Week - Old Version
« Reply #330 on: 28/02/2006 20:24:38 »
Neil - that was almost bad enough to be mistaken for 1 of mine!

On a serious note, a friend of mine set himself up as a Hollergram. Instead of singing the message to people, he shouted it at them!
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Re: Question of the Week - Old Version
« Reply #331 on: 01/03/2006 11:24:49 »
Eth, Mine don't even come close...[:)]

Hollergram .....agghhhhh !!..see ?

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

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Offline DoctorBeaver

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Re: Question of the Week - Old Version
« Reply #332 on: 01/03/2006 14:11:42 »
Or what about a Holagram who just says "hello" to you in Spanish?
Fledgling science site at http://www.sciencefile.org/SF/content/view/54/98/ needs members and original articles. If you can help, please join.

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Offline Ray hinton

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Re: Question of the Week - Old Version
« Reply #333 on: 01/03/2006 15:58:57 »
or a HALOGRAM,just does it while you tend your flocks by night [:0]

its the drugs,y-know.
its the drugs,y-know.

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Offline DoctorBeaver

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Re: Question of the Week - Old Version
« Reply #334 on: 01/03/2006 16:36:07 »
That's silly
Fledgling science site at http://www.sciencefile.org/SF/content/view/54/98/ needs members and original articles. If you can help, please join.

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Offline ukmicky

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Re: Question of the Week - Old Version
« Reply #335 on: 02/03/2006 00:39:53 »
quote:
originally by ukmicky
 At the bottom of the arc i would have thought

 
quote:
The simple answer is at bottom dead centre.


Chris

HMMM its not often i get things right but occasionally i astound myself and others and produce the goods.

Where's my congratulations, where's my prize. i want my goody bag [:D]

Michael
« Last Edit: 02/03/2006 00:40:53 by ukmicky »

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Offline clouded.perception

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Re: Question of the Week - Old Version
« Reply #336 on: 08/03/2006 06:59:13 »
Charmeleons have different coloured cells in their skin. To change colour, they 'open up' the cells with the correct pigment, thus showing that colour (or combination of colours) on their skin.

I can picture in my mind a world without hate, a world without war.
And I can picture us attacking that world, because they'd never expect it.
I can picture in my mind a world without hate, a world without war.
And I can picture us attacking that world, because they'd never expect it.

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Online chris

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Re: Question of the Week - Old Version
« Reply #337 on: 08/03/2006 08:52:28 »
That's very nice, but this week's question is about HOLOGRAMS!

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx
I never forget a face, but in your case I'll make an exception - Groucho Marx

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Offline JimBob

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Re: Question of the Week - Old Version
« Reply #338 on: 08/03/2006 18:09:40 »
Split a laser beam, record the interference pattern made when this beam shines on an object from two different directions. (It's doned with smoke and mirrors.) Project the record to make the same interferance pattern and shine coherent light on it. Presto, a 3D Hologram.

Perhaps this was ignored because it is so easy. These thngs have been around since the 50's. (I wasn't around then - I was a Science Attache at the Klingon Embassy in Star Cluster 4289-F.)  

Jim
The mind is like a parachute. It works best when open.  -- A. Einstein

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Offline ampwelder

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Re: Question of the Week - Old Version
« Reply #339 on: 08/03/2006 19:13:27 »
Hologram come from the greek words "whole" (holo) and writing (gram).
  A lazer is used to photograph an object (instead of incoharent-normal- light) it does this in such a way that enough information can be recorded to give the imprestion of a 3-D object. This is an illusion as the holograph is actually only 2-D like a regular picture.
 

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Offline NakedScientist

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Re: Question of the Week - Old Version
« Reply #340 on: 24/03/2006 08:15:51 »
ANSWER TO LAST WEEK'S QOTW:

"WHAT IS A HOLOGRAM, AND HOW ARE THEY MADE?"

Holograms are made using lasers. You need a laser because the light that is produced is "coherent". That is, all of the light waves in the beam are synchronised, and of the same amplitude (size), so they diffract (bend) identically. You can demonstrate this by shining a laser at a prism. Compared with white light, which splits up into its composite wavelengths because each bends by a different amount upon entering the prism, when a laser is fired at a prism just a single band of light is produced.

To make a hologram a laser beam is fired at a beam splitter. This sends part of the light beam, termed the reference beam, to a piece of photographic film. The other part of the light beam, let's call it the image beam, is directed at, and subsequently reflected off, the object which you intend to turn into a hologram. It, too, then shines onto the same piece of photographic film as the reference beam mentioned above.

When the reference beam and the image beam meet at the photographic plate they have travelled different distances and their light waves no longer line-up with each other precisely. As a result they "interfere" with each other with some waves adding together constructively to make brighter patches and other parts of the wave behaving destructively and cancelling each other out to make darker areas. This is called an intereference pattern, and it's what is subsequently recorded onto the photographic film. This is the hologram, the 3D representation of the original object.

TNS


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Offline NakedScientist

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Re: Question of the Week - Old Version
« Reply #341 on: 24/03/2006 08:17:17 »
Ok, here's this week's QOTW:

"HOW DO STINGING NETTLES WORK ?"

TNS
« Last Edit: 24/03/2006 08:39:32 by NakedScientist »

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Offline wim

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Re: Question of the Week - Old Version
« Reply #342 on: 12/04/2006 19:31:30 »
Stinging Nettles ,each of it’s leaves are about 10 cm long, roughly heart-shaped and have large teeth around the leaf edge. They also have tiny hollow hairs on the main stem, leaf stems and on veins on both upper and lower sides of the leaves.
When a human brushes by the plant and it touches their skin, the tiny hollow hairs break off and release formic acid, histamine, acetylcholine, serotonin (5-hydroxytryptamine), plus some unknown compounds. These irritate the skin and cause white itchy spots to appear.

grtz
 

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Offline daveshorts

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Re: Question of the Week - Old Version
« Reply #343 on: 12/04/2006 20:06:13 »
A hologram is a pattern that when light is reflected (or transmitted depending on the hologram) from it, the light reflecting from the parts of the pattern interfere with each other to produce light leaving as if it came from a 3D object.

Doing this by drawing the pattern from the start is immensely difficult ( although a company in Cambridge is developing a projector that works on a similar idea http://www.lightblueoptics.com/ ) however if you shine light reflected from an object and light coming directly from a laser the two light beams will interfere producing a pattern. Luckly this is the pattern you need to make a hologram.

If you used non-coherent light (light where you can't predict whether it is going to be a peak or a trough from one moment to the next) it would produce an interference pattern that was not stable for long enough to take the photo, so you would actually get an averaging of lots of different interference patterns which won't produce a hologram.

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Online chris

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Re: Question of the Week - Old Version
« Reply #344 on: 28/04/2006 11:57:27 »
Here's this week's QOTW:

"HOW DOES A SOLAR CELL TURN SUNLIGHT INTO ELECTRICITY?"

Have a go, below...

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx
I never forget a face, but in your case I'll make an exception - Groucho Marx

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Offline Hadrian

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Re: Question of the Week - Old Version
« Reply #345 on: 28/04/2006 12:06:05 »

The Structure of a Solar Cell
Over 95% of all the solar cells produced worldwide are composed of the semiconductor material, silicon. As the second most common element in the earth’s crust, silicon has the advantage of being available in large quantities. Furthermore while the material is been processed, it does not have an effect on the environment. [http://www.solarserver.de/wissen/photovoltaik-e.html] Another reason for the use of silicon for solar cells is that the energy needed to ionize silicon electrons matches well with the energy of photons coming from the sun. If the photons had less energy (if the solar spectrum were more red), there would not be enough energy to free the electrons, and if the photons had more energy (if the solar spectrum were more blue or ultraviolet), then all the energy above what is needed to break the electrons free would be lost as heat. [http://www.astropower.com/how_solar_cells_work.htm]

To produce a solar cell, the semiconductor (silicon) is “doped” or contaminated. “Doping” is the intended introduction of chemical elements, which can obtain excess positive charge carriers (p-conducting semiconductor layer) or negative charge carriers (n-conducting semiconductor layer) from the semiconductor material. If two differently contaminated semiconductor layers are combined, a p-n-junction results.

At this junction, an interior electric field is built up, which leads to the separation of the charge carriers that are released by light. Through metal contact, an electric charge can be tapped. If the outer circuit is closed, meaning a user is connected, then direct current flows. A transparent anti-reflection film protects the cell and decreases the reflective loss on the cell’s surface. [http://www.solarserver.de/wissen/photovoltaik-e.html]
 



What you do speaks so loudly that I cannot hear what you say.

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Offline razorbill

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Re: Question of the Week - Old Version
« Reply #346 on: 14/05/2006 23:19:25 »
I have'nt progressed that far yet...I still get Duckbumps!
 

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Offline Cut Chemist

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Re: Question of the Week - Old Version
« Reply #347 on: 17/05/2006 05:40:49 »
How fast is warp speed??
 

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Offline JimBob

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Re: Question of the Week - Old Version
« Reply #348 on: 21/05/2006 03:38:22 »
Only Scotty and Sulu (and Spock) know. It isn't scientific, it is scince fiction, believed to be fasteer than the speed of light.


The mind is like a parachute. It works best when open.  -- A. Einstein
The mind is like a parachute. It works best when open.  -- A. Einstein

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Online chris

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Re: Question of the Week - Old Version
« Reply #349 on: 16/06/2006 21:56:58 »
Now here's a question that I think you'll have fun with:

"Why does a mirror reverse things in the horizontal, but not the vertical axis?"

Answers below please...

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx
I never forget a face, but in your case I'll make an exception - Groucho Marx