[ukmicky]

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[]

Michael                 HAPPY NEW YEAR                    

[DoctorBeaver]

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.

[chris]

Why is it visible on a cold day, but not on a hot day ?

Chris

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

[ariel]

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!

[DoctorBeaver]

I refer the honourable gentleman to the answer I gave some moments ago []

[ariel]

psh, if you're referring to me..im not a gentleman :-)

[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.

[DoctorBeaver]

Ariel - It was to Chris, not you

[ariel]

yay

[chris]

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.

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

[chris]

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



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

[chris]

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.

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

[Soul Surfer]

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!

[ukmicky]

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                    

[Soul Surfer]

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.

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

[ukmicky]

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                    

[ScooterTrash]

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.

[ScooterTrash]

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.

[chris]

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

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

[ukmicky]

At the bottom of the arc i would have thought.

Michael