The Naked Scientists

The Naked Scientists Forum

Author Topic: Question of the Week - Old Version  (Read 178901 times)

Offline NakedScientist

  • Moderator
  • Sr. Member
  • *****
  • Posts: 355
    • View Profile
    • http://www.thenakedscientists.com
Re: Question of the Week - Old Version
« Reply #225 on: 23/11/2004 10:26:08 »
There's not much I can add to gsmollin's excellent synopsis regarding planetary spin.

So here's this week's QOTW :

IF THE RETINA NEEDS LIGHT TO 'SEE', HOW DOES IT SEE THINGS THAT ARE 'BLACK' ?

Fire away...

TNS
« Last Edit: 23/11/2004 10:27:39 by NakedScientist »
 

Offline neilep

  • Withdrawnmist
  • Naked Science Forum GOD!
  • *******
  • Posts: 20602
  • Thanked: 8 times
    • View Profile
Re: Question of the Week - Old Version
« Reply #226 on: 23/11/2004 11:27:18 »
Last time I looked I discovered that I'm not an Optician or eye doctor, but would the lack of light in itself be a way for the brain to construct the object that is not reflecting the light ?, if light is being received from everything but the object which is black then the the 'gap' itself is the construct ....even items which are black do reflect something, enough for the brain to construct outlines, edges, impressions, contours etc etc

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

Offline DrN

  • Hero Member
  • *****
  • Posts: 815
    • View Profile
Re: Question of the Week - Old Version
« Reply #227 on: 23/11/2004 20:42:10 »
I agree with Neil. Black absorbs light, rather than reflecting it, so it must be an absence of light. hence 'darkness' at night being perceived by our eyes as black.
 

Offline NakedScientist

  • Moderator
  • Sr. Member
  • *****
  • Posts: 355
    • View Profile
    • http://www.thenakedscientists.com
Re: Question of the Week - Old Version
« Reply #228 on: 29/11/2004 10:46:33 »
ANSWER TO LAST WEEK'S QOTW :

"IF THE RETINA NEEDS LIGHT TO 'SEE', HOW DOES IT SEE THINGS THAT ARE 'BLACK' ?"

The answer to this question lies within the retina itself. The retina consists of a sheet of cells, several layers thick, at the back of the eye. The top layer contains the photoreceptors or 'rods and cones', which contain light-sensitive pigment called rhodopsin comprising a protein - opsin - fused to retinal (the light sensitive part of the molecle) which is derived from vitamin A.

There are about 110 million rods in each eye (which see in black and white) and about 6.5 million cones (which see in colour - 62% are red cones, 32% are green, and 2% are blue). These photoreceptors connect to 'bipolar cells' which then link up with retinal ganglion cells. The retinal ganglion cells are responsible for transmitting the image to the brain. Here's a retinal schematic :



Intuitively one would think that when light shines on a photoreceptor it activates it, but in fact the opposite is true.  Light shining on a photoreceptor actually switches it OFF. When a photon of light hits a rod or cone it causes the rhodopsin to dissociate into its retinal and opsin components. This leads to the production of a chemical messenger which turns off the flow of sodium ions into the cell, hyperpolarising it, and hence making it less active.

The increased activity seen in the absence of light is referred to as the 'dark current' and, paradoxically, the retina is far more active in the dark than it is in the light.

So you do actually actively 'see' black, because the lack of light hitting photoreceptors makes them much more active.

Here's an overview of retinal physiology :
http://sky.bsd.uchicago.edu/lcy_ref/synap/retina.html

TNS
« Last Edit: 29/11/2004 10:52:18 by NakedScientist »
 

Offline NakedScientist

  • Moderator
  • Sr. Member
  • *****
  • Posts: 355
    • View Profile
    • http://www.thenakedscientists.com
Re: Question of the Week - Old Version
« Reply #229 on: 29/11/2004 10:51:26 »
HERE'S THIS WEEK'S QOTW

"HOW DOES WINDSCREEN DE-ICER SPRAY WORK ?"

TNS
 

Offline DrPhil

  • Full Member
  • ***
  • Posts: 91
    • View Profile
Re: Question of the Week - Old Version
« Reply #230 on: 29/11/2004 12:01:21 »
I have tried several different brands and found that they don't work very well. :)
 

Offline chris

  • Neilep Level Member
  • ******
  • Posts: 5335
  • Thanked: 65 times
  • The Naked Scientist
    • View Profile
    • The Naked Scientists
Re: Question of the Week - Old Version
« Reply #231 on: 29/11/2004 13:13:32 »
It worked okay for me the other day !

Sorry to hear your's works less well, but how does it work in theory ?

Chris

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

Offline chris

  • Neilep Level Member
  • ******
  • Posts: 5335
  • Thanked: 65 times
  • The Naked Scientist
    • View Profile
    • The Naked Scientists
Re: Question of the Week - Old Version
« Reply #232 on: 29/11/2004 13:17:02 »
It worked okay for me the other day !

Sorry to hear your's works less well, but how does it work in theory ?

Chris

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

Offline DrPhil

  • Full Member
  • ***
  • Posts: 91
    • View Profile
Re: Question of the Week - Old Version
« Reply #233 on: 29/11/2004 15:22:13 »
Works well in theory but not in practice.
 

Offline DrN

  • Hero Member
  • *****
  • Posts: 815
    • View Profile
Re: Question of the Week - Old Version
« Reply #234 on: 29/11/2004 21:38:12 »
is it like adding salt to water reduces the temperature at which it freezes, by interfering with the hydrogen bonding? so de-icer would presumably do the same. I remember that the salt experiment reduced the freezing point by 10 C (probably added to saturation), so based on this theory, whether it worked or not would depend on how cold it was! using salt, it would mean the temp would have to be below -10 C before it didn't work. presumably the solvents in de-icer take the temp threshold further.
 

Offline DrPhil

  • Full Member
  • ***
  • Posts: 91
    • View Profile
Re: Question of the Week - Old Version
« Reply #235 on: 30/11/2004 15:29:16 »
My guess??... Depression of the freezing point is due to a lowering of the concentration of water molecules. As the deicing chemical dissolves in a little water, the particles are randomly distributed amongst the water molecules.  The particles simply get in the way of the water molecules when they attempt to form the highly ordered pattern of the solid phase.  Hence a lower temperature is required to get the solution to freeze.
 

Offline DrN

  • Hero Member
  • *****
  • Posts: 815
    • View Profile
Re: Question of the Week - Old Version
« Reply #236 on: 01/12/2004 09:33:00 »
yes, thats a more coherent way of putting it! I looked at my can of de-icer and its essentially propan-2-ol, and it works up to -15 C. the only thing is - how does the propanol dilute frozen, solid, water? it must break the bonds somehow. chemistry was never my strong point.
 

Offline DrPhil

  • Full Member
  • ***
  • Posts: 91
    • View Profile
Re: Question of the Week - Old Version
« Reply #237 on: 01/12/2004 11:08:09 »
I know that salts have an exothermic heat of solution (or is it heat of hydration; I'm not a chemist either) that assists in melting the ice. But that doesn't explain how the non salt-based products work.

I also know that aircraft deicers are usually applied hot. The hot liquid melts the ice and then the freezing point depression properties of the solution prevent re-freezing. But that doesn't explain how the deicers that we may use on our cars might work. If you're like me you store the stuff in the trunk of your car and it probably starts out at the same temperature as the ice it's trying to melt.

Then there are the glycols which have a couple of “-OH” (hydroxyl) groups that can break up the hydrogen bond in water. I suppose that this could break apart the ice crystals.
« Last Edit: 01/12/2004 13:05:46 by DrPhil »
 

Offline NakedScientist

  • Moderator
  • Sr. Member
  • *****
  • Posts: 355
    • View Profile
    • http://www.thenakedscientists.com
Re: Question of the Week - Old Version
« Reply #238 on: 07/12/2004 11:27:19 »
ANSWER TO LAST WEEK'S QOTW :

"HOW DOES WINDSCREEN DE-ICER SPRAY WORK ?"

You're almost there. De-icer is indeed iso-propanol, an alcohol not greatly dissimilar to the alcohol in a bottle of gin or whisky.

Alcohols dissolve because they have a hydroxyl (OH) group attached to the molecule. This chemical group is very polar (pardon the pun in the context of de-icer). The oxygen atom loves electrons and pulls them towards itself very strongly, including the electron from the hydrogen that is bonded to it.

This makes the hydrogen a little bit positive, and the oxygen a little bit negative and enables it to take part in a process called hydrogen bonding. This is what makes water 'sticky' and why you can bend a stream of water with the static electricity on a comb, because all of the molecules cling together by the positive hydrogens attracting the negative oxygens on adjacent molecules.

So the alcohol can dissolve in water because it can take part in the same hydrogen bonding process. But because the alcohol molecule is much bigger than a water molecule, and a funny shape, it prevents the water molecules lining up so easily to get close enough together to form a solid crystal i.e. freeze. To do that you need to make the conditions much much colder. This is how antifreeze works. A big ethylene glycol molecule links up with lots of water molecules (and dissolves quite happily), but because it is a funny shape it prevents big regular ice crystals from forming, so even if you car radiator does begin to freeze the best it can do is form slush which won't burst your pipes.

So how does the de-icer melt the ice to start with? Well, the alcohol in the tin is concentrated. When you add it to the ice on the window, the diluting effect of the ice and concentrated alcohol mixing produces a little bit of heat which speeds up the melting process. Then the alcohol and water mix thoroughly, the alcohol dissoves in the water, lowering its melting point and preventing re-freezing.

Occasionally you can make the ice re-freeze but this is usually on an exceptionally cold day with a particularly thick layer of ice, so the concentration of alcohol in the water remains too low.

So that's how de-icer works (in theory !)

TNS
 

Offline NakedScientist

  • Moderator
  • Sr. Member
  • *****
  • Posts: 355
    • View Profile
    • http://www.thenakedscientists.com
Re: Question of the Week - Old Version
« Reply #239 on: 07/12/2004 11:30:13 »
Here's this week's QOTW

"WHAT MAKES YOUR JOINTS 'CRACK' FROM TIME TO TIME ?"

TNS
 

Offline neilep

  • Withdrawnmist
  • Naked Science Forum GOD!
  • *******
  • Posts: 20602
  • Thanked: 8 times
    • View Profile
Re: Question of the Week - Old Version
« Reply #240 on: 07/12/2004 19:52:34 »
quote:
Originally posted by NakedScientist

Here's this week's QOTW

"WHAT MAKES YOUR JOINTS 'CRACK' FROM TIME TO TIME ?"

TNS




I heard once that it was something to do with a build up of gas that 'pops'....but knowing my history of answering these questions I'm bound to be wrong......hmmm...defeatist or what ?

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

Offline DrPhil

  • Full Member
  • ***
  • Posts: 91
    • View Profile
Re: Question of the Week - Old Version
« Reply #241 on: 07/12/2004 20:45:20 »
joint surrounded by fluid
overextending joint = decrease in pressure
decrease in pressure = cavitation
cavitation = bubbles
bubbles burst = noise
 

Offline NakedScientist

  • Moderator
  • Sr. Member
  • *****
  • Posts: 355
    • View Profile
    • http://www.thenakedscientists.com
Re: Question of the Week - Old Version
« Reply #242 on: 18/12/2004 11:48:57 »
ANSWER TO LAST WEEK'S QOTW :

"WHAT MAKES YOUR JOINTS 'CRACK' FROM TIME TO TIME ?"

Dr. Phil has the correct answer. Joints are mobile articulations between bones. The ends of the bones are covered by a slippery layer of cartilage, rather like anatomical teflon, which is lubricated by a thin liquid called synovial fluid. The joint is enclosed by membranes and supporting tissues that retain and maintain the fluid, stabilise the joint, and also help to determine the directions in which it can move.

Because the fluid is held within an enclosed space, when the joint moves in certain directions it sometimes squashes the fluid on one side of the joint, and creates a partial vaccuum in the fluid on the other side of the joint.

Just as water boils at a lower temperature at the top of a mountain because the atmospheric pressure is lower at altitude, lowering the pressure in joint fluid causes small vapour bubbles to form (from the water in the synovial fluid). When these bubbles then subsequently collapse on themselves again they do so with a 'pop', which is the sound you hear.

This process is referred to as 'cavitation', and is responsible for the 'pitting' effect you see on boat propellers and hydrofoils. When the propeller cuts the water it creates zones of low pressure which yield vapour bubbles. The collapse of these bubbles against the propeller surface releases energy which damages the blades.

Fortunately for us, it happens too infrequently to cause harm to our joints !

TNS
« Last Edit: 18/12/2004 11:52:37 by NakedScientist »
 

Offline NakedScientist

  • Moderator
  • Sr. Member
  • *****
  • Posts: 355
    • View Profile
    • http://www.thenakedscientists.com
Re: Question of the Week - Old Version
« Reply #243 on: 18/12/2004 11:53:24 »
HERE'S THIS WEEK'S QOTW :

"WHY DOES ICE FLOAT, WHEN MOST SOLIDS ARE HEAVIER THAN LIQUIDS ?"

TNS
 

Offline neilep

  • Withdrawnmist
  • Naked Science Forum GOD!
  • *******
  • Posts: 20602
  • Thanked: 8 times
    • View Profile
Re: Question of the Week - Old Version
« Reply #244 on: 18/12/2004 13:13:10 »
cos water is well weird and when it feezes it becomes less dense than liquid water... i know something strange happens at 4 degrees C, something do do with a crystal lattice and hydrogen ......and HEY !!!... I kind of got the above  (joints cracking etc)question partially right in my own way.....HMMMPTHH !!!


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

Offline gsmollin

  • Hero Member
  • *****
  • Posts: 749
    • View Profile
Re: Question of the Week - Old Version
« Reply #245 on: 21/12/2004 21:28:24 »
Yea, Neil just can't get no respect. He hits the buzzer first and credit goes to some guy with a "Dr" moniker. Maybe if he put his answer in the form of a question...
 

Offline neilep

  • Withdrawnmist
  • Naked Science Forum GOD!
  • *******
  • Posts: 20602
  • Thanked: 8 times
    • View Profile
Re: Question of the Week - Old Version
« Reply #246 on: 21/12/2004 21:44:48 »
quote:
Originally posted by gsmollin

Yea, Neil just can't get no respect. He hits the buzzer first and credit goes to some guy with a "Dr" moniker. Maybe if he put his answer in the form of a question...



Thanks for the support gsmollin !!...I feel so neglected sometimes ;)!!...but you've cheered me up...thanks.....

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

Offline tups

  • First timers
  • *
  • Posts: 2
    • View Profile
Re: Question of the Week - Old Version
« Reply #247 on: 06/01/2005 15:21:10 »
Water in its liquid form forms networks and chains of water molecules that are connected together by hydorgen bonds, a kind of chemical bond in which a hydrogen atom is "shared" between two more electronegative atoms. Water always does a balance act : on the one hand thermal motion makes the networks smaller and expands the volume a single water molecule needs, ie regular thermal expansion. However, in an ideal network, every water molecule binds to two others with its hydrogen atoms, and accepts bonds from two more water molecules, and so forms a tetrahedric structure, which is very loosely packed, has, in other words, quite large holes in it. This ideal lattice is solid crystalline ice. It is thus less dense than when the networks break down into smaller and smaller chunks. The maximum density is reached at 4 degrees, where decreased networking starts to be offset by thermal expansion.
And that's the story of water. If it didn't form such a weird crystal structure, ice wouldn't float, oceans would regularly have frozen completely during the history of earth, and we wouldn't be alive today.
Lucky us ... :-)

tups
 

Offline Donnah

  • Ma-Donnah
  • Neilep Level Member
  • ******
  • Posts: 1756
    • View Profile
Re: Question of the Week - Old Version
« Reply #248 on: 20/01/2005 04:17:01 »
quote:
Originally posted by NakedScientist

ANSWER TO LAST WEEK'S QOTW :

"WHAT MAKES YOUR JOINTS 'CRACK' FROM TIME TO TIME ?"

Dr. Phil has the correct answer. Joints are mobile articulations between bones. The ends of the bones are covered by a slippery layer of cartilage, rather like anatomical teflon, which is lubricated by a thin liquid called synovial fluid. The joint is enclosed by membranes and supporting tissues that retain and maintain the fluid, stabilise the joint, and also help to determine the directions in which it can move.

Because the fluid is held within an enclosed space, when the joint moves in certain directions it sometimes squashes the fluid on one side of the joint, and creates a partial vaccuum in the fluid on the other side of the joint.

Just as water boils at a lower temperature at the top of a mountain because the atmospheric pressure is lower at altitude, lowering the pressure in joint fluid causes small vapour bubbles to form (from the water in the synovial fluid). When these bubbles then subsequently collapse on themselves again they do so with a 'pop', which is the sound you hear.

This process is referred to as 'cavitation', and is responsible for the 'pitting' effect you see on boat propellers and hydrofoils. When the propeller cuts the water it creates zones of low pressure which yield vapour bubbles. The collapse of these bubbles against the propeller surface releases energy which damages the blades.

Fortunately for us, it happens too infrequently to cause harm to our joints !

TNS
Does that mean that someone whose joints crack every time they bend them is more likely to have joint problems later in life?

"Remember, if you ever need a helping hand, you will find one at the end of each of your arms." - Audrey Hepburn
 

Offline chris

  • Neilep Level Member
  • ******
  • Posts: 5335
  • Thanked: 65 times
  • The Naked Scientist
    • View Profile
    • The Naked Scientists
Re: Question of the Week - Old Version
« Reply #249 on: 20/01/2005 08:40:18 »
I haven't heard any evidence that cracking joints are more prone to arthritis later in life, but I'll look into that.

Chris

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

The Naked Scientists Forum

Re: Question of the Week - Old Version
« Reply #249 on: 20/01/2005 08:40:18 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums