From: David Lauter (dyakov@ix.netcom.com)
Date: Tue Feb 15 2000 - 19:11:33 PST
From: "David Lauter" <dyakov@ix.netcom.com> Subject: Re: Pinhole Digest #353 - 02/15/00 Date: Tue, 15 Feb 2000 19:11:33 -0800 Message-ID: <01bf782b$8796d840$69ea6ed1@dyakov.ix.netcom.com>
OK, so why is silver so special? Why don't other ionic compounds do this
thing with light? Something else must be doing something to silver besides
the developer. But I am glad to know that it all begins when light knocks
off an electron from the halides.
By the way, my students in doing an "ion exchange" lab noticed these light
effects to be very fast with silver thiosulfate (small amount of white
precipitate quickly turned yellow to purple to black, second place went to
silver chloride (white precipitate turned purple) and third place to silver
iodide (yellow precipitate didn't change color but got clumpy). Can anyone
explain my students' observations?
Thanks
Farmer David
-----Original Message-----
From: Pinhole Listserv <pinhole@exploratorium.edu>
To: Pinhole Listserv <pinhole@exploratorium.edu>
Date: Tuesday, February 15, 2000 12:20 AM
Subject: Pinhole Digest #353 - 02/15/00
Pinhole Digest #353 - Tuesday, February 15, 2000
Photoelectron Physics
by <SFPhysics@aol.com>
Re:copy machine electrons
by <pauld@exploratorium.edu>
blue LED's
by <pauld@exploratorium.edu>
Silly quantum mechanics?
by "Deborah Hunt" <dhunt@exploratorium.edu>
Aim High summer job
by "Deborah Hunt" <dhunt@exploratorium.edu>
photoelectric effect
by "MC elover" <mcelover@yahoo.com>
re: photelectric effect
by "Heidi Black" <blackh@exchange.esuhsd.org>
----------------------------------------------------------------------
Subject: Photoelectron Physics
From: <SFPhysics@aol.com>
Date: Mon, 14 Feb 2000 05:48:49 EST
> I have a few questions on how light reduces substances. When light turns
> silver halides black or purple I understand that the silver is reduced,
but
> what is oxidized, the halide? Any other details of the chemistry of
> photography would be appreciated.
> Also when a copy machine plate is charged up by light, where are the
> electrons coming from?
> Thanks
> David Lauter
>>
Hello David:
The silver halides (chloride, bromide, and iodide) when hit with an
energetic
photon of the correct energy will cause the chemical bond between atoms to
break. It is the developer that reduces the silver ions so they may be
captured by silver sulfide which is also in the film emusion. Going from
basic to acidic, the fixer bath uses acid to wash away the remaining
unexposed silver halide crystals.
AgCl + photon ---> Ag(+1) + Cl(-1) then:
Ag(+1) + developer(-1) ----> Ag(0) reduded metalic silver/dark area of light
exposure
[Remember the definition of reducing agent, anything that will cause the
loss
of oxygen, a gain of hydrogen, or a gain of electrons.]
The photosensitive drum in the dry (xerography) copier is coated with a
selenium layer. When a photon of the correct value hits the electron cloud
of a selenium atom, a quantum jump occurs to free an electron. The surface
is not conductive so the free electron ends up as a static charge where the
photon hit. As the uncharged paper passes the drum the paper picks up the
charge and on to the next stage where electrostatic ink is drawn to the
charged areas of paper.
Al Sefl
Guaranteed to get everything 50% correct.
----------------------------------------------------------------------
Subject: Re:copy machine electrons
From: <pauld@exploratorium.edu>
Date: Mon, 14 Feb 2000 13:39:25 GMT
Hi David
The electrons sprayed onto the drum in a copy machine come from PGE
of course ;-)
They used to use rabbit fur to generate a static charge. But these days they
transform the line voltage to a high voltage, and spray electrons off of
high
voltage points onto the drum. When Light hits the drum (of selenium for
example.) the drum becomes a conductor (see Al Sefl's reply) this allows the
charge to leave the drum where light hits the drum. The toner with an
opposite
charge sticks to the drum at places that retain charge, i.e. where it was
dark.
So dark toner is deposited where there was darkness on the image of the
page.
Paul D in Sweden
I'll follow up later with the distribution of colloids with height. It is
exponential, isn't everything?
> I have a few questions on how light reduces substances. When light turns
> silver halides black or purple I understand that the silver is reduced,
but
> what is oxidized, the halide? Any other details of the chemistry of
> photography would be appreciated.
>
> Also when a copy machine plate is charged up by light, where are the
> electrons coming from?
>
> I also have a question for Paul Doherty. A few months ago he explained
the
> distribution of colloids in a gravitational field. I only remember a few
> main points. I believe the distribution depends on the weight of colloids
> and their random 'Brownian' motion . I then lost him as soon as he said
> that the mathematics was easy. Paul, could you write down the 'easy'
math?
> Thanks
> David Lauter
>
>
> --------------------------------------------------------------------------
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----------------------------------------------------------------------
Subject: blue LED's
From: <pauld@exploratorium.edu>
Date: Mon, 14 Feb 2000 13:47:18 GMT
The price of blue LED's is plummeting.
You can get them at Radio Shack for under $3.
Digi-key is much cheaper.
Check out the white LED's while you're at it. They are now $6 each and the
price
is dropping fast.
Paul D
In Sweden
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----------------------------------------------------------------------
Subject: Silly quantum mechanics?
From: "Deborah Hunt" <dhunt@exploratorium.edu>
Date: Mon, 14 Feb 2000 09:15:36 -0800 (PST)
The Well-Intended, but Not Quite Interactive,
Schrodinger's Cat
A Rather Silly Experiment in Quantum Mechanics
http://www.phobe.com/s_cat/s_cat.html
---------------------------------------------------
Deborah Hunt
Internet Resource Specialist
Exploratorium
3601 Lyon Street
San Francisco, CA 94123
Voice: 415-353-0485
Fax: 415-561-0370
email: dhunt@exploratorium.edu
----------------------------------------------------------------------
Subject: Aim High summer job
From: "Deborah Hunt" <dhunt@exploratorium.edu>
Date: Mon, 14 Feb 2000 09:30:03 -0800 (PST)
Subject: aim high - site director opening:
From: "Vivian Altmann" <viviana@exploratorium.edu>
Date: Thu, 10 Feb 2000 12:37:25 -0800
If anyone knows of folks who might need a summer job in education
- Aim High is a wonderful organization. Read on:
>Dear friends of Aim High:
>
>It looks as though we will have one, possibly two, site director openings
>for summer, 2000. This position is a co-director position for one of the
>Aim High sites in San Francisco. This is a wonderful opportunity for
>someone interested in school leadership and urban education.
>
>The position is a seven-week commitment for the summer and additional
>responsibilities during the academic year.
>
>Aim High is a tuition-free academic summer school for middle-school
>students from low-income families. We will be celebrating our 15th summer
>this summer and we will be located at four, or possibly five, sites in San
>Francisco. Aim Highh also has an extensive environmental education program
>in the Marin Headlands. For more information, please check out our website
>at: www.aimhigh.org
>
>If interested, please send a letter and resume to Alec Lee, c/o
>Lick-Wilmerding High School, 755 Ocean Avenue, San Francisco, CA 94112
>
>Many thanks,
>
>Alec Lee
>Director/Aim High
>
>Alec Lee - Director
>Aim High - San Francisco
>"Reach for a Dream"
>tel: (415) 333-5836 fax: (415) 333-9443
>e-mail: alee@lwhs.org
>c/o Lick-Wilmerding High School
>755 Ocean Avenue; San Francisco, CA 94112
>
----------------------------------------------------------------------
Subject: photoelectric effect
From: "MC elover" <mcelover@yahoo.com>
Date: Mon, 14 Feb 2000 15:17:32 -0800 (PST)
hello
i was planning on using different color LED's to help
demo the photoelectric effect/ the quantum hypothesis.
upon reflection, i don't really think my kids will
get it, unless i tell them what's going on, in which
case they'll just have to take my word for it.
does anyone have any other suggestions for
photoelectric effect demos or any other interesting
ways to talk about quanta of light, i.e. photons?
thanks
--eric
__________________________________________________
Do You Yahoo!?
Talk to your friends online with Yahoo! Messenger.
http://im.yahoo.com
----------------------------------------------------------------------
Subject: re: photelectric effect
From: "Heidi Black" <blackh@exchange.esuhsd.org>
Date: Mon, 14 Feb 2000 16:25:24 -0800
Eric,
Maybe you could work something like this into an activity....
I had a student measure the voltage output from a solar cell (one
without a diffuser on top) while shining a laser onto it. She compared
the voltage using the beam, and then using the beam after being expanded
differing amounts. Now , (I'm not as up on this as I should be) maybe
if you had a blue laser....
Heidi
----------------------------------------------------------------------
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