From: SFPhysics@aol.com
Date: Fri Feb 18 2000 - 01:10:44 PST
From: SFPhysics@aol.com Message-ID: <a3.264220b.25de6694@aol.com> Date: Fri, 18 Feb 2000 04:10:44 EST Subject: Photoelectrons Part II
> 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.
Dave, you should have taken my class rather than just been in my homeroom!
;-) Other compounds abound with light created properties. Why do you think
a plastic garbage can eventually just falls apart from being out in the sun?
UV damage to the long hydrocarbon chains (plastics) makes them become brittle
and inflexible when electron bonds break. Many compounds that do not use
silver are photographic chemicals that Eastman Kodak has developed over the
last 70 years. These modern color films have little or no silver in them but
use photosensitive dyes. In the early history of photography Silver was
chosen as the easiest metal to use with the most sensitivity and desirable
characteristics (replacing the highly poisonous mercury previously used).
All metals will produce a photoelectron when hit with just the right photon
of correct energy. Silver exhibited the best properties for the chemistry of
black and white photography.
> 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
I oversimplified my explanation of silver halide photography the other day
and as Dr. Paul would point out, "But it is more complicated than that!"
Modern photographic film is an emulsion in a gel base consisting of silver
halides and silver sulfide crystals which catalyze the photographic image
from the light exposure. On a molecular level the photon hitting the silver
halide causes metallic silver to form on the surface of a sulfide crystal.
The chemical bonds in your silver thiosulfate are simply more sensitive and
tend to self catalyze once only a few bonds are broken. The silver chloride
did liberate some metallic silver, the purple coloration you noted but
without a catalyst it was a less rapid reaction. The silver iodide reaction
to light is so slow that you would not normally observe much of a change
during a short lab period.
> 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
>>
Eric here is where the teacher turns experimental Physicist:
We all know the amount of energy a photon carries is proportional to the
wavelength (lambda). Silicon photocells are not uniformly sensitive to our
visual frequencies. Electrons in silicon take a quantum jump out of their
electron clouds when hit with photons of a wavelength less than 10^-6th metre
in the "near" infrared. As the wavelength gets shorter the voltage drops off
as the UV absorbent coating on the cell takes over (protects the cell from
sunburn). Assuming that the sensitivity is not relatively flat, you
theoretically could take a bright flashlight and consecutively put a red
filter, then an orange filter, then a yellow filter, then a green filter,
then a blue filter, and come up with results indicating which wavelength of
photons the photocell was most sensitive to. Running the photocell through a
spectrum of the sunlight from a prism might be helpful in that the cell would
show energy in areas the eye could not detect. I have done this to show IR
content in sunlight and the students are amazed at this "invisible" energy.
Since we are in a Seattle rainy season in San Francisco, an overhead
projector will work just fine.
As for the light emitting diodes (LEDs), these are great for showing that you
can put in electrons and get photons out. However, I have always preferred
large neon lamps where both electrodes can easily be seen. Students are
amazed to see the orange light appearing in the space around an electrode. I
have a socket with a diode that delivers pulsating DC so that only one
electrode glows at a time. You can reverse the plug to show that when
ionizing electrons are gathered the gas gives up a photon. I have a neon
wand that will even allow the flow of electrons to be bent in a strong
magnetic field. Lastly, there are always the gas discharge tube sets that
show how different gases, each with their own electron shell configuration,
give different spectra; and, don't forget that a Bunsen burner flame test
series for metals will show different colors too. All the students need to
know at the start is that various colors in excited gases indicate how
electron shells react to energy changes. I use the analogy that a dropped
marble makes a wave in a quiet pond. It's late and I am probably
clear as mud so I'll quit here!
Al Sefl
Guaranteed to be correct 50% of the time.
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