woelen - 29-1-2008 at 13:50
This is more a physics experiment than a chemical experiment. Some of you may know the effect already, but it still remains quite interesting.
Especially when different light sources are used (different LEDs of different color), the effect is quite nice.
http://woelen.scheikunde.net/science/physics/exps/scattering...
No special and difficult to obtain chemicals are needed, only hardware store acid and sodium thiosulfate (photo fixer solution also is suitable).
microcosmicus - 29-1-2008 at 15:07
Another version of this experiment is to consider the scattering by a single
particle as opposed to a gas of scatterers. For instance, place a little bit
of fine dust in a tube of water and illuminate from the side. Then one sees
the individual particles appear bright dots. Alternatively, one can shine a
light through a piece of glass (such as a stirring rod) --- with a little looking
about, one can usually locate a microscopic bubble which will serve as a
scattering centre. Moving around, one sees the angular dependence of the
scattering.
One can improve the demonstration by using a monochromatic light source
such as a laser pointer. Then the patterns one observes become much
sharper because one is not mushing together diffraction patterns for
different wavelengths. In particular, for suitable combinations of particle
size and refractive index, one can easily see minima and maxima in the
diffraction pattern ---- measuring the exact location of these is a way of
precisely determining the size of a tiny sphere.
Finally, it is worth mentioning that the definitive treatise on this
subject was written by one of W. Oelen's compatriots:
W. C. van de Hulst
Light Scattering by Small Particles
1957, reprinted by Dover Books
Not only does this require no special chemicals, it requires no special
equipment --- most scattering experiments I can think of require particle
accelerators or other such special equipment.
[Edited on 29-1-2008 by microcosmicus]
[Edited on 29-1-2008 by microcosmicus]
12AX7 - 29-1-2008 at 15:42
Incidentially, a laser pointer gives you two things: not only is it monochromatic, it is also coherent! Diffraction experiments can give very
interesting results.
Nice example of Rayleigh scattering, Woelen. In a pinch, milk works -- although I suppose that doesn't have the same chemical background to it.
Tim
Ozone - 29-1-2008 at 18:16
Very Nice, Woelen!
As usual, you have presented an extremely nice, easily done experiment which can be used to explain some rather complicated science. If ever I need to
explain this (and I might, since I am into polymers), this is the way I will do it.
I think with the new UV diodes (or a N2 laser), we could perhaps demonstrate (with the proper, non-absorbing colloid) the scattering of yet smaller
wavelengths via fluorescence, say of a paper coated with a high yielding fluorophore placed orthogonal to the source? Particle size and shape (if
lasering) are also a factors.
The standard colloid used for nephelometry is formazin (standard, but poisonous), given here (page six):
http://www.hach.com/fmmimghach?/CODE%3AL7045550%7C1
Kaolin can also be used, and it's both cheap and safe
Nice,
O3
Quote, from post below, The_Davster:
"0.0375M cerrous nitrate solution, when mixed with an equal volume of 0.5M hexamine... "
Sweet! I'll have to try this.
[Edited on 30-1-2008 by Ozone]
The_Davster - 29-1-2008 at 21:01
I was going to post something remarkably similar, but focusing on the tyndall effect and using nanocrystals for illustration.
I guess great minds think alike
0.0375M cerrous nitrate solution, when mixed with an equal volume of 0.5M hexamine, and the solution is allowed to sit for a length of time between
100 and 600 minutes, causes growth of ceria nanocrystals. These are completly invisible to the naked eye, but when a lazer is shot through the
solution, a perfect beam can be seen in the solution, and scattering is very noticable.
-jeffB - 30-1-2008 at 06:47
Interesting! Can you do something "between 100 and 600 minutes" to quench the reaction and stabilize the nanocrystals, or do you only have that
window in which to experiment?
Google returns a lot of hits on "ceria nanocrystals". Apparently you can make them by simply refluxing cerium(III) nitrate with NaOH for a few
minutes. I wonder what other interesting things you can do with them?
YT2095 - 30-1-2008 at 09:02
Excellent experiment perfectly executed too! 
it`s nice to see those LEDs in use also, if you want, I have a 150mw near IR/Visible laser here that I can send over too, it`s not much brighter to
look at than a simple 2mw red laser pen (our eyes aren`t Linear), but I expect that it will be the Last to be extinguished when set up in the way of
those leds.
I`ll send the Goggle filters too (just tape them to your normal Lab goggles).
again, Fantastic work!