The strength of the Tyndall "I", that is the intensity of the scattered
light to the light beam is given by the following equation:
I = kNV^2/w^4
where N is the number of particles, V is the volume of each particle,
and w is the wavelength of the light. From this we can see the following:
If a particle doubles in diameter, its volume will go up by a factor of
8, and the intensity of the Tyndall from that particle will go up by 8^2
= 64, so the Tyndall goes up by the 6th power of the diameter. However,
if you are comparing the same ppm solutions with different particle
sizes, the number of particles will go down by a factor of 8, if you
double the size. Thus the intensity of the Tyndall from two colloids
with the same ppm, but a 2:1 difference in particle size will be 8:1,
that is it will vary to the 3rd power. That is why a bright Tyndall is
usually more indicative of particle size than concentration.
Now for a typical colloid the wavelength, is more dramatic. The
intensity will vary inversely by the 4th power of the light's
wavelength. For example, if you take blue laser of 330 nm and a red one
of 660 nm, that ratio will be 2:1, and the intensity of the Tyndall from
them would be that the blue one would be 2^4 or 16 times brighter.
Now from this is appears that the ratio of light scattered from small
and large particles will have the same ratio independent of the
wavelength of the light. This is true for the general case of most sols
which are of non-conductive particles. However silver and gold are
different, being very good conductors, they have resonances, somewhat
like an antenna. As it turns out silver particles of the following
sizes will have the following maximums on the Tyndall wavelengths:
38 nm - 470 nm
47 nm - 490 nm
90 nm - 560 nm
118 nm - 600 nm
The fwhm of these runs approximately 1/2 of their peak wavelength.
If you look at
http://products.mercola.com/Images/home-tanning-beds/wavelength-chart.jpg
to see what colors the wavelengths are we find that red and amber sill
scatter much more by the 118 nm particles than the blue, and that for 38
nm particles, they will scatter blue better. From the chart I have the
ratio is rather large, 5:1 for the 38 nm between blue and amber and
almost 5:1 for the 118 between amber and blue. Thus there is a 25:1
difference between these two wavelengths and particle sizes. So it does
appear by using different colored light, and Tyndall intensity, to get a
crude handle on particle size and concentration over and above the
"color" of the sol. Only problem is that when the particle sizes get
below 30 or so nm, they both absorb and scatter UV light which cannot be
seen at all.
Marshall
Neville Munn wrote:
Now you're touching on a subject that I've been pondering for a while
but can't seem to find anything of value to read in the public domain.
You mentioned red laser becoming less apparant, but 405nm blue DOES
show up. I'm still curious to know if there are any methods of home
determination of particle size approximation using laser lights, red
or green or blue or whatever other colour which *could* indicate a
particular nm range in the colour spectrum. If light can be used to
determine approximate particle size is it possible one can make use of
differing laser colours to approximate particle size in solution?
Praps you could help me out with this:
I got three red lasers,
(1) 650nm+/-10 maximum output 1mW.......{bright beam passing thru liquid}
(2) 650nm+/-10 maximum output 5mW.......{beam of lesser brightness}
(3) 630-650nm maximum output 1mW........{no beam at all}
Is there anything you can tell me regarding the differences in beam
strength between these lasers, all shone thru the same solution?
Particularly the last one with no beam at all.
N.
------------------------------------------------------------------------
Date: Tue, 13 Apr 2010 00:44:19 -0700
From: [email protected]
Subject: Re: CS>where is everyone?/..."TAITP~WBSF"?
To: [email protected]
FYI: Using a Blue 405-nm Laser, or even Green
LD...can-detect smaller...Size!
Have gone down to 385-nm (non-laser) light source, using special TRIX
& PMT.
(Not sure Particle(s) Size PEAKING at 405 nm, but-maybe...Marshall knows?)
Tyndall will become LESS apparent (using RED), but 405~Blue
still Detecting!
As long as it's only-slightly Yellow (and don't Smell/Taste
like P*ss), it's GQQD
to use for ANY application you wanted, but for INTERNAL
use...It's UP-to user?
------------------------------------------------------------------------
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