PURPOSE: To show Rayliegh scattering with a colloidal
sunset demonstration.
DESCRIPTION: The light from a slide projector serving as a white light
source passes through an aquarium tank and hits a nearby screen. A circular metal
slide is inserted into the slide projector to produce an image shaped like the sun.
The aquarium tank is filled with 6750 g water and 135g hypo (Sodium Thiosulfate). The speed of the colloidal sunset reaction is EXTREMELY SENSITIVE to the temperature of thw water/hypo mixture. Therefore, it is important that the aquarium is filled with the water the evening before the demo is performed to insure that on the day of the demo the water has sat long enough overnite and has precisely acheived equilibrium with room temperature. If the water is slightly warmer than room temperature then the colloidal reaction will occur much too quickly, and vice versa, if the water is slightly colder than room T then the colloidal reaction will form much too slowly.
At the time of the demo add .5 ml concentrated HCl or Sulfuric acid is
carefully to the hypo/water mixture. When the chemicals mix they begin to form a
suspension of sulfur particles which act as scattering centers for the light, especially
blue light at first. This leaves the light on the screen with a yellowish tint. As time
passes the sulfur particles grow in size, becoming larger, and they scatter more light.
But, in additon to scattering more light, the larger the colloidal particles grow
the wavelength of the selectively scattered light becomes longer, changing the light on
the screen to a bright red. Ultimately most light is scattered, leaving no light on the
screen.
It takes about 5 minutes for the screen to go dark red. While that is happening you can
talk about the sky being blue, the sun red at sunset, and even why the moon takes on a red
tint when it is near the horizon and during a lunar eclipse.
The scattered light can be seen to be polarized by rotating a polaroid sheet in the light
path.
EQUIPMENT: as photographed.
SETUP NOTES:
TYNDALL'S EXPERIMENT - COLLOIDAL SUNSET
D. Tattersfield, Projects and Demonstrations in Astronomy, p.109.
Haym Kruglak, A Simplified Sunset Demonstration, TPT 11, 559, (1973).
Marla H. Moore, Blue Sky and Red Sunsets, TPT 12, 436-437, (1974).
Jay S. Huebner, Tricks of the Trade: "A Golden Oldie" - Projecting a Sunset, TPT
32, 147 (1994).
E-Qing Zhu and Se-yeun Mak, Demonstrating Colors of Sky and Sunset, TPT 32, 420-421
(1994).
Sutton, Demonstration Experiments in Physics, L-46 Scattering of Light, 387-388.
Why is the sky blue? (Part I)
By David Harris
You've asked this question before, or heard somebody else ask it. It seems like the most frequent of frequently asked
questions. But last time you heard the question, did you really get the answer?
In case you missed it and are too embarrassed to ask the question of somebody else, here is the lowdown.
The first thing to say is that the color of the sky has nothing to do with a reflection off the water as many people think! In fact,
it works the other way around; water often appears blue because it is reflecting the sky. This doesn't get us any closer to the
answer but at least it puts one myth to rest.
Just in case you don't know this yet, light from the sun is made up of a spectrum of colors that all continuously merge into
each other. The human eye can't see the infinity of distinctions between different colors and, to our eyes, sunlight is made up
of roughly seven colors: red, orange, yellow, green, blue, indigo and violet. The difference between the different colors, on a
physical level as opposed to a perceptual one, is that different colors are light of different wavelengths and frequencies. The
blue end of the spectrum has the highest frequency and the red end has the lowest frequency.
Although light seems to travel straight through air without any interruption, the air molecules as well as any contaminants such
as water droplets or smog impurities "scatter" light. That means they deflect the light so that it changes direction. However,
different colors are more affected by scattering than others. Blue light is scattered most and red light is scattered least.
Another point to remember is that we only see light because it hits our eyes. This may seem obvious but it is easy to forget
when we think about the exact details of how light travels and how we observe things.
We now have the basic ideas we need to combine to explain why the sky is blue. If there were no atmosphere, the sky
would seem dark like in space because the only light coming into your our eyes would be that coming in a straight line from
stars. But on Earth, the atmosphere is scattering light all the time. This means some light that normally would have missed our
eyes is actually deflected straight towards us. Because blue light is scattered more, the light coming into our eyes after
deflection is more likely to have been blue light rather than red light.
So when we look into the sky but not directly towards the sun, there light coming into our eyes appears blue and is actually
the blue component of light that has come from the sun.
Now you may ask a trickier question: why is the sky red at sunset?. It turns out that the answer uses the same basic physics
but the situation is slightly different. You maybe able to work it out yourself or else you can wait until next week for the
second part of the story.
By David Harris
Last week we talked about why the sky is blue in the daytime. The second most frequently asked question, following close
behind the first, is "If the sky is blue, why are sunsets red?"
The answer comes from exactly the same physics just applied to a slightly different scenario. Remember that blue light scatters
more than other colors of light. This means that during the day, the indirect light coming to your eyes is more likely to be blue
than any other color.
At sunset, the situation is slightly different. Most of the light you notice on the horizon around the setting sun is coming toward
you fairly directly. In this case the blue light has actually been scattered away from you, leaving just the reds, oranges and
yellows. If you are observant you would have noticed that even though the sunset is red, the sky above you generally has a
blue tinge to it.
So the color of the sky really depends on how close to a straight path the light has to travel from the sun to your eyes. The
more direct, the redder the light, the longer the path, the bluer the light.
Just in case you are not convinced by this explanation, or if you are wondering how to do an experiment based on this idea,
here is something to try. You can simulate the sky with all the molecules acting as scattering particles on a much smaller scale.
The amount of scattering that causes the sky to take on a color only occurs because there is so much atmosphere for the light
to pass through. To do a table-top experiment, you need to increase the amount of scattering. The best way to do this is to use
a large container of water made of a transparent material such as a plastic or glass. Try to find the longest container you can.
Depth and width are not so important.
Add a few drops of milk to the water and mix it in properly. Be careful not to add too much milk. You really only need a small
amount. For the largest containers even a few teaspoons will be too much. You can always add more milk later but it's pretty
hard to remove it once it's there!
Now you just have to take a torch and shine it in one end of the container. Have a look at the diagram below. Now all you
have to do is look at the container from different angles. You should notice that the color of the water appears to change from
blue to red along the length of the container. This is because the blue light is the first to be scattered and then toward the end of
the container, most of it is gone, leaving just the red. Try doing the experiment in a darkened room if you have trouble seeing
the colors. I couldn't believe how well this worked when I first tried it!
If you look in the opposite end of the container toward the torch, you should see the torch appearing much the same color as
the sun during the day. The color you see around it depends a lot on exactly how much milk you have added but it can range
from a red sunset to a fairly blue sky color.
Read back over last week's article as well as this weeks and see how the ideas all combine together to explain what you can
see. There are a lot of different effects you can see in the skies and a lot of them have to do with this idea of light scattering.
Keep an eye out for more and you'll soon discover you can begin to explain them yourself.
Light Scattering
Background
The phenomenon of light scattering is encountered widely in everyday life. For example light scattering by particles in the atmosphere gives rise to the blue color of the sky and the
spectacular colours that can sometimes be seen at sunrise and sunset. These are all examples of static light scattering since the time-averaged intensity of scattered light is observed.
In general, interaction of electromagnetic radiation with a molecule leads either to absorption (forms the basis of spectroscopy) or scattering the radiation. Scattering results from the
interaction of the electrons in the molecules with the oscillating electric field of riadiation. Thus a dipole is induced in the molecules which oscillates with the electric field. Since an
oscillating dipole is a source of electromagnetic radiation, the molecules emit light, the scattered light. Almost all of the scattered light has the same wavelength as the incident radiation
and comes from elastic (or Rayleigh) scattering.
Experimental Setup
Our light scattering hardware setup consists of a commercial equipment for simultaneous static and dynamic experiments by ALV-Laservertriebsgesellschaft (Langen, Germany). We use
the blue line (488 nm) of a Coherent 70/2 Innova Ar ion laser at a power output of 0,1-200 mW. The primary beam's intensity and position is monitored by means of a beam splitter and
a four-segment photodiode. The thermostated sample cell is placed on a motor-driven presicion goniometer (±0,001°) which enables the photomultiplier detector to be moved accurately
from 20° to 150° scattering angle.
Static Light Scattering
small molecules, i. e. point scatterers
In static light scattering experiments the time-averaged (or 'total') intensity of the scattered light is measured, and for solutions is related to the time-averaged mean-square excess
polarizability which in turn is related to the time-averaged mean-square concentration fluctuation. The reduced integrated scattering intensity Kc/R(q) is calculated from the absolute
photon count which is recorded simultaneously with the measurement of the TCF (see below). K is an optical constant (among some constants it depends on the square of the refractive
index increment of the solute and the square of the refractive index of the solvent), c is the mass concentration of the solution, and R(q) is the Rayleigh ratio. For the calculation of the
latter the instrument is calibrated by measuring the scattered intensity from toluene.
Refractive index increment measurements of the samples are carried out with a Brice-Phoenix differential refractometer, equipped with a 488 nm interference filter. This apparatus is
calibrated with solutions of common salts or poly(ethylene glycol).
large molecules
In the section above it was assumed that the solvent and solute molecules act as point scatterers, what means that they are much smaller then the wavelength of the incident light. This
assumption may apply to solvent molecules in most cases but it is inappropriate for polymer solute molecules or aggregates (e. g. micelles). If the dimension of the solute molecule
exceeds 1/20 of the wavelength a remarkable interference of the scattered light from one molecule occurs. It can be shown that the phase difference in the scattered light vanishes only at
zero scattering angle. In order to account for such interference effects, a particle scattering factor P(q) is introduced and is given by the ratio P(q)=R(q)/R(q=0). Since scattered
intensity at zero angle cannot be detected, analytical expressions for P(q) are required, so that an appropriate extrapolation to zero can be performed.
Conclusion: The angle dependency of Kc/R(q) contains information about the shape of the solute molecules.
Dynamic Light Scattering (Photon Correlation Spectroscopy)
Whilst static light scattering measurements provide a wealth of information (e.g. weight-average molar mass (Mw), second osmotic virial coefficient (A2) and z-average radius of gyration
(<s2>z)), still more can be obtained by considering the real-time random (i. e. Brownian) motion of the solute molecules. This motion gives rise to a Doppler effect and so the scattered
light possesses a range of frequencies shifted very slightly from the frequency of the incident light (this phenomenon is called quasi-elastic scattering). These frequency shifts yield
information relating to the movement (i. e. the dynamics) of the solute molecules. A very popular means of monitoring the motion of solute molecules is to record the real-time fluctuations
in the intensity of the scattered light in terms of the intensity time-correlation function.
The intensity time-correlation functions (TCF) g2(t) are recorded with an ALV-5000 multi tau digital correlator with 256 channels. The installed software (CONTIN2DP) allows an
on-line inverse Laplace transformation of the TCFs and yields the distribution of contributing relaxation times in the investigated sample. Further the TCFs can be analyzed in terms of a
cumulant or stretched-exponential fit
Dynamic light scattering measurements yield hydrodynamic properties of the solute, e. g. the z-average of the translational diffusion coefficient (Dz) which is related to the hydrodynamic
radius by the Stokes-Einstein equation.