To demonstrate how light waves passing through a medium can be used to determine the sizes of particles within the
Time Required: 1 hour
Saturn System Analogy: Rings and Atmospheres of Saturn and Titan
Keywords: Aerosols, Obscuration, Particles, Reflection, Ring Spokes, Scattering, Shadow
- Laser pointer (preferred; small focusable flashlight will work if laser is not available)
- Two large binder clips
- Two clear plastic or glass water bottles or cups (walls should be vertical), 50-100 millimeters in diameter (bottled-water or soft drink bottles are good as long as they have some non-corrugated surface)
- Tap water
- Milk (1/20 teaspoon per 12 ounces of water)
- Eye dropper
- Flour (less than or equal to 1/4 teaspoon -- a "pinch" -- per 12 ounces of water)
- Lazy Susan turntable (optional but helpful)
- Masking, duct, or electrical tape
Our everyday view of the world relies on the reflection of light from the objects around us. This reflection is called "backscatter" when it is coming from small objects, and especially when the light source is behind the observer. Very tiny objects, approximately the size of the light source's wavelength, also send light forward, that is, continuing from the source and away from the observer. This effect, called "forward scattering," is useful to scientists in determining the sizes of particles in planetary atmospheres and ring systems.
Adhere a piece of tape to one side of each container. Fill one container with water and place it on a Lazy Susan. In the
other container, prepare a highly dilute solution of milk, thoroughly mixed so the water is just slightly whitened.
(Start with 1/20 teaspoon of milk per 12 ounces of water; find the right proportions by experimentation in advance.)
Use the binder clips as legs for the laser pointer, with one of them holding the laser's switch in the "on" position. Place the
laser and sample bottle on the Lazy Susan.
CAUTION: BE CAREFUL NOT TO PROJECT THE LASER BEAM INTO ANYONE'S EYES. EYE DAMAGE CAN RESULT!
Align the laser pointer so that the beam passes through the water bottle and projects onto the piece of tape on the
far side of the container. (The tape will ensure that the laser beam is not projected farther into the room and perhaps into
Darken the room, if possible. Project the laser beam through the container of plain water. Observe the brightness of the
beam in the water as the Lazy Susan is rotated. The beam should pass straight through and be invisible or nearly so from all directions except directly along the beam.
Next, project the beam through the dilute milk solution. Laser light scattering from tiny particles of milk will delineate
the laser beam. The intensity of the beam is stronger or weaker according to the scattering properties of the milk
particles (primarily their size) as the assembly is turned in front of fixed observers. Observers should note how the beam
reaches maximum brightness when they are looking in nearly the direction it is coming from.
Mix flour with the plain water (less than or equal to 1/4 teaspoon flour per 12 ounces of water) in the first container.
Project the laser beam through the dilute flour solution. The scattering properties of the milk and flour solutions are different because there is greater variation in flour particle size than in milk particle size. Store-bought milk is homogenized
(its particles are reduced to the same size) so the cream stays in solution. With either mixture, notice how the beam intensity diminishes with distance (looking from the side).
As an everyday terrestrial example, recall that bright headlights in fog may or may not help drivers, depending on particle size. Reflections from large fog droplets make night visibility with bright headlights poorer than with dimmed headlights.
Additional Experiments and Questions
Try other materials that will remain suspended in liquid for useful amounts of time. Corn meal, corn starch, oat bran, silt
from a local stream bed, glitter, salt, and sugar will provide varying results. Try transparent carbonated beverages, including their foams, and smoke trapped in a jar. Which work? Which don't? Why? Estimate particle sizes in gelatin based
on its scattering properties. Can you detect different particle shapes based on scattering?
A simple photometer can be used to compare the brightness of the beam as a function of the viewing angle. Mount a solar
cell on one end of an empty toilet paper tube so the tube shades the light-sensitive surface from ambient light. Attach
the leads to a millivolt meter (or multimeter). With the sample bottle centered on the Lazy Susan, align the laser on
the Lazy Susan so it shines through the center of the bottle and onto the solar cell.
Record voltage measurements every 30 degrees around the full circle for each sample bottle. Plot the voltage as a proportioned line segment every 30 degrees around a point. Diluted milk will produce a pattern like the 1-micrometer pattern
illustrated on the front of this Educational Brief.
Several vendors offer light-measuring photometry systems that acquire data and plot it under computer control. Such
systems can be adapted for quantitative measurements of the sample bottles in this activity. Computerized data acquisition
is common in many laboratories.
Is the color of the daylight sky related to sunset colors and scattering?
Because scattering is a phenomenon dependent on both the wavelength of the wave being scattered and on the size of the scatterer, much can be learned by working in well-separated parts of the electromagnetic spectrum. Where light waves tell us about the sizes of small particles, radio waves can tell us about the sizes of objects ranging in size from golf balls to houses.