Tyndall Effect

Age

Elementary, Middle School, High School

Format

Hands-on

Materials

   Aquarium
   Water
   Milk
   Stirring Rod
   A white-light source that will produce a steady beam of light (flashlight, slide projector, etc.)
   Red LASER pointer (optional)
   A reflecting surface to make the beam of light visible (mirror, flat board, etc.) 

Safety Precautions

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience.

Always wear a lab coat and safety goggles. Do not drink any of the solution of water and milk. Do not shine a light source into anyone’s eye.

Preparation

1. Fill the aquarium with water

2. Position the aquarium so that the long end is visible to the audience

3. Point the light source through the aquarium longways – do not turn it on yet

4. Position the reflecting surface on the opposite side of the light source

5. Have a carton of milk open and ready

Demonstration

Switch the light source on so that the light shines through the aquarium water and hits the reflective surface on the other side. You should not be able to see a beam of light in the water, but it should show up on the reflective surface – demonstrating that the light travels through the water.

Keeping the light source turned on, add a small amount of milk to the aquarium. Use a stirring rod to mix the water and milk. As the water thickens, the beam of light should become visible in the mixture. If done slowly, the addition of milk will appear to widen the beam. Note that too much milk will entirely block the beam of light.

If you are using a flashlight or slide projector, as opposed to a LASER, you will notice several odd coloration effects. Objects within the beams path through the water will appear blue, while the light that passes through to the reflective surface will appear red.

What to Say

Ask the audience why they can see a circle of light from a flashlight once it hits a surface, but not while it is traveling through the air. Shine a white light source through the aquarium to demonstrate this principal. Ask them if they can think of any situations in which the beam of light is visible in the air. Suggest headlights traveling through fog as one such situation.

Add milk to the aquarium and ask the audience why the beam becomes visible. Explain that as light travels through the thick milk particles, some of the light is interrupted in its path and bounced off toward the observer (scattered). As you slowly add more milk and stir the mixture, demonstrate that increasing the particle density in the water widens the beam.

Point out that the light has become slightly discolored and no longer appears white. Ask the audience what color the scattered light is and what color the relatively-unscattered light is. Remind them that the light we see throughout the sky is blue and that the light which appears to come directly from the sun is slightly red – especially at sunset. Explain that the short-wavelength blue light is scattered more, and so is not allowed to continue along the path of the beam. Long-wavelength red light passes through the mixture in the aquarium and hits the reflective surface, just like sunlight.

If you have a LASER pointer available, ask audience members if they know how it works. Explain that LASERs direct a single wavelength of light into one focused beam; unlike flashlights, which direct light of all wavelengths into a diffuse beam. Point to some inanimate object with the LASER pointer to demonstrate that the beam is not visible – just as with the flashlight beam. Shine the LASER pointer through the aquarium to demonstrate that the Tyndall effect works with the LASER. Because the LASER uses long-wavelength red light, the light is not scattered very much and the beam is very skinny.

Why It Is

The Tyndall effect is caused by the scattering of light waves traveling through a colloid dispersion- a mixture of a solid and a liquid. This effect is not noticeable when light travels through water, because a solution of water is purely liquid. The effect is substantial for light traveling through a water-milk mixture because solid particles from the milk are dispersed throughout the liquid water. This particular type of colloid dispersion is classified as an emulsion.

Light waves passing through the emulsion are scattered in different ways depending on their wavelength. White light from a projector or flashlight contains light waves of all wavelengths. Those waves of a lower wavelength, on the blue end of the spectrum, are scattered (bounced away) from the beam towards the viewer, causing the beam to appear blue. A thicker emulsion containing more milk particles will scatter light more, creating a thicker apparent beam. The scattering of blue wavelengths leaves red ones less affected and explains why the beam resulting from the water and hitting the reflective surface appears red.

Fog, a type of liquid aerosol colloid dispersion, also produces this effect. When headlights are turned on while driving through fog, the beam of light is visible. The Tyndall effect also explains why Earth’s sky appears blue from Earth’s surface. As sunlight passes through the atmosphere, atmospheric particles scatter blue wavelengths throughout the sky. The beam of light coming directly from the sun, however, appears slightly red.

Real Life Examples

Fog through headlights, the color of the sky and the sun