Tyndall Effect: Difference between revisions

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==Age ==
{| class="wikitable" style="color:black; background-color:#ddd; margin-left: auto; float:right"
| [[Physics]]:
| Tyndall Effect, Micro-Particles
|-
| Grade Range:
| [[Elementary School]], [[Middle School]], [[High School]]
|-
| Format:
| [[Hands-on]]
|}


Elementary, Middle School, High School
This demonstration is fun for students at hands-on events, especially since it can be seen from across the room. Have paper towels on hand in case students stick fingers in the water!


== Format ==
Hands-on


== Materials ==
== Materials ==


    Aquarium
* Glass Fish Tank
    Water
* Water
    Milk
* Milk
    Stirring Rod
* Large Spoon
    A white-light source that will produce a steady beam of light (flashlight, slide projector, etc.)
* Red Laser Pointer
    Red LASER pointer (optional)
* Small Mirror
    A reflecting surface to make the beam of light visible (mirror, flat board, etc.)
* Flashlight or Lamp


== Safety Precautions ==
== 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.
Please read the General Safety Precautions section of the [[Demonstration Safety]] page before performing this demonstration.
 
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 ==
== 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.
# Set the fish tank on a table and fill it 3/4 of the way with water. Slowly add milk to the water drop by drop while stirring, and use the laser pointer after each drop to make sure you get a dispersion that lets you see the laser. Set the mirror on the other side of the tank so the laser can be reflected back through.
 
# As students come up, let them come up and shine the laser through the solution. Ask them for ideas on why you can see the laser beam. What allows the laser to be visible?
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.
# Have students to shine the lamp through the solution. They will see that the solution will appear blue, and the light bouncing off the mirror is red! What causes the separation of color?
 
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.
== Why This Works ==


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.
The ''Tyndall Effect'' is when light waves are scattered while traveling through a material or a liquid suspension. When we shine the laser through the solution, we can see the laser beam in the solution rather than just where it enters and exits. This is because some of the light is striking the particles in the solution and being reflected back towards us. Our milk/water suspension has tiny particles in it from the milk's proteins, sugars, and fat. These ''Micro-particles'' are extremely tiny, being 10,000 times smaller than a centimeter! Since these particles are extremely small, they have a big impact on what we see. This effect is substantial for light traveling through a liquid because the size of the solid particles that are suspended in the liquid can affect what wavelengths of light can pass through it.


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.
White light from a lamp or flashlight contains light waves of all wavelengths. The waves of a shorter wavelength (blue, indigo, violet) are ''Scattered'' by the micro-particles from the milk. This means that these colors of light bounce off of the particles instead of continuing straight through the solution, and instead are reflected out of the fish tank. This is why the solution will appear to have a faint blue color from the light, instead of appearing to be a pure white! A thicker emulsion, or solution, will scatter more of the short wavelengths, creating a thicker apparent beam and a more defined color. The long wavelengths of light (red, orange, yellow) are less affected by the microparticles, and are able to travel all the way through the solution. This is why the beam that strikes the mirror appears red, even though we are using a white light!


== Real Life Examples ==
== Additional Information ==


Fog through headlights, the color of the sky and the sun
* 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 becomes visible due to the droplets suspended in the air.
* This demonstration can benefit from different types of suspended particles. Try it with things such as tiny sparkles, powders, and watercolor paints!
* This demonstration pairs well with other optics demos, such as the [[Light Box]] and the [[Box of Optics]] demonstrations.

Latest revision as of 20:13, 3 October 2016

Physics: Tyndall Effect, Micro-Particles
Grade Range: Elementary School, Middle School, High School
Format: Hands-on

This demonstration is fun for students at hands-on events, especially since it can be seen from across the room. Have paper towels on hand in case students stick fingers in the water!


Materials

  • Glass Fish Tank
  • Water
  • Milk
  • Large Spoon
  • Red Laser Pointer
  • Small Mirror
  • Flashlight or Lamp

Safety Precautions

Please read the General Safety Precautions section of the Demonstration Safety page before performing this demonstration.


Demonstration

  1. Set the fish tank on a table and fill it 3/4 of the way with water. Slowly add milk to the water drop by drop while stirring, and use the laser pointer after each drop to make sure you get a dispersion that lets you see the laser. Set the mirror on the other side of the tank so the laser can be reflected back through.
  2. As students come up, let them come up and shine the laser through the solution. Ask them for ideas on why you can see the laser beam. What allows the laser to be visible?
  3. Have students to shine the lamp through the solution. They will see that the solution will appear blue, and the light bouncing off the mirror is red! What causes the separation of color?


Why This Works

The Tyndall Effect is when light waves are scattered while traveling through a material or a liquid suspension. When we shine the laser through the solution, we can see the laser beam in the solution rather than just where it enters and exits. This is because some of the light is striking the particles in the solution and being reflected back towards us. Our milk/water suspension has tiny particles in it from the milk's proteins, sugars, and fat. These Micro-particles are extremely tiny, being 10,000 times smaller than a centimeter! Since these particles are extremely small, they have a big impact on what we see. This effect is substantial for light traveling through a liquid because the size of the solid particles that are suspended in the liquid can affect what wavelengths of light can pass through it.

White light from a lamp or flashlight contains light waves of all wavelengths. The waves of a shorter wavelength (blue, indigo, violet) are Scattered by the micro-particles from the milk. This means that these colors of light bounce off of the particles instead of continuing straight through the solution, and instead are reflected out of the fish tank. This is why the solution will appear to have a faint blue color from the light, instead of appearing to be a pure white! A thicker emulsion, or solution, will scatter more of the short wavelengths, creating a thicker apparent beam and a more defined color. The long wavelengths of light (red, orange, yellow) are less affected by the microparticles, and are able to travel all the way through the solution. This is why the beam that strikes the mirror appears red, even though we are using a white light!

Additional Information

  • 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 becomes visible due to the droplets suspended in the air.
  • This demonstration can benefit from different types of suspended particles. Try it with things such as tiny sparkles, powders, and watercolor paints!
  • This demonstration pairs well with other optics demos, such as the Light Box and the Box of Optics demonstrations.
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