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Description:

This demonstration uses simple classroom tools to show how atmospheric scattering changes the colour of the light from sun during a sunset.

 


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Possible Incorporated Topics:
  • Rayleigh Scattering

  • Atmosphere composition



Theory:

     The red colour we see during a sunset is namely due to the scattering of light from the sun off of particles in the atmosphere.

     The type of scattering responsible for this is called Rayleigh scattering and occurs when the particle doing the scattering is much smaller than the wavelength of radiation being scattered. An incoming wave is absorbed by the particle and another wave of the same wavelength is reemited in a variety of directions, or "scattered". The amount of scattering that occurs is inversely proportional to the fourth power of the wavelength of the wave. For this reason, light with shorter wavelengths such as violet, blue and green are scattered more efficiently than colours like yellow, orange and red.

     Rayleigh scattering in the Earth's atmosphere happens when the light from the sun is scattered off of oxygen and nitrogen particles in the atmosphere. Violet light, followed by blue have the shortest wavelengths in the visible spectrum, and are therefore scattered the most, making the sky appear blue.

     During a sunset, the light from the Sun must go through much more atmosphere before it can reach us. The increase in atmosphere means there are far more particles to scatter the light, so the violet and blue light is completely scattered away. Now yellow orange and red light can be scattered, causing the sky to take on these colours. The more atmosphere the light from the Sun must go through, the redder the sunset appears. Pollutants in the air caused by industry can add to the number of scattering particles, so areas with high levels of air pollution may experience more vibrant sunsets.

     In this demonstration, coffee creamer is added to water slowly, creating more particles for the light to be scattered off of. Therefore, the colour of the light going through the water and projected onto the screen becomes darker and more red in colour as the creamer is added.

 


Apparatus:

  • Small beaker

  • Water

  • Coffee creamer

  • Overhead projector and screen

  • Stir Stick



Procedure:

1. Fill the beaker half way with water

2. Turn on the overhead projector and place the beaker on it. Observe the colour of light through the beaker on the wall or screen.

3. Dip the stir stick into the coffee creamer and then stir it into the water in the beaker. It can take some practice to get the right amount of creamer in the water since too little won't produce a noticeable change in the colour and too much will saturate the water right away.

4. Repeat step 3, observing the change in colour with each succesive addition of creamer. Repeat until the water is saturated and the colour of the projected water no longer changes.