MadSci Network: Earth Sciences

Re: What experiments can be used to show why the sky is blue?

Date: Thu May 3 09:35:42 2001
Posted By: Sidney Chivers, , Nuclear Engineering, retired
Area of science: Earth Sciences
ID: 988161795.Es

"As the sun's radiation ... passes through the lower atmosphere, it is 
scattered by any particles that are roughly the same size as the wavelength of 
the radiation.  The selective scattering results in the blueness of the sky, 
since the short, blue wavelengths of the light spectrum are more easily 
scattered than the longer, red wavelengths.  But when the air is contaminated 
with large numbers of motes, such as smoke particles, then slightly longer 
light wavelengths are also scattered, and the sky becomes a lighter color, more 
gray than blue."

Pages 36 to 37 of The Air Around Us by T. J. Chandler.

Simulating something on the scale of the atmosphere visible from a point can be 
a challenge and that is probably as good a reason as any to measure 
contaminants in the local atmosphere and then speak of the effects of the 
contaminants on the scattering of light - the color of the sky.

The following web references are about simulation of light scattering

If you are interested in something on a bench-scale, you might consider 
scattering light through an aquarium filled with a liquid, except that the 
observable phenomena are more related to simple absorption.  With a little 
experimentation, you may be able to demonstrate a similar scattering effect by 
beginning with a liquid of a given color and then observing changes in color as 
you add an appropriately sized particulate.

I have seen a demonstration in which a laser light is shined through an 
aquarium and produces a range of colors, as seen from the side of the aquarium, 
but I have not been able to locate that reference.

Here's a web reference with a sky color chart that may prove useful for any 
observations you want to record.

You might be interested in a nigrometer, a simple instrument which can be used 
for such as determination of the length a column of air must be to scatter 
light to the same extent as occurs in the entire depth of the atmosphere.  That 
application is described on pages 266 to 267 of M.G.J. Minnaert's Light and 
Color in the Outdoors.

I hope this helps.

Thanks for your question.


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