MadSci Network: Earth Sciences

Re: Why is it rare to see a purple or pink sunset?

Date: Wed Nov 28 20:03:20 2001
Posted By: Richard Goode, Faculty, Physical Sciences, Porterville College
Area of science: Earth Sciences
ID: 1005497224.Es

Hello Carly

First, lets talk about light. White light is a combination of all the 
colors of the spectrum (rainbow). When light passes from one material (air) 
into another material (water) it is bent because it speed changes. This is 
called refraction and is the reason your straw looks bent in a glass of 
water. Each color of light is bent at a different angle. Short wavelength 
light (violet) is bent the most and long wavelength light (red) is bent the 
least. This allows us to see a rainbow when it rains or when we shine light 
through a prism. 
The color of the sunset has to do with the way light from the sun enters 
our atmosphere. When the sun is overhead and the angle that the light 
enters is small, the shortest wavelength light (violet and blue) are bent 
slightly and are scattered by the air molecules and dust in the air to give 
us a blue sky. If there is a lot of dust in the air, more light is 
scattered making the sky appear less blue and sometimes even close to 
white. After a rain when the air is clean the sky is a much brighter blue. 
When the sun is setting, the light is passing through much more air than 
when the sun is overhead. More air allows more scattering of the light. 
Since the red light is scattered last the sunsets appear reddish. The more 
stuff in the air the prettier the sunsets. Since purple is at the shorter 
wavelength end of the spectrum, it is scattered first and little is left 
for the sunset. Sometimes when the conditions are right and there is 
reflection of the light off clouds high up in the atmosphere we can see 
some of the shorter wavelengths giving us those rare pinks and purples. 

I have attached some web sites that explain this topic more. 
 Light and Color


Why is the Sky Blue?

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