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What Color Is Your Sunset?
“Do you know that the more colorful your sunset is, the more polluted is the atmosphere?” That was what our physics teacher once told us. Well, how true is this? Stay on, and you will find out.
Of Light Waves and other Stories
Visible light occurs as waves with varying wavelengths. Red has the longest wavelength while violet has the shortest. All the other colors of the rainbow have intermediate wavelengths between red and violet. When all the wavelengths are seen together, light appears as white.
When white light enters a prism, it becomes separated into its individual waves which we see as the colors of the rainbow. Water droplets in the atmosphere also act like a prism and that’s why we see a rainbow in the sky when the atmosphere is filled with water vapor.
Of Sunlight, Sunsets, and Colors
The sun is the source of these light waves. The color of objects that we see around us is the color of the light waves that are reflected by the object. All the other colors are absorbed by the object. Thus the green color of a leaf is the reflected green wave from the leaf surface.
The different light waves plus the composition of our atmosphere explains why the sky is blue during daytime but orange-red at sunset.
a) Why is the Sky Blue?
When the sun is high up in the sky, its light travels only a short distance through the atmosphere before it reaches our eyes. Thus we see white light, the sum of all light waves.
Since our atmosphere consists mostly of nitrogen and oxygen molecules, these molecules absorb and scatter the light waves selectively through the principle of Rayleigh scattering . What does this mean?
Well, simply, nitrogen and oxygen molecules are smaller than most of the light wavelengths except for the blues and violets which have the shortest wavelengths. When the blue and violet wavelengths bump into these nitrogen and oxygen molecules, they are absorbed and then scattered in different directions by these molecules. These scattered blue and violet give the blue color to the sky.
If that is so, shouldn’t the sky be more violet than blue then, since violet is the shortest wavelength and therefore the one most affected by this scattering effect? Well, it could be so except for the fact that we see blue better than violet because of the nature of the color receptors in our eyes. Our color receptors are most sensitive to the wavelengths red, green and blue or RGB.
By the way, “Rayleigh scattering” is named after Lord John Rayleigh, an English physicist, who first described it in the 1870's.
Ok now, so that explains why the sky is blue. What about the color of sunsets?
b) Why do we have Fiery Sunsets?
During sunsets and sunrises (for that matter), the sun is low in the horizon and thus light travels through a longer distance of atmosphere. The same Rayleigh scattering occurs, but since so much scattering of the shorter wavelengths has already occurred by the time light reaches our eyes; only the reds, oranges and yellows or the longer wavelengths remain for us to see. Thus, our sunsets and sunrises are yellow-orange-red.
In general, sunset colors are more intense than sunrise colors. Why so? Well, because there are more dust particles in the atmosphere in the afternoon than early in the morning. At night, most dust particles settle down, so the atmosphere is clearer in the morning except of course if there are some night fires around.
So it’s true then that more pollution contributes to more colourful sunsets? Oh, no, no no... not so fast on that. This is actually where some confusion sets in.
c) Pollution Does Not Make For Colorful Sunsets
Air pollution a.k.a smog adds particles to the atmosphere. Yes, that’s true. However, we have to differentiate smog (or smoke + fog) produced by volcanic explosion and some forest fires, from smog caused by emissions from factories, vehicles, burning of plastics, and other unnatural materials.
While volcanic explosions release particles of the same size into the atmosphere and indeed contribute to more colourful sunsets as what happened when Pinatubo erupted several years ago, man-made pollutant particles are of totally different sizes. Particles from CFCs, car exhaust, factory emissions, etc. come in various sizes. Size of particles affects the colors produced during sunsets.
Ashes from volcanic eruptions are big enough to scatter not just the blue and violet wavelengths but also the longer yellows as well. Thus, what remains in the sky and what reaches our eyes are the fiery oranges and reds. Particles from various man-made pollutants on the other hand are of different sizes; some big, some small. What happens here is a totally different kind of scattering and not the selective Rayleigh scattering we encountered earlier. The keyword here is “selective” scattering.
When there are particles of various sizes in the atmosphere, scattering of light becomes indiscriminate. Let me repeat, “indiscriminate”! Smaller particles will scatter the shorter wavelengths while bigger particles will scatter the longer wavelengths.
So what color remains in the sky after all this scattering? Nothing! Nothing remains but a hazy, grayish, dull color ... a sunset with no colors! Can you imagine that? This is what happens when there is too much particles (of the pollutant kind) in the atmosphere.
Just think about it, if pollution creates colourful sunsets, then we should experience the most colourful sunsets in the most polluted cities, right? But from experience, we see the most colourful and fiery sunsets in the least polluted areas especially near bodies of water.
Colorful or Colorless Sunsets?
So my dear physics teacher, I have to make a qualification; pollution does not make colorful sunsets; rather, it’s particles of a certain kind that make colorful sunsets. I think you just wanted to be dramatic then.
Pollution makes colorless sunsets not colorful ones!
So, what color is your sunset?
A fellow hubber answered this question in this hub: Beautiful-Endings-Because-of-a-Day