Understanding Rayleigh Scattering
And
Blue Sky Phenomenon
You may watch the summary of this post as video here.
Overview
Look at the awesome picture that was taken at Kona, Hawaii. No doubt, the backdrop of the clear blue sky is absolutely amazing. Have you ever wondered what makes the sky so blue? What is that phenomenon that causes the sky to appear blue?
In order to understand the blue-sky phenomenon, we need to first review the phenomenon of light scattering.
Light Scattering
When light rays pass through a medium, most of them pass straight through. But some of them get scattered away.
Scattering of light occurs when some of the light particles (photons), while passing through a medium, get absorbed by the particles hanging in the medium or by the actual molecules of the medium, and then get re-emitted in a different direction from their original direction.
This phenomenon is known as the scattering of light.
Rayleigh Scattering
During the light scattering process, if the scattered photon has the same amount of energy as the original incident photon, then it is known as elastic scattering or Rayleigh scattering. (You can read about inelastic scattering known as the Raman scattering here.)
The blue-sky phenomenon is due to this Rayleigh scattering. Lord Rayleigh (1842-1919) of England discovered this in 1871.
Rayleigh's Discovery
As you may know, the visible spectrum consists of several colors – violet & blue (400 nm) at the low end and red (700 nm) at the high end. If a particle can equally scatter every color, then the sky should be just white. But it is blue mostly. Why is that?
Rayleigh discovered that the intensity of the scattered light is inversely proportional to the fourth power of the wavelength. It means that lower the wavelength, higher the intensity of scattered light.
Since blue light is at the lower end of the spectrum with low wavelength, it gets scattered almost 10 times more than red.
Hence, what you see when you look at the sky is the highly scattered color, which is blue.
Then, why does the sky turn red/orange during sunrise or sunset? Keep reading.
Red-Orange sky
At sunset or sunrise, the sunlight enters the atmosphere at an inclined angle. Therefore, the sunlight has to travel quite a long distance in the atmosphere before reaching our eyes. Consequently, most of the short wavelength colors (violet to yellow) are scattered away.
What reach our eyes are the higher wavelength colors – orange and red. That explains why the sky appears orange or red at sunset or sunrise.
Blue Ocean
Another related question that comes to our mind is why is the ocean blue? Is it just a reflection of the blue sky? Lord Rayleigh too thought so soon after his discovery of why the sky looks blue. But this is not the actual reason.
In this case, the ocean acts like a light filter. It absorbs the red part of the visible spectrum completely. However, the higher energetic blue side of spectrum is able to penetrate deeper into water. In fact, it can reach up to 650 ft. So, what we see is the blue colored water.
The ocean may also appear green, red or in other hues. It is due to light bouncing off from the sediments or light reflecting from the bed of shallow water.
Conclusion
The blue-sky phenomenon is due to the Rayleigh scattering (elastic scattering) of light. Blue color gets scattered 10 times more than the red side of the spectrum. That is the reason we see blue color all over the sky.
Oceans look blue not because of scattering or reflection but because of the absorption phenomenon of light. Red side of the spectrum get absorbed completely while the blue color, as it is more energetic, is able to reach deeper making the water look blue.