Understanding Rayleigh Scattering
And
Blue Sky Phenomenon
You may watch the summary of this post as video here.
Overview
Check out these amazing photos—one from Kovalam Beach in India and another from Kona, Hawaii in the USA. The brilliant blue sky in the background is truly breathtaking.
But have you ever thought about why the sky looks so blue? What causes this vibrant color? To figure it out, let’s start by looking into how light scatters in the atmosphere.
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.
Why does the sky appear red or orange during sunrise and sunset? The following explanation provides insight.
Red-Orange sky
During sunrise and sunset, sunlight enters Earth’s atmosphere at a low angle. As a result, the light must traverse a greater distance through the atmosphere before reaching the observer. In this process, shorter wavelength colors (ranging from violet to yellow) are predominantly scattered out of the line of sight. The remaining light consists mainly of longer wavelengths—orange and red—which is why the sky exhibits these hues during these times.
Blue Ocean
Another common question is: why does the ocean appear blue? Many assume it’s simply reflecting the sky, an idea once shared by Lord Rayleigh after his work on why the sky is blue. However, that’s not actually the case.
The real reason lies in how the ocean acts as a filter for light. It completely absorbs the red part of the visible spectrum, while the more energetic blue wavelengths penetrate deeper—sometimes reaching depths of up to 650 feet. As a result, what we perceive is the blue color of the water.
The ocean can exhibit a range of colors, including green, turquoise, red, and other hues. These variations are typically a result of light reflecting off sediments or the sea floor in shallow waters.
Conclusion
The blue appearance of the sky is primarily attributed to Rayleigh scattering, an elastic scattering process wherein shorter wavelengths of light, such as blue, are scattered approximately ten times more readily than longer red wavelengths. This selective scattering accounts for the predominance of blue hues observed across the sky.
Conversely, the blue coloration of oceans results not from scattering or reflection, but from the absorption characteristics of water. Longer wavelengths on the red end of the spectrum are absorbed efficiently, while the higher energy blue wavelengths penetrate deeper, imparting a blue tone to the water.