And the night sky is not completely dark
By Hoang Long, weekly reflection for Introduction to Psychology course, Fulbright 2023
Our ancestors used to say, "Seeing is believing - Trăm nghe không bằng một thấy". This phrase highlights how humans trust what they see, but observation is limited to the biological processes of sensation and perception, which only interpret a small part of reality. In the following blog, I will explore this idea through the example of the illusion of the blue sky.
The blue sky of Vũng Tàu, Việt Nam
To understand the color of the sky, we need to consider the three processes that contribute to this phenomenon: (1) the sun's light reaching the Earth, (2) the light passing through the atmosphere, and (3) our vision interpreting the color of the sky. The illusion of a blue sky is a result of these three processes working together.
When researching each of these processes in detail, everything is not as simple as you think.
White light does not actually exist. Sunlight consists of a spectrum of colors, and the human eye is only capable of perceiving a portion of that spectrum. The color white is produced by the combination of colors within the spectrum that are visible to the human eye [1].
If air were transparent, we could see space directly during the day. On the moon, the sky is black in the morning. This is because the moon has almost no atmosphere, so there is no scattering of sunlight, and space appears black. On Earth, however, the scattering of sunlight by our atmosphere causes the blue color of the sky.
Photo NASA, Apollo 11. The surface of the moon has almost no atmosphere, so we see the sky during the day as the black color of space.
The blue color of the sky is caused by the scattering of light as it passes through the Earth's atmosphere. Shorter waves, such as blue and violet, are more easily scattered. This phenomenon is called Rayleigh Scattering [2]. Although the sky color is a mixture of blue and violet, we only perceive the blue color due to the structure of the human eye.
Source: https://www.space.com/why-is-the-sky-blue