Everyone knows that the sky is blue, but not everyone understands why the sky appears blue. Why is the sky often blue rather than another color? Above the Earth’s surface lies a very thick layer of atmosphere that envelops the planet. This layer of air is colorless, so where does the “sky blue” color come from?
Explaining the Reason for the Sky’s Blue Color
Visible light is a part of the electromagnetic spectrum that the human eye can perceive. Light from the sun or electric bulbs is referred to as white light.
Sunlight consists of seven colors: red, orange, yellow, green, blue, indigo, and violet. Each color corresponds to a specific wavelength, frequency, and carries different energy levels. Violet light has the shortest wavelength in the visible spectrum, meaning its frequency and energy are the highest. Conversely, red light has the longest wavelength, the lowest frequency, and carries the least energy.
Light in the Atmosphere
Light travels through space in a straight line unless it is disturbed. When light enters the atmosphere, it continues on its path until it encounters small dust particles or gas molecules that impede it. What happens to the light from this point depends on its wavelength and the size of the objects it interacts with.
Dust and water particles in the air are larger than the wavelengths of visible light. When light strikes these larger particles, it is scattered in many different directions or absorbed by the obstacles. Since different colors within light are reflected from the particles in the same direction, the light reflected from these obstacles remains white light and contains all the original colors.
In addition to dust and water, the atmosphere also contains gas molecules. These gas molecules are smaller than the wavelengths of visible light. If white light interacts with gas molecules, the process is not as straightforward as with dust or water droplets.
When light hits gas molecules, “part” of it may be absorbed by the gas molecules. Subsequently, these gas molecules will re-emit light in many different directions. The term “part” is used here because some wavelengths in white light (corresponding to certain colors) are more easily absorbed than others. In other words, shorter wavelengths (like blue) are absorbed more than longer wavelengths (like red).
British physicist John William Strutt, 3rd Baron Rayleigh (1842-1919), proposed the equation that determines the Rayleigh scattering coefficient, explaining the reason behind the blue sky.
The process described above is known as Rayleigh scattering. This phenomenon is named after its discoverer: Lord John Rayleigh, a British physicist. In 1871, Rayleigh formulated an equation calculating the scattering coefficient of a substance that is inversely proportional to the fourth power of the wavelength of light (denoted as lambda). In other words, light with shorter wavelengths is scattered more than light with longer wavelengths.
We can now answer the initial question: The blue color of the sky is due to Rayleigh scattering.
Since the wavelength of light (100~1000 nm) is greater than the size of gas molecules (10 nm), we can apply the Rayleigh scattering formula to the phenomenon of light scattering in the Earth’s atmosphere.
When light enters the Earth’s atmosphere, most of the longer wavelengths are not absorbed by gas molecules, allowing them to pass through. A small amount of red, orange, and yellow light may be affected by the atmosphere. However, a significant amount of short wavelengths is absorbed by gas molecules. Short wavelength light that is absorbed will then be scattered in many different directions.
At this point, blue light will scatter throughout the sky. During the day, no matter where you stand and look, some of the scattered blue light always heads towards your eyes. Thus, when you look up, the sky will always appear blue.
If you pay closer attention, you’ll notice that as you look toward the horizon, the sky seems to be a lighter color. This is because, to reach your position, the scattered blue light must pass through more layers of air. A portion continues to scatter in various directions. Therefore, less blue light from near the horizon reaches you compared to the amount of blue light from directly above your head.
Another noteworthy point is that if you have been following along, you may wonder: The wavelengths of violet and indigo are even shorter than blue, so shouldn’t the sky be violet? The answer is ready for you.
So why isn’t the sky violet? That’s the shortest wavelength!
A primary reason is due to the function of the human eye in perceiving colors. The human eye is sensitive to light wavelengths ranging from 380 to 740 nm. The normal retina contains 10 million rod cells that detect light and 5 million cone cells that detect color. Each cone cell contains pigments that respond to different types of wavelengths. There are three main types of cone cells corresponding to short, medium, and long wavelengths. We need to utilize all three types of cells to see colors accurately.
Each cone cell responds to wavelengths with maximum sensitivity at: 570 nm for long wavelengths, 543 nm for medium wavelengths, and 442 nm for short wavelengths. However, these three types of cone cells can respond to a broad range of wavelengths that overlap. This means that two different spectra can cause the same response in the cone cells.
Two different spectra that produce the same response in the cone cells are termed isomeric variants. Returning to the sky, when the sky is a mix of blue and violet, the cone cells in the human eye react to this mixture as a blend of blue and white. Ultimately, the signals sent to the nervous system are perceived as blue. This is similar to the technique of mixing red and green to create yellow.
However, some animal species do not perceive the sky as blue as humans do. Besides humans and some primates, most other animals have only two types of cone cells in their retinas. Therefore, these animals, such as birds, perceive the sky as violet.
Why Do We See the Sun as Yellow?
On Earth, we see the sun during the day as yellow. If you go into space or on the Moon, you will see the Sun as white. Why is that? It’s simply because: In space, there is no atmosphere to scatter sunlight.
On Earth, some short wavelengths of sunlight (blue or violet) are absorbed by gas molecules and removed from the direct beam of light that reaches the human eye from the sun. Thus, the remaining colors appear together as yellow.
Finally: Why is the Sunset Red?
When the sun begins to set, the light must travel a longer distance through the atmosphere before reaching your eyes. At this time, more light will be reflected and scattered. The less direct sunlight reaches you, the less luminous the sun appears. During this moment, the sun’s color begins to change, shifting from yellow during the day to orange and then to red.
The main reason is: Although blue light is still scattered like during the day, it is scattered multiple times as it must pass through a thick layer of air before reaching the human eye. Additionally, the longer wavelengths (orange, yellow) in the light beam directly hitting your location are increasingly diminished. These longer wavelengths have to travel a longer distance compared to daylight to reach your position directly. What remains is red light, which is less scattered and is transmitted more directly to the eyes.
As a result, you will see the sky gradually turning red. After the Sun has set below the horizon, we do not see the Sun’s light directly; however, if there are clouds high in the sky, they will reflect the red light down to the ground, creating the beautiful scenery of a sunset.
Conclusion
Finally, we have found answers to the initial questions. Once again, phenomena that seem obvious hide within them many complex issues. Truly, anything has its cause. Naturally, humans are constantly researching day and night to try to explain many phenomena around us that previously had no answers. This is the desire of all of us, especially scientists. Each person has many “why” questions of their own.
