Sunlight looks bright and clear, but it’s made of multiple colors called the color spectrum, and they combine to produce white light. The sunlight or white light is a mix of all the rainbow colors. A rainbow is the visible color of the spectrum, all separated. Each color in the spectrum has a different wavelength. Some light travels in short waves, while others travel in long ways.
Also, light travels from the sun to the earth. It has to travel through the Earth’s atmosphere. The atmosphere we live in called the troposphere, comprises oxygen, nitrogen, and other molecules like water vapor. The Sun is close to the horizon at sunrise, night, and sunset. So the sunlight travels a large distance in the Earth’s atmosphere. Red light is scattered the least, and blue and green light gets scattered more. Due to the large wavelength, red light scatters less. So it reaches our eyes, which is why the Sun appears red at sunset and sunrise.
Why is the sky red at night, sunrise, and sunset?
The sky appears red at sunrise and sunset due to atmospheric scattering. Here’s a simplified explanation of why the sky can turn red during these times:
Sun’s Position: During sunrise and sunset, the sun is lower on the horizon compared to midday when it is higher in the sky. This means sunlight has to pass through a larger portion of the Earth’s atmosphere.
Increased Path Length: When sunlight travels through a larger portion of the atmosphere, it encounters more air molecules and particles. This longer path length causes more scattering of shorter wavelengths (blue and green) and enhances the transmission of longer wavelengths (red and orange).
Rayleigh Scattering: Rayleigh scattering occurs when light interacts with molecules in the atmosphere, causing shorter wavelengths to scatter more. During sunrise and sunset, the blue and green light is scattered away from our line of sight, leaving predominantly longer wavelengths to reach our eyes.
Red Light Dominance: As the shorter wavelengths are scattered away, the remaining longer wavelengths, particularly red and orange, become more visible. This dominance of longer wavelengths contributes to the red, orange, and pink hues observed during sunrise and sunset.
Particle and Dust Effects: The presence of particles, dust, and pollutants in the atmosphere can further enhance the scattering and filtering of light during sunrise and sunset. These particles can scatter and reflect sunlight, adding color effects and intensifying the red hues.
Atmospheric Conditions: The quality of the air, humidity, and the presence of clouds can also influence the appearance of the sky during sunrise and sunset. Moisture and particles in the atmosphere can scatter and refract sunlight differently, creating variations in color and intensity.
The light from the Sun enters our atmosphere and is scattered about the air. The air is filled with nitrogen, oxygen, argon, water vapor, and particles like smoke, ash, pollution, etc. These gases serve as filters or prisms to give us different types of light. The rainbow of colors that make up light blue and purple are scattered.
The sun is closer to the horizon, meaning the light has to travel far from the atmosphere. Long wavelengths give us red and orange colors, whereas short wavelengths give us blue and indigo colors. When sunlight hits these atmospheric molecules, it bounces off them and gets scattered in all directions. It’s called Reilley Scattering.
The scientific reason is they have the shortest wavelength. The other colors can make it passes all those bumps in the atmosphere. The bumps are nitrogen, oxygen, water vapor, etc. At noon or midday, the Sun is high in the sky, and light doesn’t have that far to travel to our eyes.
So not all of the blue light is scattered, and that’s why the sky is blue, but at sunset, the light has to travel ten times more atmosphere. We see the long wavelengths of light and the short wavelengths of blue are scattered out. That’s why we see the yellow, orange, and red colors.
During the autumn equinox, day and night are equal. Then, each day gets shorter as the sun’s rays approach the northern hemisphere at a more extreme angle. The southern hemisphere is starting to get the more direct Sun but a more extreme angle in the northern hemisphere. It means that now the light from the Sun has to travel even farther at sunset to make it to our eyes. There’s a better chance that more blue light is getting scattered out during that trip and a better chance that we’ll see a yellow, orange, or red sunset.
What causes the red sky at night? A good sunset depends on the atmosphere’s high pressure because it squashes all those gas molecules. So you get more blue light scattered and red light hitting your eye. It’s going to travel overnight eastwards.
Why are the colors of fall and winter sunsets so vivid? During the fall in winter, the weather patterns allow for dryness. It means with clean air, the spectrum of colors makes it through without getting scattered by those particles in the air. You would have seen some pretty spectacular sunrises and sunsets where the sky is on fire, and the colors are vivid. How does that happen? The best way is for the air to be fairly clear for brightly colored sunrises and sunsets.
Experimental explanation by physics: Here in the picture, we have a converging or convicts lens L1. A strong white light earth source is kept at this converging lens’s focus. The cardboard, there is a hole in it. So after refraction, we will get the parallel beam of light.
The parallel light beam passes through this transparent glass tank containing clear water. When it falls on the cardboard, only a few rays can pass through this hole because all other parts do not allow the light to pass to the site. So some rays of light pass through this hole.
We have another converging lens, another convex lens, which converts these parallel raids onto the screen. Take about 200 grams of sodium sulfate and Heibel, and dissolve in about two liters of clean water in the tank. Guess what? First, it was pure water, but now the sodium mixture also fits with water in two liters of clean water. Then we add about one to two milliliters of concentrated sulfuric acid to this water.
Then, the fine microscopic sulfur particles start precipitating in the tank in about two to three minutes. As the sample particles form, we can observe the blue light from the glass tank’s three sides because of scattering.
So when the beam of light passes through this glass tank, the fine sulfur particles precipitated in the glass tank scatter the light of short wavelengths such as blue light—because of that, scattering by the colloidal sulfur particles. We observe the blue light from the three sides of the glass thing from this side and then decide which side is opposed.
So when you observe the color of the transmitter light from the glass’s forward side, it is from this site facing the circular hole. We will observe at first an orange-red color. We will observe bright crimson-red colors on the screen.
Quiz: Why does the sky red during sunset, sunrise, and night?
a) Due to the reflection of light.
b) Refraction of light.
c) Dispersion of light.
d) Scattering of light.
The correct answer is it’s due to the scattering of light.
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Craig F. Bohren & Eugene Edmund Clothiaux. Fundamentals of Atmospheric Radiation: An Introduction with 400 Problems. Wiley-VCH. p. 427.
David K. Lynch; William Charles Livingston. Color and light in nature. Cambridge University Press.
National Weather Service. “Chapter 3: Radiation and Temperature”. Anchorage, Alaska: NOAA.
Darula, S.; Kittler, R. “General Sky Standard Defining Luminance Distributions.” Proc. Conf. eSim 2002. Montreal.
Nave, C. R. “Red Sunset, Green Flash.” Georgia State University. HyperPhysics.