Science Behind It: Why Sky Is Blue?

Have you ever looked up on a clear day, marveled at the vast expanse of blue above, and wondered why the sky wasn’t green or red?

This deceptively simple question delves into the fascinating interplay between chemistry and physics that governs the colors we perceive in our environment.

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The sky’s blue tint is the result of a complex interaction between sunlight and the Earth’s atmosphere, largely explained by the phenomenon of Rayleigh scattering.

This article uses basic concepts of light, atmospheric composition, and human perception to explore how the sky appears blue.

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the nature of sunlight

To understand why the sky is blue, you must first consider the properties of sunlight. Sunlight or solar radiation consists of a spectrum of electromagnetic waves of different wavelengths.

The spectrum includes visible light (the range of wavelengths our eyes can detect), ranging from violet (about 380 nanometers) to red (about 750 nanometers).

Image: NASA

Although sunlight appears white to us, it is a combination of all these colors, each contributing differently to the overall perception of color.

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When sunlight enters the Earth’s atmosphere, it interacts with molecules and small particles present in the air. These interactions are crucial in determining the color of the sky we see.

composition of earth’s atmosphere

The Earth’s atmosphere is a mixture of gases, mainly nitrogen (about 78%) and oxygen (about 21%), with trace amounts of argon, carbon dioxide, water vapor and other gases.

In addition, the atmosphere contains tiny particles such as dust, pollen and soot. The size and distribution of these molecules and particles play an important role in how sunlight is scattered.

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Rayleigh Scattering: Main Mechanisms

The main process responsible for the blue color of the sky is Rayleigh scattering, named after the British physicist Lord Rayleigh, who first described it in the 19th century.

Rayleigh scattering occurs when light interacts with particles that are much smaller than its wavelength, especially gas molecules in the atmosphere.

The main characteristics of Rayleigh scattering:

1. Wavelength dependence: Rayleigh scattering is highly dependent on the wavelength of light. The intensity of scattered light is inversely proportional to the fourth power of the wavelength. This means that shorter wavelengths (blue and violet) are scattered more than longer wavelengths (red and yellow).
2. Scattering angle: The smaller the angle, the more efficient the scattering, meaning the light is scattered in all directions, but the intensity varies with angle.

Given these properties, blue light has a shorter wavelength (~450–495 nm) and is scattered about ten times more than red light (~620–750 nm). Even though violet light has shorter wavelengths and scatters more, the sky doesn’t look purple to our eyes for a number of reasons.

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Human perception of color

Our perception of sky color is influenced not only by the physics of light scattering, but also by the biology of our eyes. The human eye contains three types of cone cells, each sensitive to a different range of wavelengths: red, green, and blue.

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The cones responsible for green and red are more sensitive than the cones responsible for blue and violet. Additionally, the atmosphere absorbs some violet light, and our brains interpret the predominantly scattered blue light as the color of the sky.

The role of atmospheric composition

While Rayleigh scattering explains the blue color of the sky, specific components of Earth’s atmosphere enhance this effect. Abundant nitrogen and oxygen molecules provide ample scatterers for incident sunlight.

Additionally, the absence of larger particles minimizes Mie scattering, which is less dependent on wavelength and would otherwise cause the sky to appear white or gray.

Mie scattering and Rayleigh scattering:

• Rayleigh scattering: Dominates molecules smaller than the wavelength of light, resulting in strong wavelength dependence and preferential scattering of shorter wavelengths.
• Mie scattering: Caused by larger particles whose size is comparable to the wavelength of light, resulting in less wavelength dependence and more uniform scattering of all colors.

In the presence of larger particles (such as fog, haze or pollution), Mie scattering becomes more pronounced, which can make the sky appear whiter or softer.

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Effect of sun position

The color of the sky changes depending on the sun’s position in the sky, illustrating the dynamic nature of light scattering. At noon, when the sun is high overhead, sunlight takes a shorter path through the atmosphere, resulting in minimal scattering and a deeper blue sky.

Instead, during sunrise and sunset, sunlight travels through a longer atmospheric path, scattering more blue and green light out of direct line of sight and allowing longer wavelengths like reds and oranges to dominate, resulting in vivid daytime views. Sunrise and sunset colors.

Other factors that affect sky color

Several other factors also affect sky color, including:

1. Atmospheric conditions: Humidity, dust, and pollution levels change scattering properties, thereby changing the intensity and hue of sky colors.
2. Geographic location: Areas near the poles or the equator may experience different sky colors due to changes in atmospheric composition and angle of sunlight.
3. Altitude: The higher the altitude, the thinner the atmosphere, the less scattering, and the bluer the sky.
4. Weather patterns: Clouds and atmospheric disturbances can affect the color of the sky by reflecting and scattering light in different ways.

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Source: https://dinhtienhoang.edu.vn
Category: Optical Illusion