To understand why the sky is blue, we need to consider the nature of sunlight and how it interacts with the gas molecules that make up the atmosphere. Sunlight, which appears white to the human eye, is a mixture of all the colors of the rainbow. Sunlight can often be thought of as an electromagnetic wave that causes charged particles (electrons and protons) in air molecules to vibrate up and down as the sun passes through the atmosphere. When this happens, the oscillating charges produce electromagnetic radiation at the same frequency as the incoming sunlight, but spread in all different directions. This deflection of the incident sunlight by the air molecules is called scattering. The blue component of the visible light spectrum has shorter wavelengths and higher frequencies than the red component. So when all colors of sunlight pass through the air, the blue part causes charged particles to oscillate faster than the red part. Blue scatters more than red because faster vibrations produce more scattered light. For particles such as air molecules, which are much smaller than the wavelength of visible light, the difference is dramatic. The acceleration of charged particles is proportional to the square of the frequency, and the intensity of scattered light is proportional to the square of this acceleration. Therefore, scattered light intensity is proportional to the fourth power of frequency. As a result, blue light scatters in other directions almost ten times more efficiently than red light.
If you look away from the sun somewhere in the sky, you will only see the light that has been redirected by the atmosphere into your line of sight. This is more common in blue light than in red, so the sky appears blue. Violet light scatters slightly more than blue light. However, there is bluer than violet sunlight entering the atmosphere, and our eyes are slightly more sensitive to blue light than violet light, which is why the sky appears blue.
The opposite happens when you watch the sunset over the horizon. The only light that is not scattered in other directions is visible. The red wavelengths of sunlight that pass through the atmosphere with little scattering reach the eye, but the strongly scattered blue light does not. The effect is amplified the further the sunlight travels through the atmosphere when the sun is above the horizon. There is more opportunity for the blue light to scatter than when the sun is overhead. This makes the sunset appear reddish. In polluted skies, small aerosol particles of sulfate, organic carbon, or mineral dust can further increase blue light scattering, making sunsets spectacular under polluted conditions.
Clouds, on the other hand, are made up of water droplets much larger than the wavelength of visible light. The way they scatter sunlight is determined by the way light is refracted and reflected internally by cloud droplets and diffracted around them. For these particles, the difference in the scattering of blue and red light is not as large as for gas molecules. As a result, our eyes receive significant stray light at all visible wavelengths, making clouds appear white instead of blue, especially when viewed against a blue sky background. A planet without an atmosphere cannot have a bright sky, because atmospheric scattering makes the sky blue
For example, photographs taken by the Apollo astronauts on the moon show them and the moon's surface bathed in sunlight, but with completely dark skies in all directions from the sun
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