The Role of Rayleigh Scattering
The sky appears blue because of a phenomenon called Rayleigh scattering. This is the scattering of electromagnetic radiation (like light) by particles that are much smaller than the wavelength of the radiation itself. Earth's atmosphere contains tiny nitrogen and oxygen molecules, which are significantly smaller than the wavelengths of visible light.
Wavelength and Light Scattering Efficiency
Visible light consists of a spectrum of colors, each possessing a different wavelength. Blue and violet light have shorter wavelengths, while red and orange light have longer wavelengths. Rayleigh scattering is far more effective at scattering shorter-wavelength light, meaning blue and violet light are scattered much more efficiently than other colors by the atmospheric molecules.
How Blue Light Dominates Our Vision
When sunlight enters our atmosphere, the blue and violet components of light are scattered in all directions. Because blue light is scattered more intensely and our eyes are more sensitive to blue than violet, a large amount of this scattered blue light reaches our eyes from all angles, making the sky appear distinctly blue. The other colors, like red and yellow, pass through the atmosphere more directly.
Explaining Sunsets and Other Atmospheric Phenomena
The same principle explains why sunsets and sunrises often display red or orange hues. When the sun is low on the horizon, its light travels through a much thicker layer of atmosphere. Most of the blue light is scattered away before it reaches our eyes, leaving behind the longer-wavelength red and orange light to dominate the view. Clouds, composed of larger water droplets, scatter all colors equally, leading to their white or gray appearance.