The light traveling from the Sun to Earth is electromagnetic radiation, composed of a spectrum of colors perceived as white light. This solar energy must first penetrate the Earth’s atmosphere, a vast envelope of gases that actively interacts with the incoming light. The atmosphere acts like a complex, natural filter, selectively altering which wavelengths are allowed to pass straight through to the surface. This filtering modifies the color and apparent position of light sources we see in the sky.
Understanding Atmospheric Scattering
The primary mechanism that determines how light is filtered by the atmosphere is a process known as Rayleigh scattering. This phenomenon occurs when light interacts with particles that are significantly smaller than its wavelength, such as the nitrogen and oxygen molecules that constitute the bulk of the air. The effectiveness of this scattering is intensely dependent on the light’s wavelength, following an inverse fourth power relationship. This means that a shorter wavelength of light is scattered much more powerfully than a longer one. Because of this physical rule, the shorter-wavelength colors at the blue and violet end of the visible spectrum are scattered approximately four times more effectively than the longer-wavelength red light.
When sunlight enters the atmosphere, the blue and violet components are immediately dispersed in all directions by the omnipresent air molecules. This widespread scattering of blue light is precisely what gives the daytime sky its characteristic color. This process of scattering essentially strips the direct beam of sunlight of its blue components. The more atmosphere the light has to travel through, the more of the shorter-wavelength light is removed from the direct path.
How Atmospheric Refraction Bends Light
Distinct from the filtering effect of scattering is the phenomenon of atmospheric refraction, which affects the light’s trajectory rather than its color composition. Refraction is the bending of light as it passes through layers of air with varying densities, much like light bending when it moves from air into water. Air density is highest near the surface and decreases with altitude, creating a gradient that continuously slows and bends light rays. When light from a distant object, like the Sun or Moon, enters the atmosphere at a shallow angle near the horizon, it must pass through the maximum amount of this dense, layered air.
This prolonged passage causes the light to bend downward, following a curved path toward the observer’s eye. The bending effect is so pronounced at low angles that it makes a celestial body appear slightly higher in the sky than its true geometric position. This means that when you see the bottom edge of the Sun just touching the horizon at sunset, the entire solar disk is already below the true horizon line. Refraction is responsible for the distortion and apparent displacement of celestial objects, clarifying that it deals with the perceived location, not the removal of specific color wavelengths.
The Color That Dominates Long Paths
The color that passes most successfully through the Earth’s atmosphere is red light, followed closely by orange light. This outcome is a direct consequence of the scattering process, or rather, the lack thereof. Red light possesses the longest wavelength in the visible spectrum, which makes it the least susceptible to Rayleigh scattering by the tiny gas molecules in the air. Because red light is scattered so infrequently, it maintains its original, straight-line path through the atmosphere with minimal deflection. When a beam of white sunlight traverses a particularly long column of the atmosphere, such as when the Sun is rising or setting, virtually all the shorter, more scatterable wavelengths are stripped away.
Visualizing the Effect on Celestial Bodies
The most compelling real-world evidence of this atmospheric filtering is observed when viewing celestial bodies near the horizon. During sunrise or sunset, the Sun often takes on a deep orange or red hue because its light is traveling through the maximum possible extent of the atmosphere. The intense filtering removes the blue, green, and yellow light, leaving the dominant red and orange wavelengths to color the solar disk. When the Moon is low on the horizon, it can appear golden, copper, or even rusty-red, depending on the amount of dust and aerosols present in the air. Additionally, the same low-angle passage through the atmosphere that causes the reddening also results in the visible flattening of the Sun or Moon. This slight distortion is caused by differential refraction, where the light from the lower edge is bent more than the light from the upper edge.