The sun and sky display a captivating array of colors, from daytime blue to dramatic reds and oranges during sunrise and sunset. These vibrant transformations are natural atmospheric events, driven by how sunlight interacts with Earth’s atmosphere. This interaction creates the visual phenomena we observe.
The Basics of Light and Color Perception
Light is a form of electromagnetic radiation that travels in waves. The length of a light wave determines its color. Visible light, the portion human eyes can detect, ranges from about 380 to 780 nanometers.
White light, like sunlight, is a combination of all colors in the visible spectrum. When white light passes through a prism, it separates into its constituent colors: red, orange, yellow, green, blue, indigo, and violet. Our eyes perceive these different wavelengths as distinct colors through specialized cone cells in the retina. These cones send signals to the brain, allowing us to distinguish millions of colors.
Why the Sky is Blue and Sun is Red at Times
Rayleigh scattering explains the sky’s blue color and the sun’s reddish appearance. This process involves light scattering by particles much smaller than its wavelength, primarily nitrogen and oxygen molecules in Earth’s atmosphere. Shorter wavelengths, such as blue and violet, scatter more efficiently than longer wavelengths like red and orange.
During the day, sunlight enters the atmosphere, scattering blue light in all directions. This scattered blue light reaches our eyes, making the sky appear blue. While violet light scatters more, our eyes are more sensitive to blue, which is why the sky does not look violet. As sunlight travels through the atmosphere, much of the blue and violet light is redirected, leaving the direct path of sunlight with a slightly yellowish tint.
At sunrise or sunset, sunlight travels through a significantly greater thickness of Earth’s atmosphere. This extended path scatters even more short-wavelength blue and violet light. Consequently, longer wavelengths, primarily red and orange light, are less scattered and penetrate the atmosphere more directly. This causes the sun and surrounding sky to take on characteristic red, orange, and pink hues.
Factors That Enhance Redness
Beyond Rayleigh scattering, other atmospheric components can intensify the sun’s red appearance. Larger particles, such as dust, smoke, and volcanic ash, influence how light is scattered. These particles are often comparable in size to visible light wavelengths, causing them to scatter light less selectively by color than smaller gas molecules. Instead, they scatter all wavelengths more evenly, which can make the sky appear hazy or whitish.
These larger particles also filter out more blue and green light, allowing red and orange wavelengths to dominate. For instance, wildfire smoke introduces fine particles, leading to vivid red or blood-orange skies at sunrise or sunset. Volcanic eruptions inject ash and sulfur dioxide into the stratosphere, which can remain aloft for years, producing intense yellow-red horizons and purple-pink glows. The 1883 Krakatoa eruption caused pronounced red sunsets globally.
Observing a Red Sun
While a red sun is beautiful, direct observation requires caution. Even when red, the sun’s ultraviolet (UV) and infrared radiation remain harmful. Looking directly at the sun, regardless of its color or position, can cause solar retinopathy, damaging the retina. This damage can occur quickly, leading to blurry vision, blind spots, or light discomfort.
Therefore, direct observation without proper eye protection, such as certified solar filters or eclipse glasses, is not recommended. While redness might suggest reduced intensity, harmful invisible rays are still present. Occasionally, a vivid red sun might signal high levels of atmospheric pollutants like smoke or dust. While striking, this can indicate air quality issues, though eye safety remains the primary concern.