The sun’s appearance changes throughout the day, particularly at sunrise and sunset. Its transformation to vivid reds, oranges, and pinks is a common spectacle. This display results from interactions between sunlight and Earth’s atmosphere, governed by principles of light.
The Nature of Light and Color
Sunlight, which appears white, is a composite of various colors, much like a rainbow. Each color corresponds to a different wavelength within the electromagnetic spectrum. Violet and blue light have shorter wavelengths, while red and orange light possess longer wavelengths. Our eyes perceive these distinct wavelengths as different colors through specialized cells called cones in the retina. These cells are sensitive to varying light wavelengths, allowing us to differentiate between millions of hues.
Atmospheric Light Scattering
Earth’s atmosphere consists primarily of tiny gas molecules, predominantly nitrogen (about 78%) and oxygen (about 21%). When sunlight enters the atmosphere, these molecules interact with the light, causing it to scatter. This phenomenon is known as Rayleigh scattering, named after the 19th-century physicist Lord Rayleigh. Rayleigh scattering is particularly effective for light with shorter wavelengths, such as blue and violet light. These shorter wavelengths are scattered much more efficiently by the small atmospheric particles compared to longer wavelengths like red and orange light.
The Journey of Red Light
The amount of atmosphere sunlight traverses influences the colors we perceive. When the sun is high, sunlight travels a relatively short path through the atmosphere. During this journey, some blue light is scattered, giving the sky its characteristic blue hue. At sunrise and sunset, the sun is low on the horizon, meaning its light must travel through a greater thickness of the atmosphere, where most shorter-wavelength blue and violet light is scattered away and dispersed. Consequently, the longer-wavelength red and orange light, which scatters less, travels more directly to our eyes, resulting in the sun’s reddish appearance.
Environmental Influences on Color
Beyond normal atmospheric scattering, larger particles in the air can intensify the sun’s red and orange hues. These particles, larger than gas molecules, include dust, smoke from wildfires or pollution, and volcanic ash. Such particles can scatter or absorb more shorter-wavelength light, allowing a greater proportion of red and orange light to pass through. For example, major volcanic eruptions can lead to vibrant red sunsets for extended periods. Similarly, smoke from distant wildfires or wind-carried dust can enhance the redness of sunsets by adding more scattering agents to the atmosphere.