Natural light, originating from the sun, is a complex blend of many different wavelengths of energy. Outside of Earth’s atmosphere, sunlight is considered spectrally neutral or “white” because it contains an equal distribution of all colors in the visible spectrum. This energy travels as electromagnetic radiation. The color we perceive is simply the human eye’s interpretation of a narrow band of these wavelengths.
Atmospheric Effects on Perceived Color
The color of natural light appears to change dramatically throughout the day due to the Earth’s atmosphere, which acts like a selective filter. This filtering is explained by Rayleigh scattering, involving the interaction of sunlight with tiny gas molecules like nitrogen and oxygen. These molecules are smaller than visible light wavelengths, causing them to scatter shorter wavelengths more effectively than longer ones.
Because blue and violet light have the shortest wavelengths, they are scattered across the sky much more intensely than the longer red and orange wavelengths. We see this scattered blue light, which is why the sky appears blue during the day. The sunlight reaching our eyes directly has had a portion of its blue light removed by this scattering, causing the sun itself to appear slightly yellowish.
At sunrise and sunset, the sun sits low on the horizon, forcing its light to travel through a much greater thickness of the atmosphere. This extended journey means nearly all of the short-wavelength blue and much of the green light is scattered away. The remaining light consists predominantly of the longer, less-scattered wavelengths. This leaves the reds, oranges, and yellows to dominate the horizon, creating the warm, vibrant colors seen during twilight hours.
Measuring Light Color Using Correlated Color Temperature
To quantify the perceived color of light, scientists use Correlated Color Temperature (CCT), expressed on the Kelvin (K) scale. CCT compares the light emitted by a source to the color of a theoretical black body radiator as it is heated. As this body heats up, it glows, shifting from deep red through yellow and white, eventually reaching blue, with each color corresponding to a specific temperature in Kelvin.
Lower Kelvin temperatures represent warmer, more yellowish light, while higher Kelvin temperatures represent cooler, more bluish light. For instance, light just after sunrise or before sunset is often measured below 3000K, appearing distinctly orange or reddish. This low CCT reflects the atmospheric filtering that removes the cooler blue components.
As the sun climbs toward midday, the light travels through the least amount of atmosphere, resulting in a higher CCT. Direct midday sunlight typically falls within the range of 5500K to 6500K, perceived as a bright, neutral white or slightly bluish light. Even an overcast sky, where light is diffused by clouds, can yield a CCT exceeding 7000K, appearing cool and blue due to the dominance of scattered light.
The Complete Electromagnetic Spectrum of Natural Light
The visible colors—red, orange, yellow, green, blue, indigo, and violet—represent only a tiny fraction of the sun’s total output. Natural light is more accurately defined by the entire electromagnetic spectrum, which includes many types of radiation invisible to the human eye. This vast spectrum ranges from high-energy gamma rays to low-energy radio waves.
The energy just beyond the violet end of the spectrum is Ultraviolet (UV) radiation, which has shorter wavelengths and higher energy. Although only about 7% of the sun’s energy reaching Earth is UV, it is responsible for effects like sunburn and Vitamin D production. Conversely, the energy past the red end of the spectrum is Infrared (IR) radiation, which possesses longer wavelengths and lower energy.
Nearly half of the sun’s total energy output, approximately 48%, reaches Earth as infrared radiation. We do not see this energy, but we perceive its presence as heat warming the Earth’s surface.