Light is a form of electromagnetic energy that travels through space in waves. Our perception of color is entirely dependent on the characteristics of these waves, which carry the energy from a source to our eyes. When white light, such as sunlight, passes through a medium like water droplets in the atmosphere, it separates into its component colors, creating the familiar visible spectrum we recognize as a rainbow. The specific color we see is directly related to a fundamental property of that light wave.
What is Wavelength?
Wavelength is the physical distance between two successive peaks or troughs of a wave as it travels. Light waves are characterized by this distance, which is typically measured in nanometers (one billionth of a meter) within the visible spectrum.
This measurement is inversely linked to both the wave’s frequency and its energy. A shorter wavelength results in a higher frequency and carries a greater amount of energy. Conversely, a longer wavelength corresponds to a lower frequency and less energy. The specific wavelength of light determines its color in the visible spectrum.
Red: The Longest Wavelength Color
The color with the longest wavelength in the visible spectrum is red. The visible light spectrum, often remembered by the mnemonic ROYGBIV, arranges the colors from longest (red) to shortest (violet) wavelength.
Red light waves occupy the range of approximately 620 to 750 nanometers. Red light possesses the lowest frequency and the least amount of energy per photon among all the colors we can see. Just beyond the red end of the visible spectrum lies infrared radiation, which has even longer wavelengths and is invisible to the human eye. The boundary of the visible spectrum is not sharply defined, but the red end marks the point where the wave’s distance becomes too long for our photoreceptor cells to register as visible light.
How Wavelength Affects What We See
The varying wavelengths of light have profound effects on how we perceive the world, especially in the presence of Earth’s atmosphere. Shorter wavelengths, such as blue and violet light, interact much more readily with the tiny gas molecules of nitrogen and oxygen in the air. This phenomenon, known as scattering, redirects the light in all directions across the sky.
This preferential scattering of shorter wavelengths is why the sky appears blue during the day. Longer wavelengths, particularly red and orange, are less affected by these small molecules and tend to travel in a straighter path through the atmosphere. This ability to penetrate the air is why red is often used for stop signs and warning lights, as the signal remains clear over a long distance.
During sunrise and sunset, sunlight must travel through a much greater thickness of the atmosphere to reach our eyes. Over this long journey, most of the shorter-wavelength blue and green light is scattered away completely. What remains are the longer-wavelength colors—red and orange—which are able to pass through the dense layer of air, creating the brilliant, warm hues of a sunset.