Visible light is the narrow band of the electromagnetic spectrum that the human eye can detect. It is a continuous wave composed of many colors, each corresponding to a unique physical wavelength. This wavelength determines the hue we perceive, differentiating colors within the visible window between the infrared and ultraviolet regions.
The Physics of Wavelength
Wavelength is the fundamental physical property defining a wave, measured as the distance between two identical points on successive waves. For visible light, this distance is typically expressed in nanometers (nm), often measured from one wave crest to the next. Wavelength is intrinsically linked to frequency, which is the number of wave cycles passing a fixed point per second.
Wavelength and frequency maintain an inverse relationship for all electromagnetic waves traveling at the speed of light. A longer wavelength results in a lower frequency. Conversely, a shorter wavelength corresponds to a higher frequency, establishing the physical basis for the colors within the visible spectrum.
Identifying the Longest Wavelength Color
The color possessing the longest wavelength within the visible spectrum is red. The visible spectrum, which contains all the colors of the rainbow, spans a range generally accepted to be from approximately 380 nanometers to 750 nanometers. Red light occupies the highest end of this range, with wavelengths typically starting around 620 nm and extending up to 750 nm.
This position places red light adjacent to the invisible infrared radiation, which has even longer wavelengths. The sequence of colors in the spectrum, based on increasing wavelength, is:
- Violet
- Blue
- Green
- Yellow
- Orange
- Red
The Inverse Relationship with Energy
Red light carries the lowest energy per photon in the visible spectrum because of its long wavelength. This is due to the inverse relationship between a photon’s wavelength and its energy. Since energy is directly proportional to frequency, the longest wavelength corresponds to the lowest frequency and thus the lowest energy.
In contrast, colors at the opposite end of the spectrum, such as blue and violet, have the shortest wavelengths and highest frequencies. This translates to the highest energy per photon. This energy difference dictates how different colors of light interact with matter.
Visible Effects of Long Wavelengths
The extended wavelength and lower energy of red light influence its behavior in the atmosphere. Red light is less susceptible to Rayleigh scattering, which is the scattering of light by small atmospheric particles like nitrogen and oxygen molecules. Shorter wavelengths, such as blue light, are scattered much more easily, which is why the daytime sky appears blue.
Because red light penetrates the atmosphere more effectively, it can travel longer distances without being scattered. This is demonstrated during sunrises and sunsets when sunlight travels through a greater depth of the atmosphere. As the light path lengthens, shorter-wavelength light is scattered away, allowing the longer-wavelength red and orange light to reach our eyes.