Definition
The arco-íris (Portuguese for “rainbow”) is an optical and meteorological phenomenon that appears as a multicoloured circular arc in the sky. It is produced by the interaction of sunlight with water droplets suspended in the atmosphere.
Physical Mechanism
- Refraction – Sunlight enters a spherical water droplet and is refracted, causing the light to spread into its constituent wavelengths.
- Internal Reflection – The light reflects once off the interior surface of the droplet.
- Second Refraction – Upon exiting the droplet, the light is refracted again, further separating the wavelengths.
These processes result in a spectrum ranging from red (outermost edge) to violet (innermost edge). The primary rainbow is observed at an angular radius of approximately 42° from the antisolar point (the direction opposite the Sun). A secondary rainbow, caused by two internal reflections, appears outside the primary arc at about 51° and displays reversed colour order.
Visual Characteristics
- Primary Rainbow: Red, orange, yellow, green, blue, indigo, violet (outer to inner).
- Secondary Rainbow: Colours reversed; typically fainter due to greater light loss during the additional internal reflection.
- Supernumerary Arcs: Fine, closely spaced bands just inside the primary rainbow, resulting from interference effects in droplets of uniform size.
- Moonbow (Lunar Rainbow): Similar phenomenon produced by moonlight; typically appears white to the naked eye because of lower illumination.
Meteorological Conditions
- Occurs when the Sun is low in the sky (typically < 42° elevation) and rain or mist is present in the opposite part of the sky.
- The Sun must be behind the observer, and the raindrops must be in front of the observer.
Geographical Occurrence
Rainbows can be observed worldwide wherever the requisite combination of sunlight and precipitation occurs. Frequent observations are reported in tropical and temperate regions with regular convective activity.
Cultural and Linguistic Context
- In Portuguese‑speaking cultures, the term arco‑íris is commonly used in literature, folklore, and everyday speech to denote both the natural phenomenon and symbolic concepts such as hope, diversity, and peace.
- The term is a compound of “arco” (arc) and “íris” (iris, the coloured part of the eye), reflecting the visual similarity between the rainbow’s arc and the eye’s coloured ring.
Scientific Significance
The study of rainbows has contributed to the development of optics, particularly the understanding of light refraction, dispersion, and interference. Historical experiments by scientists such as René Descartes and Isaac Newton employed rainbow observations to quantify the refractive index of water and to demonstrate that white light comprises a spectrum of colours.
