Rainbow (optics)
A rainbow is an arc or circle of colored light formed when sunlight or moonlight is refracted, reflected, and dispersed inside water droplets.
Refraction: The change in a wave’s direction and speed as it passes between materials. Light bends as it enters and exits each droplet, setting the rainbow’s viewing angle.
Angle of incidence: The angle between an incoming ray and the normal to the surface it strikes. The entry angle helps determine the paths that concentrate light into the rainbow.
Primary rainbow: The brighter, inner rainbow produced by one internal reflection within water droplets. Its color order and roughly 42-degree radius follow from a single internal reflection.
Halo (optical phenomenon): A ring or arc around the Sun or Moon produced by light interacting with ice crystals. Halos can resemble rainbows but usually form through refraction in ice rather than reflection inside water droplets.
Roy G. Biv: A mnemonic for the conventional sequence red, orange, yellow, green, blue, indigo, and violet. The mnemonic packages a continuous spectrum into the familiar named bands of a rainbow.
Dispersion: The separation of a wave into components because different wavelengths travel differently through a medium. Water bends each visible wavelength by a slightly different amount, spreading white sunlight into colors.
Index of refraction: A dimensionless measure of how much light slows in a material relative to vacuum. Water’s wavelength-dependent index sets the bending angles that separate rainbow colors.
Secondary rainbow: A fainter outer rainbow produced by two internal reflections within water droplets. Its reversed color order and larger radius distinguish it from the primary bow.
Sundog: A bright colored patch beside the Sun, caused by refraction through ice crystals. Its position beside the Sun contrasts with a rainbow’s location opposite the light source.
Visible spectrum: The range of electromagnetic wavelengths detectable by the human eye. Rainbow colors occupy only part of the broader electromagnetic spectrum.