A rainbow is fundamentally a continuous spectrum of light, not seven discrete colored bands. When sunlight enters water droplets in the air, it is refracted (bent) upon entry, reflected internally, and refracted again upon exit. Because different wavelengths bend at slightly different angles—red bending least and violet bending most—white light spreads into a spectrum with red on the outer edge and violet on the inner edge.

The idea that a rainbow has exactly seven colors—red, orange, yellow, green, blue, indigo, and violet—is a historical convention established by Isaac Newton in the 17th century. Newton initially identified fewer colors but later added orange and indigo to bring the total to seven, a choice driven by non-scientific reasoning. He was influenced by the ancient Greek belief that seven was a sacred or harmonious number, and he specifically wanted the spectrum's color count to match the seven notes of the Western musical scale.

In reality, the human eye can distinguish hundreds of different hues within a rainbow's arc. Moreover, indigo is particularly difficult for most people to distinguish from blue or violet, and many modern color models omit it entirely, using a six-color division instead. Some sources also note that the spectrum extends into ultraviolet and infrared ranges, which are present in the rainbow but invisible to human vision.

Therefore, the seven-color rainbow is better understood as a cultural and pedagogical legacy than as a strict scientific fact about the nature of light or rainbows themselves.