Rainbows are one of nature’s most beautiful and inspiring phenomena, often appearing after a rain shower when sunlight passes through water droplets in the atmosphere. A striking feature of rainbows is that they are typically described as having exactly seven distinct colors: red, orange, yellow, green, blue, indigo, and violet. But why seven? The answer lies in the physics of light, the way the human eye perceives color, and even the cultural history of how colors have been categorized. Exploring why rainbows have exactly seven colors connects optics, perception, and history in fascinating ways.
Rainbows form when sunlight interacts with water droplets through three main processes:
This separation produces a continuous spectrum of colors, from long-wavelength red to short-wavelength violet, creating the arc we see as a rainbow.
Sunlight contains all visible wavelengths of light, ranging roughly from 380 nanometers (violet) to 750 nanometers (red). When dispersed, this produces a continuous spectrum with countless possible shades. In reality, a rainbow does not have just seven colors—there is an infinite range of wavelengths blending into each other. The “seven colors” are a human simplification.
The reason rainbows are traditionally described as having seven colors dates back to Sir Isaac Newton in the 17th century. While experimenting with prisms, Newton discovered that white light could be split into a spectrum of colors. He initially identified five main colors: red, yellow, green, blue, and violet. Later, influenced by his belief in the harmony of the natural world, he added orange and indigo to make the total seven—matching the seven musical notes, seven known planets, and other symbolic sevens in nature and culture.
According to Newton’s classification, the rainbow is divided into the following seven colors:
| Color | Approximate Wavelength Range (nm) |
|---|---|
| Red | 620–750 |
| Orange | 590–620 |
| Yellow | 570–590 |
| Green | 495–570 |
| Blue | 450–495 |
| Indigo | 425–450 |
| Violet | 380–425 |
Although the rainbow is a continuous gradient, the human brain tends to group colors into categories. Our eyes contain three types of cone cells sensitive to short, medium, and long wavelengths. This trichromatic vision allows us to distinguish between different hues, but our perception is influenced by cultural and linguistic conventions. Because Newton defined seven categories, people have been taught to see seven bands, even though there are no sharp boundaries between them.
Not all cultures identify exactly seven rainbow colors. Some traditions count fewer, while others identify more. For example:
This shows that the “seven colors” of a rainbow are not a universal fact of nature but a convention shaped by science and history.
Indigo is the most debated of the seven rainbow colors. Many scientists and educators argue that indigo is difficult to distinguish from blue and violet, and modern depictions of rainbows often omit it. Nonetheless, indigo remains part of the traditional seven-color sequence, reflecting Newton’s original categorization rather than strict visual perception.
Rainbows actually extend beyond the visible spectrum. Infrared light (longer than red) and ultraviolet light (shorter than violet) are also present but cannot be seen by the human eye. Some animals, such as birds and insects, can perceive ultraviolet patterns in nature, meaning their “rainbows” would look different from ours.