1. Why It Is Hard to Say Where One Rainbow Color Ends and the Next Begins
If you try to count the colors of a rainbow from one end to the other, you might stop between blue and green. It is hard to draw a line showing where blue ends and green begins. Canon’s explanation for children states that the boundaries in a rainbow are ambiguous, forming bands where each color blends vaguely into the next [1]. Depending on how you look, you might see colors like blue-green between blue and green, and it can appear that there are more than seven colors [1].
Imagine a gentle slope with seven flags planted on it. The slope continues without breaks, and the position of the flags is decided by people. However, the location of the flags is not fixed by the rainbow’s color sequence itself. It is clearer to think of the seven colors as divisions that people placed into a continuous band.
2. How Raindrops Bend Sunlight and Split It into Colors
A rainbow begins when sunlight enters round raindrops in the air. When light enters water, it bends, and because the amount of bending differs by color, whitish sunlight separates into different colors [2]. The separated light reflects inside the drop and then exits [2].
In the primary rainbow created this way, red appears on the outside and violet on the inside. It is seen at an angle of about 42 degrees from the point opposite the sun, known as the antisolar point [5]. The number "7" does not appear anywhere in this explanation of the mechanism. The band of colors forms simply because light bends differently depending on its color. The reason for counting the band as seven parts was separate from the physics of light.
3. How Many Colors Did People See in Rainbows Before Newton?
It is often explained that before Newton published his work, rainbows were considered to have 3 or 5 colors [1]. However, this varied by region and era, so it does not mean everyone counted them this way.
Newton’s own words tell a slightly different story. Another manuscript by Newton suggests that he listed five colors—red, yellow, green, blue, and purplish violet—as the basis, while mentioning orange and indigo alongside "innumerable intermediate stages" [6]. The English Wikipedia article on rainbows explains that Newton initially divided the spectrum into 5 colors and later added orange and indigo [5]. Therefore, for Newton, seven was not a number "decided by nature." He divided it into seven by adding orange and indigo to match musical scales. The reason for adding them is explored in the next section.
4. Why Newton Chose Seven Colors: The Link to Musical Scales
According to Canon’s explanation, Newton chose seven colors because he concluded that the width of each color band corresponded to the intervals between notes in a musical scale [1]. In Newton’s book *Opticks*, there is a description where he divides a circle into seven parts, assigning red, orange, yellow, green, blue, indigo, and violet in order, and matching their ratios to the intervals of a musical scale [4].
Orange and indigo are said to correspond to the positions of semitones in the scale. Newton reportedly thought that violet was like a repetition of red. This is similar to how a note one octave higher returns to the same name in a musical scale [3]. The important point is that Newton did not "discover" seven colors; he "divided" the color band following musical scales. Canon’s explanation notes that Newton himself knew there were infinite colors [1]. The number seven came from applying the ruler of music to the band.
5. How the Number of Rainbow Colors Differs by Country and Language
Some Japanese articles give examples such as 6 colors in the United States and the UK, 5 colors in Germany, and 3 colors in Taiwan [7]. A column in the Ryukyu Shimpo also writes that the US uses 6 colors by not separating indigo, while Germany and Russia use 5 [8]. Thus, "rainbows have seven colors" is primarily a Japanese convention [7].
However, these are guidelines based on textbooks and customs, not a claim that every citizen sees it this way. The English Wikipedia article on rainbows explains that the number of colors distinguished and their names depend on the language used [5]. The light seen is not different; rather, the counting method and names are different.
In Japan, after Western science and education systems were introduced following the Meiji era, the seven-color theory was adopted into textbooks, establishing the current seven colors [9]. Until the Edo period, rainbows were described using 3 or 5 colors [9]. It is necessary to distinguish between when seven colors spread in Japan and when they were first introduced.
6. How to Make a Rainbow with Water Spray and Look for Color Boundaries
On a sunny day, stand with the sun behind you and spray fine water with a spray bottle. If successful, you will see a small rainbow in front of you. If you see it, try counting the colors yourself. You can also give your own name to the color between blue and green. Whether you count 7 or 6, the band itself does not change. Do not look directly at the sun.
Ask people close to you how many colors they see in a rainbow; the answers may differ. This difference is likely due to counting methods and names, rather than differences in the light itself.
If you want to learn more, Newton’s *Opticks* is the original English text. Libraries and history of science books may have Japanese explanations. The materials used in this article are listed in the source notes.