Rainbows appear in that order because sunlight refracts when entering a water droplet, disperses into its different wavelengths, and reflects internally before exiting, separating the colors.
How the phenomenon occurs
The process takes place in three stages:
- Refraction: light enters the droplet and changes speed, bending as it does so.
- Dispersion: each color bends at a different angle depending on its wavelength.
- Internal reflection: light strikes the inner wall of the droplet and exits again, completing the arc.
The optical exit angle is approximately 42 degrees relative to the line connecting the observer's position with the sun's shadow, which explains why we see the arc in the shape of a semicircle.
Order of the colors
The order of the colors, from longest to shortest wavelength, is: red, orange, yellow, green, blue, indigo and violet. This order occurs because the dispersion of light in water depends on wavelength: colors with longer wavelengths (red) bend less, while those with shorter wavelengths (violet) bend more.
| Color | Approximate wavelength (nm) |
|---|---|
| Red | 620‑750 |
| Orange | 590‑620 |
| Yellow | 570‑590 |
| Green | 495‑570 |
| Blue | 450‑495 |
| Indigo | 425‑450 |
| Violet | 380‑425 |
This phenomenon is visible when the sun is low in the sky and rain or dew is present, since a combination of direct light and water droplets in the air is needed for refraction and dispersion to be sufficient.
Rainbows can be double, in which case the second arc shows the colors in reverse order due to a second internal reflection within the droplet.
