A circular rainbow, also known as a full-circle rainbow or a glory, is a spectacular optical phenomenon created when sunlight interacts with water droplets in the atmosphere. Though most commonly seen as an arc, a rainbow is inherently circular.
The Science Behind Rainbows
The formation of a rainbow, whether an arc or a full circle, is a complex interplay of light and water droplets. Here's a breakdown of the key stages:
1. Light Entering the Raindrop (Refraction)
Sunlight, which appears white, is actually composed of a spectrum of colors. When sunlight encounters a spherical raindrop, it first enters the droplet. As light moves from one medium (air) to another (water), it changes speed and direction. This bending of light is called refraction. Different colors of light bend at slightly different angles, causing them to separate.
2. Light Reflecting Inside the Raindrop (Reflection)
After entering the raindrop, the light travels to the back inner surface of the droplet. Here, it undergoes total internal reflection, bouncing off the inside back wall as if it were a mirror.
3. Light Exiting the Raindrop (Second Refraction)
Finally, the reflected light travels back through the raindrop and exits it, bending once more as it passes from water back into the air. This second refraction further separates the colors, making them more distinct.
The critical factor in rainbow formation is that the light exits the raindrop at a very specific angle relative to the angle it entered. For visible rainbows, this angle is consistently between 40 and 42 degrees away from the angle the light initially entered the raindrop. This precise angular relationship is what defines the shape of the rainbow.
Why Rainbows Appear Circular
For an observer, a rainbow forms because all the raindrops that are at this specific 40 to 42-degree angle relative to your eye and the sun appear to illuminate. Since this angle is constant, the collection of all such raindrops forms a perfect circle (or part of one) with the observer's eye at its center. The center of the rainbow circle is always directly opposite the sun.
Let's visualize the light's journey:
| Stage | Description | Key Phenomenon |
|---|---|---|
| 1. Refraction In | Sunlight enters the raindrop, bending and beginning to separate into colors. | Light Bending |
| 2. Reflection | Light hits the back inner surface of the raindrop and bounces back. | Internal Bounce |
| 3. Refraction Out | Light exits the raindrop, bending again and fully separating into its component colors (spectrum). | Color Dispersion |
| 4. Observation | The separated light reaches the observer's eye at a precise angle (40-42 degrees) from the incoming sunlight, forming the rainbow arc or circle. | Rainbow Visibility |
Conditions for Seeing a Full Circle Rainbow
While a rainbow is inherently circular, we usually only see an arc or a half-circle from the ground. This is because the horizon (the ground) blocks the lower portion of the circle.
To observe a full circular rainbow, two main conditions are usually required:
- Elevated Vantage Point: The most common way to see a full circular rainbow is from an elevated position, such as from an airplane flying above the clouds, or from a very high mountain. From such a height, there's no ground to obstruct the lower part of the circular arc.
- Sun Position: The sun must be behind the observer and low in the sky, preferably just above the horizon. The lower the sun, the higher the center of the rainbow circle will be relative to the observer, making more of the arc visible above the ground. If the sun is very low in the sky, either just before sunset or just after sunrise, we can see a half circle. When the sun is directly overhead, a rainbow cannot be seen from the ground at all, as the entire circle would be below the horizon.
In essence, a full circle rainbow is not a different type of rainbow, but rather the complete manifestation of the same optical phenomenon that produces the familiar arc, simply seen under optimal viewing conditions.
For more detailed information on light and optics, you can explore resources from organizations like NASA or National Geographic.
