Secondary rainbows are notably dimmer than primary rainbows because the light forming them undergoes an additional reflection inside the raindrop, which significantly spreads the light out more and results in greater light loss. This process inherently reduces the intensity of the light reaching your eyes.
Understanding the Difference in Formation
The key to a secondary rainbow's dimness lies in the path light takes through a raindrop.
Primary Rainbow Formation
A primary rainbow is formed when sunlight enters a raindrop, undergoes one internal reflection off the back surface, and then exits the raindrop. This single reflection is efficient, concentrating the light into a bright arc.
Secondary Rainbow Formation
In contrast, a secondary rainbow forms when sunlight enters a raindrop, undergoes two internal reflections before exiting. Each reflection within the raindrop causes some light to be transmitted out of the raindrop, rather than being reflected inward. Therefore, the more reflections light undergoes, the less light remains to form the visible rainbow.
Factors Contributing to Dimness
Several factors, stemming from the double internal reflection, make secondary rainbows less vibrant:
- Increased Light Loss: With each internal reflection, a portion of the light exits the raindrop instead of staying inside. For a primary rainbow, one reflection means less light loss. For a secondary rainbow, two reflections mean twice the opportunity for light to escape, significantly reducing the intensity of the light that eventually reaches an observer's eye.
- Wider Angular Spreading: The double reflection causes the light to be dispersed over a wider angle in the sky (approximately 50-53 degrees for a secondary rainbow, compared to 40-42 degrees for a primary). When the same amount of light is spread across a larger area, its perceived intensity decreases, making the rainbow appear dimmer.
- Inverted Color Order: A unique characteristic of secondary rainbows is their inverted color order. While a primary rainbow displays colors from red on the outside to violet on the inside, a secondary rainbow shows violet on the outside and red on the inside. This inversion doesn't directly cause dimness but is a visual consequence of the double reflection, further distinguishing it from its brighter counterpart.
Comparing Primary vs. Secondary Rainbows
To summarize the differences that contribute to the varying brightness:
| Feature | Primary Rainbow | Secondary Rainbow |
|---|---|---|
| Reflections | One internal reflection | Two internal reflections |
| Brightness | Brighter, more vivid | Dimmer, fainter |
| Light Loss | Less | More (due to additional reflection) |
| Angular Width | ~40-42 degrees from the antisolar point | ~50-53 degrees from the antisolar point (wider spread) |
| Color Order | Red on top/outside, Violet on bottom/inside | Violet on top/outside, Red on bottom/inside (inverted) |
| Location | Lower in the sky, brighter | Higher in the sky, above the primary, fainter |
How to Spot a Secondary Rainbow
While dimmer, secondary rainbows can often be seen by those who know where to look. They typically appear as a fainter, wider arc above the brighter primary rainbow. Look for the inverted color order to confirm you've found one. They are best observed when the sun is low in the sky and there's a significant amount of rain, providing ample raindrops for the necessary double reflections.
For more information on the physics of rainbows, you can explore resources like NASA's Earth Observatory or physics educational sites.
