Secondary rainbows are significantly dimmer than primary rainbows primarily because the light forming them undergoes an additional reflection inside the raindrop, which causes more light to be lost and spread out.
The Core Reason: Extra Reflection
The fundamental difference between primary and secondary rainbows lies in the number of internal reflections light undergoes within a raindrop before it reaches your eyes. This difference directly impacts their brightness.
- Primary Rainbow: A primary rainbow is formed when sunlight enters a raindrop, refracts (bends) as it enters, reflects once off the back inner surface of the droplet, and then refracts again as it exits, creating the familiar arc. This single reflection is highly efficient in terms of light preservation.
- Secondary Rainbow: In contrast, a secondary rainbow is formed when sunlight refracts as it enters the raindrop, reflects twice inside the droplet, and refracts again while exiting. This additional reflection causes the light to spread out more and results in a larger, dimmer rainbow with the order of colors reversed.
Light Loss and Spreading
Each time light reflects inside a raindrop, some of its energy is lost. This loss occurs due to several factors:
- Absorption: A small amount of light energy is absorbed by the water itself.
- Scattering: Not all light reflects perfectly; some is scattered or diffused in various directions.
- Transmission: A portion of the light that hits the back surface of the raindrop actually passes through the water-air interface and exits the droplet, rather than reflecting back internally. This transmitted light does not contribute to the rainbow.
When light undergoes a second internal reflection for a secondary rainbow, it experiences these losses twice, significantly diminishing its intensity compared to light that only reflects once. The extra reflection also causes the light to spread out more broadly over a larger angular area, further reducing its perceived brightness. Think of it like spreading the same amount of paint over a much larger canvas; the color appears fainter.
Visual Differences at a Glance
Here’s a quick comparison highlighting the key distinctions between primary and secondary rainbows:
| Feature | Primary Rainbow | Secondary Rainbow |
|---|---|---|
| Brightness | Much Brighter (more intense and vivid) | Much Dimmer (fainter, often harder to spot) |
| Reflections | One internal reflection | Two internal reflections |
| Color Order | Red on the outside of the arc, violet on the inside | Violet on the outside of the arc, red on the inside |
| Angular Radius | Approximately 40-42 degrees from the anti-solar point | Approximately 50-53 degrees from the anti-solar point |
| Appearance | Often stands alone, clearly visible | Appears outside the primary, wider, and hazy |
| Dark Band | No dark band between it and the secondary (Alexander's Dark Band is between them) | Appears outside the Alexander's Dark Band |
For more details on light and optics, you can explore resources like those from NASA or educational science museums like the Exploratorium.
Observing Secondary Rainbows
Spotting a secondary rainbow requires the right conditions: strong sunlight and sufficiently large raindrops. You will typically see it as a faint, wider arc positioned outside the brighter primary rainbow. Look closely, and you might even notice the reversed color order, with violet on the outside.
In essence, the additional internal reflection and the resulting increased light loss and spreading are why secondary rainbows are consistently fainter than their primary counterparts.
