The colour pattern in a secondary rainbow is inverted compared to a primary rainbow, with red on the innermost side and violet on the outermost side.
Understanding the Secondary Rainbow
A secondary rainbow, often appearing fainter and wider than the primary rainbow, forms due to a different light path within the raindrops. While a primary rainbow results from a single internal reflection of sunlight within water droplets, a secondary rainbow is created by two internal reflections inside the raindrops. This double reflection is the key reason for its inverted colour pattern and reduced brightness.
Colour Order in Detail
The specific order of colours in a secondary rainbow is a striking reversal:
- Red: Appears on the innermost edge of the arc.
- Orange
- Yellow
- Green
- Blue
- Indigo
- Violet: Appears on the outermost edge of the arc.
This sequence can be remembered as an inverse of the common "ROYGBIV" mnemonic for primary rainbows, or simply as VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red) read from outer to inner.
Primary vs. Secondary Rainbows: A Comparative Look
Understanding the differences between primary and secondary rainbows helps to appreciate their distinct appearances and formation mechanisms.
| Feature | Primary Rainbow | Secondary Rainbow |
|---|---|---|
| Formation | Single internal reflection of sunlight in raindrops | Two internal reflections of sunlight in raindrops |
| Brightness | Brighter and more vivid | Fainter and less distinct |
| Arc Position | Appears lower in the sky, at about 40-42 degrees | Appears higher in the sky, at about 50-53 degrees |
| Colour Order | Violet innermost, Red outermost | Red innermost, Violet outermost (inverted) |
| Arc Width | Narrower | Wider |
| Dark Band | Lies outside the primary arc, towards the secondary | Lies between the primary and secondary arcs (Alexander's Dark Band) |
For more scientific details on rainbow optics, explore resources on light refraction and reflection in water droplets. Learn about Rainbow Optics
How to Spot a Secondary Rainbow
While not as common or as bright as primary rainbows, secondary rainbows can be observed under the right conditions:
- Look for Faintness: They are typically much dimmer than primary rainbows, making them harder to spot.
- Position Relative to Primary: The secondary rainbow always appears above the primary rainbow, further away from the sun's anti-solar point.
- Inverted Colours: Once spotted, confirm it's a secondary rainbow by noting the reversed colour order – red on the inside, violet on the outside.
- Ideal Conditions: Look for them during powerful rain showers with bright sunshine directly opposite you, as these conditions maximize the chance of seeing both arcs.
The phenomenon of the secondary rainbow is a beautiful testament to the complex interaction of light with water droplets, offering a slightly different perspective on the spectrum of colours.
