Primary rainbows are sometimes referred to as first-order bows. First-order rainbows are produced when light is reflected once as it passes through the interior of each raindrop.
Secondary rainbows are second-order bows. Second-order bows are produced when light is reflected twice as it passes through the interior of each raindrop.
- Each subsequent order of rainbows involves an additional reflection inside raindrops.
- Higher-order bows get progressively fainter because photons escape droplets after the final reflection. As a result, insufficient light reaches an observer to trigger a visual response.
- Each higher-order of bow gets progressively broader spreading photons more widely and reducing their brightness further.
- Only first and second-order bows are generally visible to an observer but multi-exposure photography can be used to capture them.
- Different orders of rainbows don’t appear in a simple sequence in the sky.
- First, second, fifth and sixth-order bows all share the same anti-solar point.
- Zero, third and fourth-order bows are all centred on the Sun and appear as circles of colour around it.