noun
secondary rainbow
The fainter, outer rainbow with reversed colors, formed by double reflection inside water droplets. Technical term in meteorology and optics.
主虹の外側に、うっすらと副虹が見えた。
A faint secondary rainbow was visible outside the primary rainbow.
副虹は主虹より色の順序が逆になっている。
The secondary rainbow has its colors in reverse order compared to the primary rainbow.
Compound of 副 (ふく, 'secondary, auxiliary') and 虹 (にじ, 'rainbow'). The reading ふくこう is a less common on'yomi variant.