How many internal reflections of light take place in the formation of primary rainbow ?

A primary rainbow is a natural spectrum formed by the dispersion of sunlight through water droplets in the atmosphere [1]. When sunlight enters a raindrop, it undergoes two refractions and exactly one internal reflection [2]. Specifically, the light refracts as it enters the drop, reflects once off the back inner surface, and refracts again as it exits [1]. This single internal reflection concentrates light at an angular deviation of approximately 42, creating the primary bow [2]. In contrast, a secondary rainbow is formed by two internal reflections, which results in a fainter appearance and reversed color order [2]. While higher-order rainbows involving three or more reflections exist, they are rarely visible to the naked eye. Therefore, the primary rainbow is characterized as a first-order rainbow because it involves only one internal reflection.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 10: The Human Eye and the Colourful World > Activity 10.2 > p. 167
2. [2] https://www.nesdis.noaa.gov/about/k-12-education/optical-phenomena/what-causes-rainbow