The optical phenomenon that is primarily responsible for the observation of rainbow on a rainy day is

A rainbow is a natural spectrum formed in the sky through the interaction of sunlight with atmospheric water droplets [1]. The primary optical phenomenon responsible for the separation of white sunlight into its constituent colors is dispersion [1]. This occurs because the refractive index of water varies with the wavelength of light, causing different colors to bend at slightly different angles as they enter and exit the droplet. While the complete process involves refraction, internal reflection, and a second refraction, it is the dispersion of light that specifically creates the visible color spectrum [2]. The water droplets essentially act as small prisms, refracting and dispersing incident sunlight before reflecting it internally [1]. Consequently, while reflection and refraction are necessary steps in the path of the light ray, dispersion is the fundamental cause of the rainbow's characteristic colors [1].

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] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 24: Hydrological Cycle (Water Cycle) > Explanation: > p. 335