The spread in colours in a rainbow on sky is primarily due to

The spread of colors in a rainbow is primarily due to the dispersion of sunlight. When sunlight enters a spherical water droplet, it undergoes refraction and is separated into its component colors—a process known as dispersion [1]. While refraction occurs as light enters and exits the droplet, and internal reflection occurs at the back surface of the drop, it is the dispersion that specifically causes the white light to split into a spectrum of colors [2]. Water droplets act like tiny prisms, bending different wavelengths of light at slightly different angles [1]. This separation is further enhanced as the light exits the droplet after one internal reflection (for a primary rainbow), resulting in the distinct bands of color visible to an observer [3]. Therefore, while multiple optical phenomena are involved, dispersion is the fundamental cause of the color spread.

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
3. [3] https://www.britannica.com/science/What-Causes-a-Rainbow