Explanation:
A rainbow is a natural phenomenon that occurs when light is refracted, reflected, and dispersed in water droplets in the atmosphere. It is formed due to a combination of dispersion and total internal reflection.

Dispersion:
Dispersion is the phenomenon where light is separated into its component colors as it passes through a medium. This occurs because different colors of light have different wavelengths and therefore different speeds when passing through a medium like water. When light enters a water droplet, it is refracted (bent) and dispersed into its component colors.

Total Internal Reflection:
Total internal reflection occurs when light traveling from a denser medium (e.g. water) to a less dense medium (e.g. air) is reflected back into the denser medium instead of being refracted. This happens when the angle of incidence of the light is greater than the critical angle, which depends on the refractive indices of the two media.

Formation of a Rainbow:
When sunlight passes through water droplets in the atmosphere, it is refracted and dispersed into its component colors. The different colors of light then undergo multiple reflections and refractions inside the droplets. Some of the light is reflected back into the droplet, while the rest is refracted out of the droplet. The refracted light then exits the droplet and spreads out, creating a circular arc of colors in the sky.

The primary rainbow is formed by a single internal reflection, while the secondary rainbow is formed by two internal reflections. The colors in the rainbow are arranged in a specific order, with red on the outer edge and violet on the inner edge.

Conclusion:
In conclusion, a rainbow is formed due to a combination of dispersion and total internal reflection. Dispersion separates the white light into its component colors, while total internal reflection causes the light to reflect back and create the circular arc of colors that we see in the sky.