The colours of a rainbow are caused by:

A rainbow is a meteorological phenomenon caused by the interaction of sunlight with water droplets in the atmosphere. The formation of a rainbow involves three main physical processes: 1. **Refraction:** As sunlight enters a spherical water droplet, it slows down and bends because light travels slower in water than in air. 2. **Dispersion:** White light is composed of different wavelengths (colors). In a medium like water, different wavelengths travel at different speeds, meaning they bend by different amounts. This separation of white light into its constituent colors (VIBGYOR) is called **dispersion**. According to Cauchy's relation, the refractive index $n$ is approximately: $$n(\lambda) \approx A + \frac{B}{\lambda^2}$$ Since violet light has a shorter wavelength than red light, it refracts more. 3. **Total Internal Reflection:** The light hits the back surface of the droplet and reflects internally. 4. **Exit Refraction:** The light refracts again as it exits the droplet, further spreading the colors. Therefore, the primary cause of the colors is the **dispersion of white light by water droplets**.