The process of splitting of white light into seven colours is called dispersion of light.
Example: Formation of Rainbow during a cloudy day.
Step 1: Sunlight passes through the raindrops.
Step 2: Some of the sunlight is reflected and some other part is refracted (passes into the water droplets).
Example: Through a window glass light reflects as well as refracts. We can observe the objects through the glass due to refraction (light passes through the glass). We can also observe the reflection as we can observe our own image by looking at the same window glass.
Step 3: Refracted light, while passing through the water splits into its components (7 colours) and appears as rainbow
White light is composed of seven colours, (VIBGYOR) Violet, Indigo, Blue, Green, Yellow, Orange, and Red.
When the above seven colours are painted on a circular disc and rotated we observe the circular disc in white colour.
When the white light undergoes refraction (passing from rarer medium to denser medium), as the speed of light changes, the white light splits into seven colours.
Related words: Refraction | total internal reflection | Lens.