Theory:

  • Place a prism on a table and keep a white screen near it.
glass-prism-3781924_960_720.jpg
  • Now, with the help of a torch, allow white light to pass through the prism. What do you see?
You can observe that white light splits into seven coloured light rays, namely, violet, indigo, blue, green, yellow, orange and red (VIBGYOR) on the screen.
 
Dispersion_of_light.png
 
Now, place another prism in its inverted position, between the first prism and the screen. Now, what do you observe on the screen?
You can observe that white light is coming out of the second prism.
YIoSq.png
 
In this activity, you can see that the first prism splits the white light into seven coloured light rays and the second prism recombines them into white light, again.
Thus, it is clear that white light consists of seven colours.
 
Splitting white light into its seven constituent colours (wavelength) on passing through a transparent medium is known as dispersion of light.
Reference:
https://pixabay.com/photos/glass-prism-color-technology-design-3781924/
https://commons.wikimedia.org/wiki/File:Dispersion_of_light.png
https://physics.stackexchange.com/questions/23401/can-anyone-explain-to-me-why-light-is-not-dispersed-into-a-spectrum-through-a-pa