Question

how to convert light dispersion by prism again into white light?

Answer 1

White light when passes through a glass prism it is dispersed in VIBGYOR. As per the question our motto is to turn that dispersed light into white undispersed state again. To achieve that take an identical glass prism with the initial which dispersed the white light at the first place.

Answer 2

white light broke into it's different components it is called dispersion of light.

there are 7 components of white light after dispersion
violet
indigo
blue
green
yellow
orange
red