White colour is not a single colour, it is the combination of seven colours commonly known as VIBGYOR. The colours that are present in white colour consist violet ,Indigo, blue, green, yellow, orange and red. Rainbow is the best example for the Dispersion of white light. When sunlight passes through water droplets present in the moist atmosphere after of before the rain, it undergoes refraction and dispersion which results in splitting of seven colours. These seven colours are then arranged according to their respective wavelengths.
The Dispersion of light can be made possible using a prism. When a beam of light is passed through one side of the prism, it get dispersed and we can see the seven colours on the screen. But if we place two identical glass prisms in such a way that their refracting surfaces are in opposite direction, we can recombine the dispersed light into former white light. While the primary prism or the first prism disperses the white colour into seven component colours, the second prism recombines the emitted colours into its original combined form. This is due to the fact that the refraction produced by the second prism is equal and opposite to the refraction produced by the first prism.