In the rainbow formation , the diagrams speaks about the light ray entering a water droplet and gets refracted .. gets internally reflected.. and gets refracted again and exits the droplet.

Now the question is .. why do the colours split when the light ray gets refracted the first time , but when it's refracted again and exits the molecule it does not combine to form white light again.

If it splits in first refraction why not join in the second ?

The same question applies to a prism too.

When the light refracts for the first time it splits into colours,because the light which enters the water drop consists of differnt wavalengths.so it splits into colours.but when the lights refracts for the second time the light already split into colours of which having monochramatic.so further it cannot split.but when it refracts for the second time the monochromatic lights incident at different positions in water drop due to their wavelengths.so they cannot combine.This applies for prism too.