Question

Why by using two prisms we can get white light?

Answer 1

when you will use one prism then white light gets convert in seven colors but when you use two prism the first one converts it in seven colors while the second one again convert seven colors to white light.

Answer 2

Heya!!

Actually, using two prims and placing them in no definite manner will not give back white light. But if u place the prisms just like in the picture i provided as an attachment below, 

what happens is in the first prism, as usual, refraction occurs and the light is dispersed into its seven colors....but when u keep an extra prism there in a way opposite to the first prism, (the  prism acts as dispersing prism..and the second one acts as a recombining prism)  the dispersed colors, recombine to a single ray of white light. 

I think u could understand better by seeing the pictures i provided.

Hope this helps u my friend :))))