index value changes

Why does sensex or nifty open at different value than its last closing? What things leads to these change?

Value of Sensex / NIFTY is derived from the market price of their constituents stocks.
Yesterday's close price of Sensex/NIFTY is derivative of closing price of their 30(or 50) companies. Next morning, all stocks of all these companies open at different price from their close price (due to developments /news/events between 3.30 pm to 9.00 am) hence sensex/nifty gets new derived price.

Thanks for you reply.

AFAIK sensex/nifty are derived based on the selected 30/50 stocks' prices and prices of these stocks varies due to demand and supply rule.

Please correct me if I am worng.

But still I am confused how does these developments/news/events related to all constituent stocks affect their prices after trading hours and who decides how much change should it make in their prices? which leads to change in index.