Can you explain how its premium will change over time if nifty increases/decreases towards your strike price?
Basically I want to know the effect of theta parameter in Options.
Delta is the metric describing option price fluctuations as a function of moves in the underlying. Theta is a function of time. It's important to not overlap the Greeks and their effects; some are independent of one another.
For ATM options near expiration, theta decay is particularly pronounced. Theta decay is not linear, but rather the rate of decay begins slowly and then drastically increases over time.
However, if an option is deeply ITM, the price of the option "looks" closer to "linear", due to the majority of its value being intrinsic. But theta's magnitude is still varying across time.
OTM options experience a deceleration in time decay. They are nearly worthless already near expiration.
And here with the magnitude of slope shown increasing:
Source: Options Industry Council (OIC)
For more information, TastyTrade (I'm unassociated) made an informative video about the topic: