Does not work like this. In EU and UK (and most of the world), one prong is at nearly zero voltage, other prong is sine wave, with amplitude 340v or so (240v RMS), with equation $v\sin(\omega t) $
US uses 120v for appliances such as TVs, lightbulbs, etc.
However, in the US you might also encounter two-phase wiring where one prong is at $\frac v 2\sin(\omega t) $ and another is at $-\frac v 2\sin(\omega t) $ , used for electric stoves that run at 240v. You might also encounter the wiring that uses 2 phases of three phase system, with voltage of about 200v between them but 120v from each to ground. Which happens because the equations are $ v \sin(\omega t) $ and $v \sin(\omega t + \frac 2 3 \pi) $ , the phase is offset by third of a circle. Ahh, and my apartment seem to be using full three-phase (with 400v between two phases) for the electric stove. Not quite sure. All in all there's a wide variety of wirings.
edit: ahh and also:
As far as I know,
Both the two prongs will act as a phase for every 1/50th second
alternatively.
i.e For the first 1/50 second one hole in the outlet must act as a
phase and in the next 1/50 second other outlet hole must act as a
phase. And this cycle must continue...
This is always incorrect, everywhere in the world. The voltage of a sine wave source in general is given by $ v\sin(\omega t + \alpha) $ where $ \frac {\omega} {2\pi} $ is the frequency and $ \alpha $ is time offset.
The 'phase' aka 'live' is any wire with such variable voltage. The 'neutral' is wire with nearly zero voltage. The role of phase and neutral never alternates. What you are thinking of is a two phase system where one phase is positive and other is negative for 1/100th of a second, then vice versa, then back again (cycle repeats in 1/50th of a second)
