In the convergent part pressure decreases and hence
velocity increases according to Bernouli's theorem . hence
Nozzles are made convergent .
convergent nozzles are used to get sonic velocity .when
requirement is more than sonic velocity convergent-
divergent nozzle are used,with a little loss due to
sepertation.

as we know when the area decreases the velocity increases
so convergent section are used to increase the velocity
and divergent section are used to reduse the friction losses

As we know from pascal law that PV=constant
and we also AV=constant which is volume flow rate so
whenever we are decreasing area at that region velocity
increases and we get required jet velocity

but where as in divergents we increase area decreasing
velocity and finally we get more pressure.