Question

The oil immersion objective is necessary to determine the morphology of prokaryotes

Answer 1

To clearly view the morphology of the prokaryotes, one needs a microscope with higher resolution i.e magnification of ×100. Use of oil for this purpose is important because it  reduces the refraction of light as it travels from air to glass. this process increases the resolution of the microscope making it possible to view morphology of bacteria.  

Answer 2

This is very true.  
Prokaryotes are microorganisms and come in different shapes that would not be visible and recognizable to the normal human eyes by any means but with the oil immersion objective, The oil immersion objective enable a person to be able to view these bacterias at a size that is 1000 times the apparent size of the prokaryotes.