In physics, epsilon naught is the permittivity of free space. Actually, this is a mathematical quantity that represents, how much electric field is permitted ( penetrated) in free space or vacuum. Similarly, epsilon of water means how much electric field is permitted in water (or it can cross the water). 1/4(pi)(epsilon naught) is 9*10? this number tells us that 9*10? field lines are crossing by a charge in a vacuum but for water, this number may change and number of field lines penetrating is also changed

Also called the permittivity of free space, it is an ideal physical constant that represents the absolute dielectric permittivity of a vacuum. In other words, epsilon naught quantifies the ability of a vacuum to allow electric field lines to flow through. It is approximately 8.854 10^-12 farads per meter.