Two metal plates having charges Q, –Q face each other at some separation and are dipped into an oil tank. If the oil is pumped out, the electric field between the plates will
Dielectric constant of a substance is the factor>1) by which the capacitance increases from its vacuum value, when the dielectric is inserted fully between the plates of the capacitor
∴ 
Where,
C=capacitance in presence of dielectric
C0=capacitance in presence of vacuumK=1)
The electric field between the plates of a capacitor when the space between the plates is filled with a dielectric of dielectric constant K is given by
Where,
Q=charge on the capacitor
A=area of metal plates
K=dielectric constant
ϵ0=permittivity of vacuum
When dipped in oil tank value of K>1
When oil is removed there is air between the plates with K~1
∴ the value of K decreases when oil is pumped out
By the formula,
So as K decrease from greater than 1 to 1 , the electric field increases.
Therefore, after pumping out oil, the electric field between the plates increases.