how capacitance of a capcitor is increased when dielectric is introdused?

The electrons within dielectric molecules are influenced by the electric field, causing the molecules to rotate slightly from their equilibrium positions. The air gap is shown for clarity; in a real capacitor, the dielectric is in direct contact with the plates.

The induced field of the dielectric is in a direction opposite to the field between the plates of the capacitor. As the field decreases, the potential difference between the plates of the capacitor also decreases as V = Ed.

Hence capacitance increases.