Capacitor is a device which is used to store the energy in it
This device can store the energy in the form of electrostatic field energy
This is given by formula
![U = (Q^2)/(2C)](https://img.qammunity.org/2020/formulas/physics/high-school/ifr4vhznxvmpvpqz1z6hd25z6u8q6omt6s.png)
As we connect a charged capacitor to a light bulb the charge on the plates of capacitor will start flowing from one plate to other plate and during this charge flow the bulb will glow
As the charge from one plate move to another plate positive charge of one plate will discharge the negative charge of another plate and this will discharge the whole capacitor with time
So with time the glow of bulb or brightness of bulb that is connected across a capacitor will decrease