The angle of incidence at which reflected light is totally polarized for reflection from air to glass (refractive index $$n$$) is :

When $$p$$-$$n$$ junction diode is forward biased then

A current $$i$$ ampere flows along an infinitely long straight thin walled tube, then the magnetic induction at any point inside the tube is

Four charges equal to -$$Q$$ are placed at the four corners of a square and a charge $$q$$ is at its center. If the system is in equilibrium the value of $$q$$ is