Ans: Threshold frequency: It is the minimum frequency of the incident light on the surface of a metal so that electrons are just ejected from the metal surface without acceleration. It is constant for a given metal but it differs metal to metal.

Work Function: The minimum amount of energy required to eject an electron just from the surface of a metal is called the work function of the metal. It is constant for a given metal but it differs metal to metal.

Ans: Cathode rays are a beam of fast moving electrons. They are not actually rays but they are particles (electrons). They are deflected in an electric field and magnetic field. Its speed is less than that of light in air and hence cathode rays aren't electromagnetic wave.