(i) Microwaves are produced by special vacuum tubes like the klystron/ Magnetron / Gunn diode.
The frequency of microwaves is selected to match the resonant frequency of water molecules, so that energy is transferred efficiently to the kinetic energy of the molecules.
(ii) (a) Associated with the greenhouse effect.
(b) In remote switches of household electrical appliances. [CBSE Marking Scheme, 2014]