Question

What is the origin of electromagnetic wave? Why it originates?( what actually an electromagnetic wave is ) ?

Answer 1

Origin of Electromagnetic Waves is oscillation of electric charge.
We know that a moving charge produce electric field and and magnetic fields (see Maxwell's equation).

Electromagnetic Waves are generated when an accelerated charge produced a combination of an alternating electric and magnetic fields which propagate out with speed of light in the form of waves or radiation.