What does an e.m. wave consists of? On what factors does its velocity in vacuum depend?

What does an e.m. wave consist of?

What does an electromagnetic wave consists of ? On what factors does its velocity in vacuum depend ?

What does an electromagnetic wave constist of ?On what factors does its velocity in vacuum depend?

On what factors does the elasticity of a material depend ?

a. If the fundamental source wave is a vibrating object, what is the fundamental source of an electromagnetic wave ? b. Give the expression for velocity of E.M waves in vacuum. c. On what factors does its velocity in vaccum depend ?