The electromagnetic theory is a united theory of electromagnetism established by James Clerk Maxwell. This theory primarily discusses the relationships between electric field and magnetic field based from previous observations and experiments related to electricity, magnetism and optics combined.

There are four phenomena involved in this theory. The first states that the force of attraction or repulsion between electric charges is inversely proportional to the square of the distance between them. The second states phenomena involves magnetic poles, stating that magnetic poles come in pairs and that either attract or repel each other as much a electric charges do. The third states that the direction of the magnetic field produced by the electric current in the wire is dependent on the direction of the current. And lastly, the fourth theory states that a moving electric field produces a magnetic field and vice versa. He was able to develop four equations relating the above stated phenomena, which nowadays is called the Maxwell's equation. Using these equations, he was also able to compute and establish that both electricity and magnetism travel at a constant speed equivalent to the speed of light. It was, therefore, concluded that the light travel at a constant speed in all directions.