The relation between electric and magnetic fields is obtained by applying Faraday's law to a plane wavefront traveling in the +x-direction with a constant speed. Consider a rectangle of height "a" having an area vector in the +z-direction. While integrating around the rectangle counterclockwise, the electric field is found to be zero along length "PQ", or perpendicular to the elements except for length "RS". It only contributes to the integral giving a non-zero value. To satisfy Faraday's law, there must be a magnetic field component in the +z-direction, which can provide a non-zero magnetic flux through the rectangle and hence a non-zero time derivative of magnetic flux. In time "dt", the wave moves a distance "c dt". While moving, it sweeps an area equal to "ac dt". During this time interval, the magnetic flux through the rectangle increases, giving the rate of change of magnetic flux. This value is substituted in Faraday's law, resulting in a relationship between E and B in terms of the speed of wave propagation in a vacuum.