Rami Arieli: "The Laser Adventure" Chapter 7 Section 5, page 1

Section 1.1 explained that electromagnetic radiation is a transverse wave.

Electromagnetic radiation is described by the electric field vector, which oscillate in the plan perpendicular to the direction of propagation of the radiation. Thus, the direction of oscillation of the electric field vector is all the possible directions in the plan perpendicular to the beam direction.

Plane of polarization of electromagnetic wave is defined as the plane created by the direction of oscillation of the electric field vector, and the direction of propagation of the beam.

Polarized wave is a wave with a definite direction of oscillation of the electric field vector.

The experimental proof that electromagnetic radiation is a transverse wave is by transmitting the beam through a polarizer, and than through another polarizer (analyzer) turned at an angle in relation to the first polarizer.

The polarizer permit transmission through it only for light that is polarized at specific direction.

It is trivial to see that only transverse wave can be polarized.

Ideal polarizer transmit 100% of the radiation when the plane of polarization of the beam is parallel to the plane of polarization of the polarizer.

Ideal polarizer transmit 0% of the radiation (blocks all the radiation), when the plane of polarization of the beam is perpendicular to the plane of polarization of the polarizer.

Practically, there is no perfect ideal polarizer, but the analysis below is based on ideal polarizer.

Most of the radiation in nature is not polarized, because it is emitted by different atoms independent of each other.

The direction of polarization of the radiation from each atom is random, thus statistically there is no preference to any direction. As a result, the direction of the oscillating electric field is randomly distributed in the plane perpendicular to the beam direction.