Introduction to polarization

Light: an electromagnetic wave

Light is a wave (imagine a wave moving on a lake), an electromagnetic wave, represented by a vector (the long arrow on the figure) where the electric field E (in blue) and the magnetic field B (in red) are orthogonal (perpendicular) to each other, and also orthogonal to the direction of propagation. These 2 fields change with time and space in a sinusoid fashion.
	In general, to represent light, we only show the electric field (and do not show the magnetic field) because it is with the electric field that detectors (eye, photographic film, CCD, etc.) interact. The wavelength of light lambda is the distance (in meters for example) between 2 maxima (or 2 minima).

Natural light

``Ordinary'' light, or natural light, is in general non-polarized: the electric field is on average oriented in an arbitrary direction, and has a very precise orientation for too short a time to be measured by instruments.

Linear polarization of light

If light is linearly polarized, the electric vector has a fixed orientation in space. When one measures linear polarization, the result is a percentage (intensity of the polarized light with respect to the total intensity of the light) and a position angle (orientation of the electric field of the polarized waves with respect to the celestial North).

Vertical polarization.	Horizontal polarization.

Circular polarization of light

In the (rarer) case of circular polarization, the electric field vector rotates in a plane perpendicular to the direction of propagation.