With the grain or against it – the polarization of electromagnetic waves

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The direction (plane) in which the electric field oscillates is also the so-called polarization direction of the electromagnetic wave. Unpolarized radiation, as emitted by the sun or an incandescent lamp, has electromagnetic waves with any orientation of the electric fields. On the other hand, linearly polarized radiation emits electromagnetic waves that have a single orientation of the electric fields, such as 30° or 90° to the propagation direction. In this case, the electric and magnetic fields oscillate in phase, i.e., their strength is zero at the same places along the propagation direction at the same times. In the case of circularly polarized radiation, on the other hand, the electric and magnetic fields have the same amplitude but are 90° out of phase. When one component reaches its maximum, the other is at its minimum value, and vice versa. Therefore, the vector resulting from the sum of both components describes a circular rotation to the left or right around the axis of the propagation direction.

Polarization is the production of a uniform direction of oscillation from otherwise irregular oscillations of the incident radiation.

The polarization of electromagnetic waves can occur in various ways. We are particularly interested in three types because they either occur naturally or have technical relevance to photography.

Electromagnetic radiation can be polarized by reflection from non-metallic surfaces. We observe this particularly often on glass, plastic or water surfaces, snow fields or even the asphalt pavement of roads. Despite the fundamental dependence of the degree of polarization on the illumination angle and surface material type, these materials often reflect in a manner that aligns a significant portion of the vibration directions parallel to their surface. Therefore, objects that we view in light reflected and polarized in such a way often appear to us a little blurred or outshined.

Additionally, the scattering phenomenon polarizes electromagnetic radiation, as demonstrated by the blue sky (refer to „The Rayleigh scattering and the sky blue„). The process of scattering involves the absorption of an electromagnetic wave by atoms in a medium, leading to their vibration and the subsequent emission of a new electromagnetic wave. This wave radiates in all directions and, in turn, forces the electrons of neighboring atoms to oscillate at the same frequency. The process continues from atom to atom, producing at least partially polarized radiation that blurs any sharp contours that may exist in the sky.

Of course, we can also send electromagnetic radiation, or what we consider light, through a filter to trim it to a uniform plane of oscillation. These polarization filters are made up of a clear plastic film that is made up of a lattice structure of long, parallel molecular chains of polyvinyl alcohol (PVA), for example.Due to their chemical nature, these substances are able to absorb the part of the radiation that is not parallel to their orientation. If unpolarized light, which contains electromagnetic waves with many different orientations of the electric fields, is incident and the filter grating is vertical, only the vertically oscillating portions of the waves can pass through. The horizontally oscillating ones are held back because of their too large expansion in this direction. Electromagnetic waves oscillating at an angle deviating from the vertical lose intensity when passing through such a filter to the extent that their polarization angle deviates from the vertical.

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