Polarization of light

Polarization is another phenomenon of waves, which should also exist in light.

We already know that light is an electromagnetic wave.

Electromagnetic waves are Transverse waves. This means that the magnetic and electric fields are perpendicular to the direction of propagation.

Consequently, light should also be transverse waves.

Polarization of microwaves

Microwaves can be polarized by forming a grid of parallel electrically conducting rods (e.g.B. metal) between the source and the receiver is. The microwaves, after passing through the metal rods linearly polarized – only the direction of vibration that corresponds to the direction of the rods passes through.

Through another grating, whose metal rods perpendicular to those of the first grating, no more signal is registered at the receiver. Microwaves can be completely shielded with two metal grids standing perpendicular to each other. For this reason, it is sufficient to equip the door of microwave ovens with an appropriate metal grid.

Polarization of light with polarization filters

If one sends light through two special plastic foils, so called polarization foils, then it shows:

If the polarization directions of the two foils are perpendicular to each other, no light passes through them.

Info: In polarization foils, carbon chains are arranged parallel to each other like lattice bars. In the same way polarizing filters, which are used in photography, or metal wires work with Hertzian waves.

So the answer to the initial question is:

Light is polarizable, so it behaves like a transverse wave.

Michael Faraday (1791-1867) sent linearly polarized light through a glass rod in 1846, through which a magnetic field could be switched off.

He found that when the magnetic field was turned on, the direction of polarization of the light was rotated and concluded that light is an electromagnetic wave.

Further experiments supported this view.

Polarization by reflection

If one photographs smooth surfaces (z.B. glass surfaces or a lake with a mirror-like surface), strong reflections usually occur.

By using a polarization filter, these often disturbing reflections are largely suppressed.

The explanation is:

Reflected light is obviously partially polarized.

If you look at a pane of glass through a polarizing filter, you find that at a certain viewing angle the reflections disappear completely. The polarization is obviously related to the angle of reflection.

The exact relationship is explained by the Brewster’s law captured:

Brewster’s law

Standing reflected and refracted beam at the interface between two transparent materials perpendicular on each other, then that reflected light completely linearly polarized.

For the angle of incidence at which this is the case:

=n" width="80" height="18" /> resp. =\dfrac>>" width="90" height="36" />

The refractive index of the second substance (z.B. Glass) when the first substance is air.

(for air applies: )

If the light comes from a different substance, both refractive indices must be considered.

This law was found by the British physicist David Brewster (1781-1868) around 1815.

The Angle" width="18" height="14" /> is also called Brewsterwinkel denotes.

This relationship is illustrated again in the following sketch:

Polarization by reflection - Brewster angle

The incoming light (left side) is not polarized, but contains all possible directions of oscillation.

The reflected light is polarized so that it oscillates perpendicular to the plane of incidence.

The refracted light, which is also polarized, oscillates on the other hand in the plane of incidence.

If the condition of orthogonality of reflected and refracted beam mentioned in the law is met not is fulfilled, then partially Polarization on.

The condition in Brewster’s law results with the help of the law of refraction:

The Law of refraction reads: =n" width="73" height="40" />

This results in:

>=\dfrac>-\alpha_

)>=\dfrac>>=tan\alpha_

" width="354" height="42" />

Liquid crystal display (LCD)

For cell phones, tablets, laptops, calculators, thermometers, digital timers, etc., you can choose one of the three starters. one uses liquid crystal displays for a long time (Liquid Crystal Display, short LCD).

The function of LCDs is based on the Polarization of light:

liquid crystal is located in seven separately switchable segments between two sealed glass plates, which are covered with crossed polarizing foil.

The incident light is reflected by a mirror. Without voltage, incident light is linearly polarized by the polarizer, rotated 90° by the liquid crystal, then passes through the analyzer, is reflected and passes through the array in the reverse direction.

The display appears bright; the light is linear polarized.

If a voltage is applied to segments, then the liquid crystal no longer rotates the plane of oscillation. The corresponding areas appear dark.

Experiment:

If you look at an LCD screen through a polarizing film, the image will appear brighter or darker depending on the direction of rotation of the polarizing filter with respect to the display.

Thus, the light emitted from an LCD screen is partially linearly polarized.

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