Polarization of light

Japanese: 偏光 - へんこう（英語表記）polarization of light

The state in which light waves vibrate in a regular direction. Light is a type of electromagnetic wave, and in a uniform medium, the vibration directions of the electric field and magnetic field and the propagation direction are all perpendicular to each other as transverse waves.

[Tanaka Shunichi]

Polarization Classification

As shown in Figure A , linearly polarized light is light in which the electric field E (and therefore the magnetic field H ) oscillates sinusoidally in a plane that includes the direction of propagation ( z axis) ( E is in the xz plane, H is in the yz plane). For this reason, it is also called plane polarized light. The oscillation of E can be expressed as

where A is the amplitude of the electric field, ν is the frequency, t is time, and δ is the initial phase. The wave in the figure propagates in the positive direction of z at a speed of v as a whole. Ordinary polarized light can be considered as a combination of two linearly polarized lights that vibrate perpendicularly to each other, and the electric fields of these lights can be expressed as

If the light is expressed as, the locus of the combined E usually rotates in a spiral shape as shown in Figure B , and this type of light is called circularly polarized light. Its projection onto the xy plane is generally an ellipse, and this light is called elliptically polarized light, but in particular, when A _x = A _y and δ _y - _{δ x} = m π/2 radians ( m is an odd integer), it becomes a circle, and this is called circularly polarized light. As with linear polarization, the light wave propagates in the positive direction of z at a speed v as a whole, so if the spiral is a right-handed screw as shown in Figure B , the point where the spiral cuts a certain z plane (for example, the xy plane in the figure) rotates clockwise over time when viewed directly against the polarized light, and this case is called right-handed elliptical polarization or circular polarization. Conversely, if the spiral is a left-handed screw, it is left-handed polarized light. In particular, when δ _y - _{δ x} = m π radians ( m is an integer), the projection onto the xy plane is a straight line, which corresponds to the linear polarization mentioned above.

In contrast, light whose vibration direction changes irregularly from moment to moment, but whose directional distribution is uniform in all directions when the average is taken over a certain period of time, is called natural light, and light that is considered to be a combination of natural light and polarized light is called partially polarized light. In contrast to partially polarized light, pure polarized light (elliptically, circularly, linearly polarized light) is sometimes called completely polarized light.

[Tanaka Shunichi]

Applications of Polarized Light

Light emitted from a normal light source is approximately considered to be natural light. Scattered light when natural light is scattered by particles or groups of particles, and light reflected or transmitted through the surface of a non-absorbing medium such as glass, generally become partially polarized. In 1808, the Frenchman Malus first discovered polarization in light reflected from a glass surface. In particular, when the angle of incidence satisfies Brewster's law, the reflected light becomes linearly polarized.

A polarizer is an element that converts natural light into polarized light. Examples are a Nicol prism and a polarizing plate, but some polarizers use reflection or transmission on the surface of a non-absorbing medium. Polarizers are also used to examine the polarization state of light, in which case they are called analyzers. A polarizer and analyzer arranged as shown in Figure C is called a polarimeter (sometimes called a polariscope), and is used to examine the change in polarization caused by a sample, usually by rotating the analyzer around the optical axis, to clarify the physical properties of the sample. A polarimeter is a device used to measure the concentration of sugar solutions. Figure C examines the change in linearly polarized light, but some also measure elliptically polarized light.

The human eye, whose retina is made of materials with anisotropic refractive index and absorptivity, acts as a kind of analyzer and can perceive the vibration direction of the electric field of the incident light. That is, when you stare at a linearly polarized white field, you see a yellow, slightly dark hourglass-shaped pattern that stretches perpendicular to the vibration direction of the electric field, as shown in Figure D , A, and has a size of 2 to 4 degrees in visual angle and a bluish area around it. This is called a Heidinger brush after its discoverer, but if the vibration direction of the incident polarized light is fixed, it will soon become invisible. Each ommatidia that make up the compound eye of a honeybee can also distinguish the direction of incident polarized light, and it is said that the direction of the sun (sunlight is polarized due to scattering by fine particles in the atmosphere) can be detected in visual angle with an accuracy of 1 to 5 degrees from the difference in the direction of polarization incident on each ommatidia.

[Tanaka Shunichi]

"Optical Technology Series 4: Polarization and Crystal Chemistry" by Yasuhiro Doi (1975, Kyoritsu Shuppan)

[Reference] | Saccharometer | Light | Brewster's Law | Polarizing Plate

Linearly polarized light (Figure A)
At any point in space ,

Linearly polarized light (Figure A)

Rotary polarization (Figure B)
At any point in space, and the combination of these ©Shogakukan

Rotary polarization (Figure B)

Polarimeter (Fig. C)
©Shogakukan ">

Polarimeter (Fig. C)

Haidinger Brush (Fig. D)
©Shogakukan ">

Haidinger Brush (Fig. D)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

光波の振動方向が規則的なものおよびその状態。光は電磁波の一種で、一様な媒質中ではその電界・磁界の振動方向、伝搬方向の三つが互いに垂直な横波である。

［田中俊一］

偏光の分類

直線偏光は図Aのように電界E（したがって磁界Hも）が伝搬方向（z軸）を含む面内（Eはxz面内、Hはyz面内）に正弦波状に振動している光で、このため平面偏光ともよばれる。Eの振動を式で表せば

で与えられ、Aは電界の振幅、νは振動数、tは時間、δは初期位相で、図の波動は全体としてzの正方向へ速度vで伝搬することになる。一般の偏光は互いに垂直方向に振動する二つの直線偏光の合成とみなすことができ、それらの電界を

で表せば、合成されたEの軌跡は普通、図Bに示すように螺旋(らせん)状に回転し、このような光は回転偏光とよばれる。そのxy面への射影は一般に楕円(だえん)で、この光を楕円偏光というが、とくにA_x=A_yでかつδ_y-δ_x=mπ/2ラジアン（mは奇数の整数）の場合は円になり、これを円偏光という。直線偏光の場合と同様に、光波は全体として速度vでzの正方向に伝搬するので、螺旋が図Bのように右ねじの場合は、あるzの面（たとえば図のxy面）を螺旋が切る点は、偏光に正対して見たとき、時間の経過とともに時計回りに回転し、この場合を右回り楕円偏光または円偏光という。逆に螺旋が左ねじの場合は左回り偏光である。とくにδ_y-δ_x=mπラジアン（mは整数）の場合はxy面への射影は直線となり、これは先に述べた直線偏光に相当する。

　これに対して、刻々に見れば振動方向が不規則な変化をしているが、ある時間の平均をとったときには、方向分布があらゆる方向に一様である光を自然光といい、自然光と偏光が合成されたとみなされる光を部分偏光という。部分偏光に対して、純粋な偏光（楕円、円、直線偏光）を完全偏光ということもある。

［田中俊一］

偏光の応用

普通の光源から出る光は近似的に自然光とみなされる。自然光が粒子や粒子群で散乱されるときの散乱光や、ガラスなどの非吸収性媒質の表面で反射や透過をする光は一般に部分偏光になる。1808年フランスのマリュスはガラス面からの反射光についてはじめて偏光を発見した。とくにブルースターの法則を満足する入射角のときには、反射光は直線偏光になる。

　自然光を偏光に変える素子を偏光子polarizer（または偏光器）といい、ニコルのプリズムや偏光板がその例であるが、非吸収性媒質表面での反射や透過を利用するものもある。偏光子はまた光の偏光状態を調べるのにも用いられ、この場合はとくに検光子analyserという。偏光子、検光子を図Cのように配置したものは偏光計polarimeter（偏光器ということもある）とよばれ、試料による偏光の変化を、普通、検光子を光軸の周りに回転して調べ、試料の物理的性質を明らかにするのに用いられる。とくに砂糖溶液の濃度を測定するために用いられるものを検糖計という。図Cは直線偏光の変化を調べるものであるが、楕円偏光の測定を行うものもある。

　人間の目は、網膜が屈折率や吸収率の異方性をもつ物質で構成されているので一種の検光子の働きをし、入射する光の電界の振動方向を知覚することができる。すなわち、直線偏光の白色視野を注視すると、図DのAのように電界の振動方向に垂直に伸びる黄色でやや暗い砂時計状の模様が見え、その大きさは視角で2～4度、また周りに青みを帯びた部分がある。これは発見者の名前をとってその形状からハイディンガー・ブラシとよばれているが、入射する偏光の振動方向を固定すると、じきに見えなくなってしまう。ミツバチの複眼を構成する各個眼も入射偏光方位を識別でき、それぞれの個眼に入射する偏光方位の相違から、太陽の方向（大気中の微粒子の散乱によって太陽光は偏光している）を視角にして精度1～5度で検知できるといわれている。

［田中俊一］

『土井康弘著『光学技術シリーズ4・偏光と結晶化学』（1975・共立出版）』

[参照項目] | 検糖計 | 光 | ブルースターの法則 | 偏光板

直線偏光〔図A〕

回転偏光〔図B〕

偏光計〔図C〕

ハイディンガー・ブラシ〔図D〕

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例

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