It is difficult to single out who was the first to discover polarized light. Ancient people could notice a peculiar spot by looking at the sky in certain directions. Polarization has many quirks, manifests itself in different areas of life, and today it is the subject of mass research and application, the reason for everything is the law of Malus.
Discovery of polarized light
Vikings may have used sky polarization to navigate. Even if they didn't, they definitely found Iceland and the wonderful calcite stone. Icelandic spar (calcite) was known even in their times, it is the inhabitants of Iceland that he owes his name to. The mineral was once used in navigation due to its unique optical properties. It played a major role in the modern discovery of polarization and continues to be the material of choice for separating the polarization components of light.
In 1669, the Danish mathematician from the University of Copenhagen, Erasmus Bartholinus, not only saw a double light, but also carried out some experiments, writing a 60-page memoir. This iswas the first scientific description of the polarization effect, and the author can be considered the discoverer of this amazing property of light.
Christian Huygens developed the pulsed wave theory of light, which he published in 1690 in his famous book Traite de la Lumiere. At the same time, Isaac Newton advanced the corpuscular theory of light in his book Opticks (1704). In the end, both were right and wrong, since light has a dual nature (wave and particle). Yet Huygens was closer to the modern understanding of the process.
In 1801, Thomas Young made the famous double slit interference experiment. He proved that light behaves like waves, and superposition of waves can lead to darkness (destructive interference). He used his theory to explain things like Newton's rings and supernatural rainbow arcs. A breakthrough in science came a few years later when Jung showed that polarization is due to the transverse wave nature of light.
Young Etienne Louis Malus lived in a turbulent era - during the French Revolution and the reign of terror. He participated with Napoleon's army in the invasion of Egypt, as well as Palestine and Syria, where he contracted the plague that killed him a few years later. But he managed to make an important contribution to the understanding of polarization. Malus' law, which predicted the intensity of light transmitted through a polarizer, has become one of the most popular in the 21st century when creating liquid crystal screens.
Sir David Brewster, renowned science writer, studied optical physics subjects such as dichroism and spectraabsorption, as well as more popular subjects such as stereo photography. Brewster's famous phrase is known: "Everything is transparent except glass".
He also made an invaluable contribution to the study of light:
- The law describing the "polarization angle".
- Invention of the kaleidoscope.
Brewster repeated Malus's experiments for many gems and other materials, discovering an anomaly in glass, and discovered the law - "Brewster's angle". According to him, “…when the beam is polarized, the reflected beam forms a right angle with the refracted beam.”
Malus Polarization Law
Before we talk about polarization, we must first remember about light. Light is a wave, although sometimes it is a particle. But in any case, polarization makes sense if we think of light as a wave, as a line, as it travels from the lamp to the eyes. Most light is a mixed mess of light waves that vibrate in all directions. This direction of oscillation is called the polarization of light. The polarizer is the device that cleans up this mess. It accepts anything that mixes light and only lets through light that oscillates in one particular direction.
The formulation of Malus's Law is: when a completely flat polarized light falls on the analyzer, the intensity of the light transmitted by the analyzer is directly proportional to the square of the cosine of the angle between the analyzer's transmission axes and the polarizer.
A transverse electromagnetic wave contains both an electric and a magnetic field, and the electric field in a light wave is perpendicular to the direction of light wave propagation. The direction of the light vibration is the electric vector E.
For an ordinary unpolarized beam, the electric vector keeps changing its direction randomly when light is passed through a polaroid, the resulting light is plane polarized with its electric vector vibrating in a certain direction. The direction of the emerging beam vector depends on the orientation of the polaroid, and the plane of polarization is designed as a plane containing the E-vector and the light beam.
The picture below shows flat polarized light due to the vertical vector EI and the horizontal vector EII.
Unpolarized light passes through a Polaroid P 1 and then through a Polaroid P 2, forming an angle θ with y ax-s. After light propagating along the x direction passes through the Polaroid P 1, the electrical vector associated with the polarized light will only vibrate along the y axis.
Now if we allow this polarized beam to pass through the polarized P 2 again, making an angle θ with the y axis, then if E 0 is the amplitude of the incident electric field on P 2, then the amplitude of the wave coming out of P 2, will be equal to E 0 cosθ and, therefore, the intensity of the emerging beam will be according to the Malus Law (formula) I=I 0 cos 2 θ
where I 0 is the intensity of the beam emerging from P 2 when θ=0θ is the angle between the transmission planes of the analyzer and the polarizer.
Light intensity calculation example
Malus' Law: I 1=I o cos 2 (q);
where q is the angle between the light polarization direction and the polarizer transmission axis.
Unpolarized light with intensity I o=16 W/m 2 falls on a pair of polarizers. The first polarizer has a transmission axis aligned at a distance of 50[deg.] from the vertical. The second polarizer has the transmission axis aligned at a distance of 20o from the vertical.
A test of Malus' Law can be done by calculating how intense the light is when it emerges from the first polarizer:
4 W/m 2
16 cos 2 50o
8 W/m 2
12 W/m 2
Light is not polarized, so I 1=1/2 I o=8 W/m 2.
Intensity of light from the second polarizer:
I 2=4 W/m 2
I 2=8 cos 2 20 o
I 2=6 W/m 2
Followed by the Malus Law, the formulation of which confirms that when light leaves the first polarizer, it is linearly polarized at 50o. The angle between this and the transmission axis of the second polarizer is 30[deg.]. Therefore:
I 2=I 1 cos 2 30o=83/4 =6 W/m 2.
Now the linear polarization of a beam of light with an intensity of 16 W/m 2 falls on the same pair of polarizers. The polarization direction of the incident light is 20o from the vertical.
Intensity of light coming out of the first and second polarizers. Passing through each polarizer, the intensity decreases by a factor of 3/4. After leaving the first polarizerthe intensity is 163/4 =12 W/m2 and decreases to 123/4 =9 W/m2 after passing the second.
Malusian law polarization says that to turn light from one direction of polarization to another, the intensity loss is reduced by using more polarizers.
Suppose you need to rotate the direction of polarization by 90o.
| N, number of polarizers | Angle between successive polarizers | I 1 / I o |
| 1 | 90 o | 0 |
| 2 | 45 o | 1/2 x 1/2=1/4 |
| 3 | 30 o | 3/4 x 3/4 x 3/4=27/64 |
| N | 90 / N | [cos 2 (90 o / N)] N |
Calculation of the Brewster Reflection Angle
When light hits a surface, some of the light is reflected and some of it penetrates (refracted). The relative amount of this reflection and refraction depends on the substances passing through the light, as well as the angle at which the light hits the surface. There is an optimal angle, depending on the substances, that allows the light to refract (penetrate) as much as possible. This optimal angle is known as the Scottish physicist David Brewster's angle.
Calculate the angleBrewster for ordinary polarized white light is produced by the formula:
theta=arctan (n1 / n2), where theta is the Brewster angle, and n1 and n2 are the refractive indices of the two media.
To calculate the best angle for maximum light penetration through glass - from the refractive index table we find that the refractive index for air is 1.00 and the refractive index for glass is 1.50.
The Brewster angle would be arctan (1.50 / 1.00)=arctan (1.50)=56 degrees (approximately).
Calculating the best light angle for maximum water penetration. From the table of refractive indices it follows that the index for air is 1.00, and the refractive index for water is 1.33.
The Brewster angle would be arctan (1.33 / 1.00)=arctan (1.33)=53 degrees (approximately).
Use of polarized light
A simple layman can not even imagine how intensively polarizers are used in the world. The polarization of the light of the law of Malus surrounds us everywhere. For example, such popular things as Polaroid sunglasses, as well as the use of special polarizing filters for camera lenses. Various scientific instruments use polarized light emitted by lasers or by polarizing incandescent and fluorescent sources.
Polarizers are sometimes used in room and stage lighting to reduce glare and provide more even illumination and as glasses to give a visible sense of depth to 3D films. Crossed polarizers evenused in space suits to drastically reduce the amount of light that enters an astronaut's eyes while sleeping.
Secrets of optics in nature
Why blue sky, red sunset and white clouds? These questions are known to everyone since childhood. The laws of Malus and Brewster provide explanations for these natural effects. Our sky is really colorful, thanks to the sun. Its bright white light has all the colors of the rainbow embedded inside: red, orange, yellow, green, blue, indigo and violet. Under certain conditions, a person meets either a rainbow, or a sunset, or a gray late evening. The sky is blue because of the "scattering" of sunlight. The color blue has a shorter wavelength and more energy than other colors.
As a result, blue is selectively absorbed by air molecules, and then released again in all directions. Other colors are less scattered and therefore usually not visible. The noon sun is yellow after absorbing its blue color. At sunrise or sunset, sunlight enters at a low angle and must pass through a large thickness of the atmosphere. As a result, the blue color is thoroughly scattered, so that most of it is completely absorbed by the air, lost and scattering other colors, especially oranges and reds, creating a glorious color horizon.
The colors of sunlight are also responsible for all the hues we love on Earth, whether it's grass green or the turquoise ocean. The surface of each object selects the specific colors it will reflect in order todistinguish yourself. Clouds are often brilliant white because they are excellent reflectors or diffusers of any color. All returned colors are added together to neutral white. Some materials reflect all colors evenly, such as milk, chalk, and sugar.
The importance of polarization sensitivity in astronomy
For a long time, the study of Malus's law, the effect of polarization in astronomy was ignored. Starlight is almost completely unpolarized and can be used as a standard. The presence of polarized light in astronomy can tell us how light was created. In some supernovae, the light emitted is not unpolarized. Depending on the part of the star being viewed, a different polarization can be seen.
This information about the polarization of light from different regions of the nebula could give researchers clues to the location of the shadowed star.
In other cases, the presence of polarized light can reveal information about the entire part of the invisible galaxy. Another use of polarization-sensitive measurements in astronomy is to detect the presence of magnetic fields. By studying the circular polarization of very specific colors of light emanating from the corona of the sun, scientists have uncovered information about the strength of the magnetic field in these places.
Optical microscopy
The polarized light microscope is designed to observe and photograph specimens that are visible throughtheir optically anisotropic nature. Anisotropic materials have optical properties that change with the direction of propagation of light passing through them. To accomplish this task, the microscope must be equipped with both a polarizer placed in the light path somewhere in front of the sample, and an analyzer (second polarizer) placed in the optical path between the objective rear aperture and the viewing tubes or camera port.
Application of polarization in biomedicine
This popular trend today is based on the fact that in our bodies there are many compounds that are optically active, that is, they can rotate the polarization of the light passing through them. Various optically active compounds can rotate the polarization of light in different amounts and in different directions.
Some optically active chemicals are present in higher concentrations in the early stages of eye disease. Physicians could potentially use this knowledge to diagnose eye diseases in the future. One can imagine that the doctor shines a polarized light source into the patient's eye and measures the polarization of the light reflected from the retina. Used as a non-invasive method for testing eye disease.
The gift of modernity - LCD screen
If you look closely at the LCD screen, you will notice that the image is a large array of colored squares arranged in a grid. In them, they found application of the law of Malus,the physics of the process which created the conditions when each square or pixel has its own color. This color is a combination of red, green and blue light in each intensity. These primary colors can reproduce any color that the human eye can see because our eyes are trichromatic.
In other words, they approximate specific wavelengths of light by analyzing the intensity of each of the three color channels.
Displays exploit this shortcoming by only displaying three wavelengths that selectively target each type of receptor. The liquid crystal phase exists in the ground state, in which the molecules are oriented in layers, and each subsequent layer twists slightly to form a helical pattern.
7-segment LCD display:
- Positive electrode.
- Negative electrode.
- Polarizer 2.
- Display.
- Polarizer 1.
- Liquid crystal.
Here the LCD is between two glass plates, which are equipped with electrodes. LCDs of transparent chemical compounds with "twisted molecules" called liquid crystals. The phenomenon of optical activity in some chemicals is due to their ability to rotate the plane of polarized light.
Stereopsis 3D movies
Polarization allows the human brain to fake 3D by analyzing the differences between two images. Humans can't see in 3D, our eyes can only see in 2D. Images. However, our brains can make sense of how far away objects are by analyzing the differences in what each eye sees. This process is known as Stereopsis.
Because our brains can only see pseudo-3D, filmmakers can use this process to create the illusion of three dimensions without resorting to holograms. All 3D movies work by delivering two photos, one for each eye. By the 1950s, polarization had become the dominant method of image separation. Theaters began to have two projectors running simultaneously, with a linear polarizer over each lens.
For the current generation of 3D movies, technology has switched to circular polarization, which takes care of the orientation problem. This technology is currently manufactured by RealD and accounts for 90% of the 3D market. RealD has released a circular filter that switches between clockwise and counter-clockwise polarization very quickly, so only one projector is used instead of two.
