What is a Polarizer?
A Polarizer is an optical device that transforms unpolarized light into polarized light. Unpolarized light refers to light waves that vibrate in multiple directions perpendicular to the direction of propagation. On the other hand, polarized light is described as light waves that vibrate in a single plane perpendicular to the direction of propagation.
The polarizer works by allowing only light waves that vibrate in a particular direction or plane to pass through, while blocking light waves that vibrate in other directions. This results in light that is polarized, meaning it has a single plane of vibration. Polarizers are used in various optical techniques and instruments, including photography, liquid crystal display technology, and polarizing sunglasses.
Working Principle of Polarizer
The working principle of a polarizer can be explained using Malus' law, which describes the relationship between the intensity of polarized light passing through a polarizer and the angle between the polarization direction of the light and the transmission axis of the polarizer.
According to Malus' law, the intensity (I) of light passing through a polarizer is proportional to the square of the cosine of the angle (θ) between the polarization direction of the light and the transmission axis of the polarizer. Mathematically, it can be expressed as:
where Io is the initial intensity of the light before passing through the polarizer.
When the polarization direction of the light is aligned with the transmission axis of the polarizer (θ=0°), the cosine of 0° is 1, and the light passes through the polarizer with maximum intensity (I=Io). This means that light waves with the same polarization direction as the transmission axis are transmitted through the polarizer without any attenuation.
On the other hand, when the polarization direction of the light is perpendicular to the transmission axis of the polarizer (θ=90°), the cosine of 90° is 0, and the intensity of the light passing through the polarizer is completely blocked (I=0). This means that light waves with a polarization direction orthogonal to the transmission axis are effectively blocked or attenuated by the polarizer.
For intermediate angles (0°<θ< 90°), the intensity of the light passing through the polarizer is reduced proportionally to the square of the cosine of the angle. This relationship allows the polarizer to selectively filter out light waves with specific polarization orientations, while allowing light waves with the desired polarization direction to pass through.
Types of Polarizers
There are mainly two types of polarizers:
- Linear Polarizer
- Circular Polarizer
Linear Polarizer
A Linear polarizer allows the passage of light waves that vibrate consistently in a single plane, while blocking those that vibrate in the perpendicular plane or reducing the intensity of light waves vibrating at different angles. If the vibrations of the light occur exclusively in one direction, without any components in other directions, the light is said to be linearly polarized.
Working of a Linear Polarizer
The concept of polarizing light involves two main axes: the transmitting axis and the absorbing axis. The transmitting axis, also known as the polarizing axis, is responsible for allowing the light to pass through. Even with the use of a single polarizer, there is a loss of at least 50% transmission, which varies depending on the grade of polarizer employed and the specific polarization requirements.
The absorbing axis, also known as the extinction axis, plays an important role in the process of polarizing light. When two linear polarizers are aligned perpendicular to each other, the absorbing axis of one polarizer is positioned parallel to the transmitting axis of the other polarizer, creating a condition called extinction. The absorbing axis of the polarizer blocks or absorbs the light waves that are aligned with its orientation. As a result, the transmitted light is greatly reduced or completely blocked, leading to near-complete light cancellation. The efficiency of a polarizer is determined by the ratio of transmission between two polarizers that have axes parallel to the crossed axes. Simply increasing the transmission rate may not always result in higher efficiency because doing so can also raise extinction transmission.
Using Malu’s law, the polarization direction of the light is parallel with the transmission axis of the first polarizer (θ=0º), hence cos θ=1. So, the light passes through it. But the transmission axis of the second polarizer is aligned perpendicular to the polarization direction of the light coming from the output of the first polarizer (θ=90º). Then cos θ=0 and therefore, the light is blocked by the second polarizer.
Methods of producing linear polarized light:
Linearly polarized light can be generated through various methods, and three mechanisms are particularly well-known. These are:
- Double Refraction or Birefringence
- Reflection
- Dichroism
Double refraction or birefringent polarizers
Certain natural crystals, such as calcite and quartz, have the ability to divide a single unpolarized beam of light into two separate beams of equal intensity, each of which is polarized. By separating these two polarized beams, it becomes possible to create a highly efficient linear polarizer.
Reflection polarizers
When a beam of unpolarized light strikes a flat, smooth, non-metallic surface at an angle, the resulting reflected beam may be partially or fully linearly polarized. The extent of polarization depends on both the angle of incidence and the refractive index of the reflecting surface. The polarizing angle, or Brewster's angle, refers to the angle at which polarization is complete and reaches 100%.
Dichroic absorptive polarizers
Most commercially available polarizers, including APIs, are dichroic polarizers that display dichroism, which is the ability to absorb light that is polarized in a specific direction. A dichroic linear polarizer can be viewed as having an indicated absorption axis and a transmission axis, which is also known as the polarizing axis. These polarizers are typically made from Stretched Polyvinyl Alcohol (PVA).
Types of Linear Polarizers
An Absorptive polarizer is an optical filter that selectively transmits light waves with a particular polarization orientation while absorbing or attenuating waves with orthogonal polarization orientations. This type of polarizer typically consists of a material or coating that absorbs the undesired polarizations, such as a dichroic dye or a thin film of metal or polymer. The absorbed light is either converted into heat or rerouted through reflection, depending on the specific design of the polarizer. Absorptive polarizers are commonly used in various optical applications, including photography, microscopy, and display technology.
The Wire-grid polarizer is a simple polarizer that consists of a regular array of parallel metallic wires. When placed at a 90-degree angle to an unpolarized incident beam, the beam can pass through the grid without much energy loss if the waves are perpendicular to the wires. And, if the waves are parallel to the wires, they will either be absorbed or reflected. That is, the polarizer allows the transmission of light waves that oscillate in a particular direction, while the remaining portion of the beam is either reflected or absorbed. For practical use, the distance between the wires must be smaller than the wavelength of the radiation. Wire-grid polarizers find their primary application in the far and mid-range of the electromagnetic spectrum, specifically for microwaves and infrared light.
- Beam-Splitting Polarizers
Beam-splitting polarizersdivide the incident beam into two polarization beams, which have differing polarization. Many common beam-splitting polarizers usually fully polarize only one of the polarization beams. Unlike absorptive or reflective polarizers, beam-splitting polarizers do not absorb or reflect the energy of the rejected polarization state, which makes them well-suited for applications involving laser light. In situations where it is necessary to study or use both polarization components simultaneously, true polarizing beamsplitters can prove to be highly beneficial.
Circular Polarizer
A Circular polarizer is a type of polarizer that transforms linearly polarized light into circularly polarized light. Circular polarization refers to the rotating direction of the electric field vector of light. It does not have a fixed orientation like linear polarization, instead circular polarization rotates as the light propagates.
It is a variation of linear polarizers with an additional quarter-wave plate that converts linearly polarized light into circularly polarized light. Circular polarizers are commonly used in various applications such as photography and displays to control glare, enhance contrast, and manage reflections.
Working of a Circular Polarizer
The input light that enters the linear polarizer filter is known as randomly polarized light (although all light is technically polarized). Once the light passes through the linear polarizer filter, it becomes linearly polarized light, with the plane of polarization being oriented in a particular direction, as opposed to being random or unpolarized.
The linearly polarized light subsequently proceeds through the quarter-wave plate. The polarization axis denotes the vector between the electrical fields (Ex and Ey) associated with the light.
The quarter-wave plate comprises a fast axis and a slow axis. Quarter Wave refers to the degree to which the slow axis delays one of the electrical fields when it traverses the wave plate. To achieve true circularly polarized light (rather than elliptically polarized light), the polarizing axis should be set at a 45º angle to the fast and slow axes. This 45º polarizer axis alignment enables the electromagnetic fields to be parallel to the fast and slow axes of the wave plate. With all of these elements in place, the polarized light then emerges from the quarter-wave plate, with one of the Ex or Ey fields shifted by one quarter of a wave.
In terms of time, the Ex and Ey electrical fields, as well as the polarizing axis that represents the vector of the two electrical fields, have a collective impact on the light that emerges from the rear of the circular polarizing filter, causing it to exhibit a rotating polarization state. Hence it is referred to as a "circular polarizer." Also, the electrical field (either Ex or Ey) that runs parallel to the slow axis of the wave plate determines which field gets shifted, and as a result, the direction in which the polarizer axis rotates, either clockwise (right-handed) or counter clockwise (left-handed).
Applications of Polarizers
One of the most common applications of polarizers is in photography. Polarizing filters are used to reduce glare and reflections from non-metallic surfaces such as water, glass, and leaves. They also enhance the color saturation and contrast of the image by selectively blocking certain polarizations of light. Polarizers are commonly used in landscape photography, where they help to create deep blue skies and enhance the color of foliage.
They are used in LCD screens, which consist of a liquid crystal layer sandwiched between two polarizers. When an electric current is applied to the liquid crystal layer, it changes the polarization of the light passing through it, creating the images seen on the screen. Polarizers play a critical role in this process by ensuring that only light with the correct polarization passes through the screen.
Polarizers find applications in optical microscopy as well, enabling scientists to study the properties of materials that interact with polarized light. Polarized light microscopy can provide valuable information about the structure, composition, and optical properties of materials, including crystals, minerals, and biological tissues.
They are used in communication systems, particularly in satellite communications and fiber-optic communications. In satellite communications, polarizers are used to ensure that the transmitted signal is polarized in a specific direction, which makes it easier to receive and process at the receiving end. In fiber-optic communications, polarizers are used to maintain the polarization of the light as it travels through the fiber, which helps to reduce signal loss and distortion.
Polarizers are used in medicine, particularly in dermatology, where polarized light, capable of penetrating deeper into the skin than unpolarized light, proves useful for diagnosing and treating skin conditions such as psoriasis, eczema, and vitiligo. Polarizers are also used in polarized light microscopy to study the structure of biological tissues. They are used in astronomy to reduce the glare and scattered light from the sky. Also, they are used to enhance the contrast of faint objects in the night sky, making them easier to observe and study.
