Introduction
In the Torah (the first part of the Hebrew Bible), it is said the first thing God said while creating our world was – “Let there be light”. Light is that important. The phrase, since, has been used as a motto of various universities and even in literature. Be it in the literal sense or a metaphorical one, light does seem to have immense value when it comes to knowledge.
In truth, there is no definitive answer to the question – “What is light?”. It is all dependent on perspective. For example, an artist defines light as nothing but a source that helps appreciate all things that are pleasing to the eye. In pure sciences, though, physicists define light as the electromagnetic radiation of a wavelength that is discernible to the human eye i.e. the light waves that can be seen by us. Electromagnetic radiations have a broad spectrum but visible light has a narrow range of 700 nanometers to 400 nanometers, which is between infrared rays to ultraviolet rays.
Light waves travel from the vacuum of outer space to reach the earth and are said to be produced via the vibration of electric charges. Light waves are also known to be transverse in nature, this means that the vibration of the charges is perpendicular to the direction of propagation of the wave.
The light emitted by the celestial bodies like the sun and stars, even the light of the candle – this light has unrestricted vibrations of charges. The direction of vibration of charges in these waves is not restricted to a single plane. This is unpolarized light. Contrastingly, when the vibrations of charges are restricted to a single plane in all the waves of the light, the light is said to be polarized. A laser is the commonest example of polarized light.
Differences between Polarized and Unpolarized light in Tabular form
| Parameter | Polarized Light | Unpolarized Light |
| Description | Polarized light is where all the vibrating light particles or charges are restricted to a single plane. | Unpolarized light is where the light particles or charges are not restricted to any plane. They are free to move in any plane. |
| Nature | Polarized light has a coherent nature. | Unpolarized light has an incoherent nature. |
| Electric Field | The electric field in polarized light oscillates in only one direction. | The electric field in unpolarized light oscillates in multiple directions. |
| Phases | In polarized light, the x-component and the y-component have a constant phase difference between them. | In unpolarized light, the x-component and the y-component, the phase difference undergoes random changes. |
| Production | When light from natural sources undergoes reflection or scattering, it gets polarized (partially). | Light that is commonly found is nature is unpolarized like sunlight or light from stars or jellyfish. |
| Intensity | The intensity of polarized light is dependent on the polarizer. | The intensity of the unpolarized light is dependent on the source of the light. |
| Applications | Lasers, infrared spectrometry. | Vision, photosynthesis. |
What is Unpolarized light?
Unpolarized light is the light that is present in abundance all around us. It is the sunlight that wakes one up every morning and the starlight that puts one to sleep at night. Even the light from the candle flame or the incandescent bulb overhead is unpolarized.
In nature, even unpolarized light when scattered by particles that are smaller than the amplitude of the wavelength of light forms partially polarized light. For example, when sunlight passed through the atmosphere to reach the surface of the earth, it is partially polarized. The extent to which it is polarized is dependent on the scattering angle. Most human eyes are unobservant to the polarization effect. Some insects, though, are receptive to this polarization and use the polarization of the sky as a navigational tool.
In unpolarized light, the wave-trains change direction unprecedentedly and randomly i.e. the phase difference between the x-component of the wave and the y-component of the wave can change unpredictably. This is what causes the nature of unpolarized light to be incoherent. Even the electric fields are not restricted to a plane – the particles oscillate in any plane they wish to as per their preference.
Unpolarized light can easily be polarized by the following processes:
- Transmission
- Reflection
- Refraction
- Scattering
Depending on the orientation of the electric field, the polarization can be of the following 3 types:
- Linear polarization: Here, the electric field and the direction of propagation are in the same plane.
- Circular polarization: Here, the electric field is comprised of 2 waves. These waves have the same amplitude and are perpendicular to each other. There is also a constant phase difference of p/2. Thus, the electric field moves circularly around the direction of propagation. Depending on the direction of the rotation, it is left-hand or right-hand circularly polarized.
- Elliptical polarization: Here, the resulting electric field forms an ellipse around the direction of propagation. The amplitudes of the two waves are unequal and the phase difference between them is not p/2. In other words, any polarized light that is not circular or linear is an elliptically polarized light.
What is Polarized Light?
Polarization is a process by which the light particles in unpolarized light are made to vibrate in a single plane. In nature, very often, unpolarized light undergoes multiple reflections, scattering and becomes partially polarized.
In polarized light, the wave trains are unidirectional i.e. the charges and particles, of all waves, all vibrate in a single plane. There is no phase difference at all in the x-component and y-component of the waves. This leads the light waves to be extremely coherent. The electric fields of the light waves are also restricted to a single plane – the particles oscillate in a single plane.
It was French physicist, Etienne Louis Malus, who discovered that when unpolarized light undergoes reflection from a non-metallic surface, it becomes partially polarized. The degree to which the light wave got polarized was dependent on the angle of incidence and the refractive index of the reflecting material. The direction of polarization is found to be parallel to the surface that reflects the light.
The intensity of the polarized light depends on the polaroid filter that is used. American physicist, Edwin Land, invented the polaroid which is comprised of sheets of long-chain hydrocarbons that have undergone heat treatment. The polaroid has varied applications depending on its use to remove scattered or reflected light. It is used in sunglasses, camera filters, telescopes etc.
Unpolarized light can easily be polarized by the following processes:
- Transmission
- Reflection
- Refraction
- Scattering
Depending on the orientation of the electric field, the polarization can be of the following 3 types:
- Linear polarization: Here, the electric field and the direction of propagation are in the same plane.
- Circular polarization: Here, the electric field is comprised of 2 waves. These waves have the same amplitude and are perpendicular to each other. There is also a constant phase difference of p/2. Thus, the electric field moves circularly around the direction of propagation. Depending on the direction of the rotation, it is left-hand or right-hand circularly polarized.
- Elliptical polarization: Here, the resulting electric field forms an ellipse around the direction of propagation. The amplitudes of the two waves are unequal and the phase difference between them is not p/2. In other words, any polarized light that is not circular or linear is an elliptically polarized light.
In nature, even unpolarized light when scattered by particles that are smaller than the amplitude of the wavelength of light forms partially polarized light. For example, when sunlight passed through the atmosphere to reach the surface of the earth, it is partially polarized. The extent to which it is polarized is dependent on the scattering angle. Most human eyes are unobservant to the polarization effect. Some insects, though, are receptive to this polarization and use the polarization of the sky as a navigational tool.
Polarized light can be manipulated in various ways. It mainly occurs due to polarizers. Polarizers are of 3 types:
- Reflective
- Dichoric
- Birefringent
Reflective polarizers are those polarizers that permit the transmission of the desired polarized light and at the same time, reflect the undesired light. An example of this kind of polarizer is wire grid polarizers – these polarizers contain multiple thin wires that are arranged in a parallel fashion. The light waves that are reflected are the ones travelling along the direction of these wires while the undesired waves that are perpendicular to the direction of wires are transmitted.
Dichoric polarizers are those that absorb the need for specific polarization of the light waves and transmit the rest of the waves. An example of this type of polarizer is the modern nanoparticle.
Birefringent polarizers are those polarizers that count on the dependence of the refractive index on the degree of polarization of the light waves. Specific or desired polarization of light can be selected depending upon the degree of polarization produced at the different refractive angles of the light waves.
Another way to manipulate polarized light is by the use of waveplates. Wave-plates are devices that do not select or discard polarized light, rather they modify the existing polarized light waves. This happens without the action of attenuation, deviation or displacement of the light waves. Wave-plates do this by delaying a component of the light wave concerning the light wave that is orthogonal to it. Due to this function of wave-plates, they create a new state of polarized light from any existing polarized light. It is most often used to form circularly polarized light from the polarized light that has a linear configuration or vice-versa.
These flexible and intriguing properties and functions of polarized light make it crucial for most imaging applications. Polarization of light waves can be used to increase or decrease the focus of laser beams, influence the wavelengths of light filters and even prevent unnecessary reflection of light. Polarization has immense applications, especially in the field of optics. The main uses of polarization are due to the property of polarizers to increase the intensity of the light or contrast it or help discover surface defects that are usually hidden from the naked eye.
- Polarizers help eliminate glare from the scattered light and are hence used in sunglasses.
- In plastic industries, polarized filters are used in performing the Stress Analysis Test. The non-amorphous materials like plastics and glass undergo stress from the temperature and pressure. this further leads the material to become birefringent and non-homogenous. This stress is analyzed using polarization.
- Polarization is used in infrared spectroscopy.
- Entertainment has seen a rise due to 3-dimensional movies. The production and showcasing of these movies happen with the help of polarization.
- In seismology, polarization helps prevent natural disasters by studying earthquakes.
- In chemistry, the chirality of organic compounds is tested using the techniques employing polarization. This property of polarization is used in food, beverage and even pharmaceutical companies.
- In physics, polarization is used to differentiate between the transverse waves and the longitudinal waves.
- Polarized microscopes are used to identify specimen thickness and birefringence.
Main Differences between polarized light and unpolarized light in points
- The most important difference between polarized and unpolarized light is the direction of vibration of the light particles. In polarized light, the light particles vibrate in a single plane in all the waves while in unpolarized light, it is quite the opposite. The light particles vibrate in any direction in the plane.
- The light wave in polarized light is very coherent while the light wave in unpolarized light is incoherent.
- In the electric field of the polarized light, the charged particles oscillate in a single plane whereas the charged particles in the unpolarized light oscillate in multiple planes.
- The phase difference observed in the x-component of the wave and the y-component of the wave in the polarized light is always constant. This is not the case in unpolarized light. Here, the phase difference between the x-component of the wave and the y-component of the wave is rather unpredictable.
- Polarized light, in nature, is obtained when the light undergoes multiple reflections or scattering or is filtered via a polarizing filter like a polaroid. In nature, the light from the sun, moon, stars, and even luminescent creatures like the jellyfish, all express unpolarized light.
- The intensity of the polarized light is dependent on the material that causes the polarization i.e. the polarizing filter. The intensity of the unpolarized light is dependent on the source of the unpolarized light.
- Artificial sources of polarized light include the laser. LED lights, incandescent lights and even fluorescent lights are mostly unpolarized. They have a small degree of polarization.
- The application of polarized light is in 3-dimensional movies and infrared spectrometry. Unpolarized light has many more applications like basic vision, photosynthesis in plants etc.
Conclusion
Light, since the beginning, has seen a tremendous transformation and polarization is just one of the few phenomena that have further empowered light itself. Polarization has advanced our knowledge and technology at the same time. We can now study matter and organisms better with the use of laser-powered optical instruments; cut through metal as if wielding a lightsaber; upgrade security with a maze of razor-sharp zig-zags that are only intense light waves and maybe even destroy space ships with just a sweep like Darth Vader’s Death Star (this technology is rather more wishful and has yet to see the light of day). We can do all that and more. Polarized or unpolarized, light is fundamental and its future seems only bright.
References
- https://en.wikipedia.org/wiki/Let_there_be_light
- https://www.britannica.com/science/light/Radiation-pressure
- https://www.britannica.com/science/light
- https://www.edmundoptics.com/knowledge-center/application-notes/optics/introduction-to-polarization/
