Modern Physics

Physical optics: history, frequent terms, laws, applications

The optical physics is studying of the optical wave nature of light and physical phenomena which comprise only from the wave model. It also studies the phenomena of interference, polarization, diffraction and other phenomena that cannot be explained from a geometric perspective.

The wave model defines light as an electromagnetic wave whose electric and magnetic fields oscillate perpendicular to each other.

The electric field ( E ) of the light wave behaves similarly to its magnetic field ( B ), but the electric field predominates over the magnetic field due to Maxwell’s (1831 to 1879) reason, who states the following:

= cB

Where c = wave propagation velocity.

Physical optics does not explain the absorption and emission spectrum of atoms. On the other hand, quantum optics addresses the study of these physical phenomena.

History

The history of physical optics begins with the experiments carried out by Grimaldi (1613-1663), who observed that the shadow cast by an illuminated object was wider and surrounded by colored stripes.

The phenomenon observed was called diffraction. His experimental work led him to elevate the wave nature of light, in opposition to Isaac Newton’s conception that prevailed during the 18th century.

The Newtonian paradigm established that light behaved like a ray of small corpuscles that moved at great speed in straight trajectories.

Robert Hooke (1635-1703) defended the wave nature of light in his studies of color and refraction, claiming that light behaved like a sound wave spreading rapidly almost instantaneously through a material medium.

Later, Huygens (1629-1695), based on Hooke’s ideas, consolidated the theory of light waves in his Traité de la lumière (1690), in which he assumes that light waves emitted by light bodies propagate through a subtle and elastic medium called ether .

Huygens’ wave theory explains the phenomena of reflection, refraction and diffraction much better than Newton’s corpuscular theory and demonstrates that the speed of light decreases as it passes from a less dense medium to a denser one.

Huygens’ ideas were not accepted by scientists at the time for two reasons. The first was the inability to satisfactorily explain the definition of ether, and the second was Newton’s prestige around his theory of mechanics which influenced the vast majority of scientists to decide to support the corpuscular paradigm of light.

Rebirth of wave theory

In the early 19th century, Tomas Young (1773-1829) led the scientific community to accept the Huygens wave model based on the results of his light interference experiment. The experiment allowed us to determine the wavelengths of the different colors.

In 1818, Fresnell (1788-1827) rethought the theory of Huygens waves based on the principle of interference. He also explained the phenomenon of birefringence of light, which allowed him to claim that light is a transverse wave.

In 1808, Arago (1788-1853) and Malus (1775-1812) explained the phenomenon of light polarization using the wave model.

The experimental results of Fizeau (1819-1896) in 1849 and Foucalt (1819-1868) in 1862 allowed us to verify that light propagates faster in air than in water, contradicting Newton’s explanation.

In 1872, Maxwell published his Treatise on Electricity and Magnetism, in which he enunciated the equations that synthesize electromagnetism. From his equations, he obtained the wave equation that allowed him to analyze the behavior of an electromagnetic wave.

Maxwell found that the speed of propagation of an electromagnetic wave is related to the propagation medium and coincides with the speed of light, concluding that light is an electromagnetic wave.

Finally, Hertz (1857-1894) in 1888 manages to produce and detect electromagnetic waves and confirms that light is a type of electromagnetic wave.

What does physical optics study?

Physical optics studies phenomena related to the nature of light waves, such as interference, diffraction and polarization.

Interference

Interference is the phenomenon in which two or more light waves overlap, coexisting in the same region of space, forming bands of bright and dark light.

Bright bands occur when several waves add up to produce a wave of greater amplitude. This type of interference is called constructive interference.

When waves overlap to produce a wave of lesser amplitude, the interference is called destructive interference, and dark bands of light are produced.

The way the colored bands are distributed is called an interference pattern. Interference can be seen in soap bubbles or oil layers on a wet road.

Diffraction

The phenomenon of diffraction is the change in the propagation direction that the light wave experiences when it affects an obstacle or opening, altering its amplitude and phase.

Like the interference phenomenon, diffraction is the result of superposition of coherent waves. Two or more light waves are coherent when they oscillate at the same frequency, maintaining a constant phase relationship.

As the obstacle becomes smaller and smaller in relation to the wavelength, the diffraction phenomenon predominates over the reflection and refraction phenomenon in determining the distribution of the rays of the light wave once it hits the obstacle. .

Polarization

Polarization is the physical phenomenon whereby a wave vibrates in a single direction perpendicular to the plane containing the electric field. If the wave does not have a fixed propagation direction, the wave is said to be unpolarized. There are three types of polarization: linear polarization, circular polarization and elliptical polarization.

If the wave vibrates parallel to a fixed line that describes a straight line in the plane of polarization, it is said to be linearly polarized.

When the wave’s electric field vector describes a circle in the plane perpendicular to the same propagation direction, keeping its magnitude constant, the wave is said to be circularly polarized.

If the wave’s electric field vector describes an ellipse in the plane perpendicular to the same propagation direction, the wave is said to be elliptically polarized.

Frequent terms in physical optics

Polarizer

It is a filter that allows only a portion of light oriented in a single specific direction to pass through it without letting waves that are oriented in other directions.

wavefront

It is the geometric surface on which all parts of a wave have the same phase.

Amplitude and wave phase

Amplitude is the maximum elongation of a wave. The phase of a wave is the state of vibration at an instant of time. Two waves are in phase when they have the same state of vibration.

Brewster’s Angle

It is the angle of incidence of light through which the wave of light reflected from the source is fully polarized.

Infra-red

Light not visible to the human eye in the spectrum of electromagnetic radiation from 700 nm to 1000 μm.

Speed ​​of light

It is a light wave propagation constant in a vacuum whose value is 3 × 10 8 m / s. The value of the speed of light varies when propagated through a material medium.

Wave-length

Measuring the distance between crest and crest or between valley and valley of the wave as it propagates.

Ultraviolet

Non-visible electromagnetic radiation with a wavelength less than 400 nm.

Laws of Physical Optics

Below are some laws of physical optics that describe the polarization and interference phenomena.

Fresnell and Arago’s Laws

1. Two light waves with linear, coherent and orthogonal polarizations do not interfere with each other to form an interference pattern.
2. Two light waves with linear, coherent and parallel polarizations can interfere in a region of space.
3. Two natural light waves with linear, non-coherent and orthogonal polarizations do not interfere with each other to form an interference pattern.

malus law

Malus’ Law states that the intensity of light transmitted by a polarizer is directly proportional to the square of the cosine of the angle that forms the axis of transmission of the polarizer and the axis of polarization of the incident light. In other words:

I = I cos 2 θ

I = Intensity of light transmitted by the polarizer

θ = angle between transmission axis and incident beam polarization axis

= Intensity of incident light

Brewster’s Law

The beam of light reflected by a surface is completely polarized, in the direction normal to the plane of incidence of light, when the angle formed by the beam reflected with the refracted beam is equal to 90°

applications

Some of the applications of physical optics are found in the study of liquid crystals, in the design of optical systems, and in optical metrology.

liquid crystals

Liquid crystals are materials that lie between a solid and a liquid state, whose molecules have a dipole moment that induces a polarization of the light that falls on them. From this property, calculator screens, monitors, laptops and cell phones were developed.

Optical Systems Design

Optical systems are often used in everyday life, science, technology and health. Optical systems allow you to process, record and transmit information from light sources such as the sun, LED, tungsten lamp or laser. Examples of optical systems are the diffractometer and the interferometer.

optical metrology

It is responsible for performing high resolution measurements of physical parameters based on the light wave. These measurements are made with interferometers and refraction instruments. In the medical field, metrology is used to constantly monitor patients’ vital signs.

Recent Research in Physical Optics

Optomechanical Kerker Effect (AV Poshakinskiy1 and AN Poddubny, January 15, 2019)

Poshakinskiy and Poddubny (1) demonstrated that nanometric particles with vibrating motion can manifest an optical-mechanical effect similar to that proposed by Kerker et al (2) in 1983.

The Kerker effect is an optical phenomenon that consists in obtaining a strong directionality of light scattered by spherical magnetic particles. This directionality requires that particles have magnetic responses of the same intensity as electrical forces.

The Kerker effect is a theoretical proposal that requires material particles with magnetic and electrical characteristics that currently do not exist in nature. Poshakinskiy and Poddubny achieved the same effect on nanosized particles, without significant magnetic response, vibrating in space.

The authors demonstrated that particle vibrations can create electrical and magnetic polarizations that interfere appropriately, as components of magnetic and electrical polarity of the same order of magnitude are induced in the particle when considering the inelastic scattering of light.

The authors propose the application of the optical-mechanical effect in nanometric optical devices, vibrating them by applying acoustic waves.

Extracorporeal optical communication (DR Dhatchayeny and YH Chung, May 2019)

Dhatchayeny and Chung (3) propose an experimental extracorporeal optical communication (OEBC) system that can transmit information about people’s vital signs through applications on cell phones with Android technology. The system consists of a set of sensors and a diode concentrator (set of LEDs).

Sensors are placed in various parts of the body to detect, process and communicate vital signs such as pulse, body temperature and respiratory rate. Data is collected through the LED array and transmitted through the mobile phone camera with the optical app.

The LED array emits light in the Rayleigh Gans Debye (RGB) scattering wavelength range. Each color and color combinations of emitted light are related to vital signs.

The system proposed by the authors can facilitate the reliable monitoring of vital signs, since the errors in the experimental results were minimal.

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