por What is physical optics?
The physical optics It is the branch of optics that studies the wave nature of light and the physical phenomena that can only be understood from the wave model. He also studies the phenomena of interference, polarization, diffraction and others that cannot be explained from geometric optics.
The wave model defines light as an electromagnetic wave whose electric and magnetic fields oscillate perpendicular to each other.
The electric field (AND) of the light wave behave in a similar way to its magnetic field (B.), but the electric field predominates over the magnetic field due to Maxwell’s relationship (1831-1879) which establishes the following:
AND=cB
Where c = Velocity of propagation of the wave.
Physical optics does not explain the absorption and emission spectrum of atoms, concepts that quantum physics addresses.
History of physical optics
The history of physical optics begins with the experiments carried out by Francesco Grimaldi (1618-1663), who observed that the shadow cast by an illuminated object appeared wider and was surrounded by colored fringes.
This phenomenon is called diffraction. His experimental work led him to propose the wave nature of light, in opposition to Isaac Newton’s conception, which prevailed during the 18th century.
The Newtonian paradigm established that light behaved like a beam of small corpuscles that moved at great speed in rectilinear trajectories.
Robert Hooke (1635-1703) defended the wave nature of light, in his studies on color and refraction, stating that light behaved like a sound wave propagating rapidly almost instantaneously through a material medium.
Later, Christiaan Huygens (1629-1695), basing himself on Hooke’s ideas, consolidated the wave theory of light in his treat of the light (1690), in which he supposed that the light waves emitted by luminous bodies propagate through a subtle and elastic medium called ether.
Huygens’s wave theory explains the phenomena of reflection, refraction and diffraction much better than Newton’s corpuscular theory, and shows that the speed of light decreases when passing from a less dense medium to a more dense one.
Huygens’s ideas were not accepted by the scientists of the time for two reasons. The first was the impossibility of satisfactorily explaining the definition of ether, and second, Newton’s prestige around his theory of mechanics influenced a vast majority of scientists to decide to support the corpuscular paradigm of light.
Revival of wave theory
At the beginning of the 19th century, Tomas Young (1773-1829) got Huygens’s wave model accepted by the scientific community based on the results of his light interference experiment. The experiment made it possible to determine the wavelengths of the different colors.
In 1818, Augustin Fresnell (1788-1827) restated Huygens’ wave theory in terms of the interference principle. He also explained the phenomenon of birefringence of light, which allowed him to affirm that light is a transverse wave.
In 1808, François Arago (1788-1853) and Étienne Malus (1775-1812) explained the phenomenon of light polarization from the wave model.
The experimental results of Hippolyte Fizeau (1819-1896), in 1849, and Léon Foucalt (1819-1868), in 1862, made it possible to verify that light propagates faster in air than in water, contradicting the explanation given by Newton. .
In 1872, James Clerk Maxwell (1831-1879) published his treatise on electricity and magnetism in which he stated the equations that synthesize electromagnetism. From his equations, he obtained the wave equation that allowed us 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, reaching the conclusion that light is an electromagnetic wave.
Finally, Heinrich Rudolf Hertz (1857-1894), in 1888, managed to produce and detect electromagnetic waves and confirmed that light is a type of electromagnetic wave.
What does physical optics study?
This discipline studies the phenomena related to the wave nature of light, such as interference, diffraction and polarization.
Interference
Interference is the phenomenon by which two or more light waves overlap, coexisting in the same region of space, forming bands of bright and dark light.
Bright bands are produced when several waves add together to produce a wave with a higher amplitude. This type of interference is called constructive interference.
When the waves overlap to produce a wave of smaller amplitude, the interference is called destructive interference, and bands of dark light are produced.
The way the colored bands are distributed is called an interference pattern. The interference can be observed in soap bubbles or in the oil layers of a wet road.
Diffraction
Diffraction is the change in the direction of propagation that a light wave experiences when it hits an obstacle or opening, altering its amplitude and phase.
Like the phenomenon of interference, diffraction is the result of the superposition of coherent waves. Two or more light waves are coherent when they oscillate with the same frequency while maintaining a constant phase relationship.
As the obstacle becomes smaller compared to the wavelength, the phenomenon of diffraction predominates over the phenomenon of reflection and refraction in determining the distribution of the rays of the light wave, once it is incident on the object. obstacle.
Polarization
Polarization is the physical phenomenon by which the wave vibrates in a single direction perpendicular to the plane containing the electric field. If the wave does not have a fixed direction of propagation, the wave is said to be unpolarized. There are three types of polarization: linear, circular, and elliptical polarization.
If the wave vibrates parallel to a fixed line describing a straight line in the polarization plane, it is said to be linearly polarized.
When the electric field vector of the wave describes a circle in the plane perpendicular to the same direction of propagation, keeping its magnitude constant, it is said that the wave is circularly polarized.
If the electric field vector of the wave describes an ellipse in the plane perpendicular to the same direction of propagation, the wave is said to be elliptically polarized.
Frequent terms in physical optics
Polarizing: It is a filter that allows only a part of the light that is oriented in a single specific direction to pass through it without letting those waves that are oriented in other directions through.
wave front: geometric surface in which all parts of a wave have the same phase.
wave amplitude and phase: the 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: angle of incidence of light by which the light wave reflected from the source is fully polarized.
Infrared: light not visible to the human eye in the spectrum of electromagnetic radiation from 700 nm to 1,000 μm.
Speed of light: propagation speed constant of the light wave in a vacuum, whose value is 3×108m/s. The value of the speed of light varies when it propagates in a material medium.
Wavelength: measure of the distance between one crest and another, or between one trough and another, of the wave as it propagates.
Ultraviolet: non-visible electromagnetic radiation with a spectrum of wavelengths less than 400 nm.
Laws of physical optics
Some laws of physical optics that describe the phenomena of polarization and interference are mentioned.
Fresnell’s and Arago’s laws
1. Two light waves with linear, coherent, 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 non-coherent and orthogonal linear polarizations do not interfere with each other to form an interference pattern.
Law of Malus
It states that the intensity of light transmitted by a polarizer is directly proportional to the square of the cosine of the angle between the transmission axis of the polarizer and the polarization axis of the incident light. In other words:
I = I0cos2θ
I = Light intensity transmitted by the polarizer
θ = Angle between the axis of transmission and the axis of polarization of the incident beam
I0 = Incident light intensity
Brewster’s Law
The beam of light reflected by a surface is completely polarized, in the direction normal to the plane of incidence of the light, when the angle between the reflected beam and the refracted beam is equal to 90°.
Physical optics 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 remain between the solid and liquid state, whose molecules have a dipole moment that induces a polarization of the light that falls on them. From this property, displays of calculators, monitors, laptops, and cell phones have been developed.
Optical system design
Optical systems are often used in everyday life, in science, technology and health. Optical systems allow processing, recording and transmitting 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 makes high-resolution measurements of physical parameters based on the light wave. These measurements are made with interferometers and refractive instruments. In the medical area, metrology is used to constantly monitor the vital signs of patients.
Recent Research in Physical Optics
Optomechanical Kerker effect (AV Poshakinskiy1 and AN Poddubny, 2019)
Poshakinskiy and Poddubny showed that vibrating nanometer particles can manifest an optical-mechanical effect similar to that proposed by Kerker 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 the particles have magnetic responses of the same intensity as the 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 particles…