Optics

Summary

Optics is the study of light. It includes the description of light properties that involve refraction, reflection, diffraction, interference, and polarization of electromagnetic waves. Most commonly, the word "light" refers to the visible wavelengths of the electromagnetic spectrum, which is between 380 and 700 nanometers (nm). Lasers use wavelengths that vary from the ultraviolet (100 nm to 380 nm) through the visible spectrum into the infrared spectrum (greater than 700 nm). Optics can be used to understand and study mirrors, optical instruments such as telescopes and microscopes, vision, and lasers used in industry and medicine.

Definition and Basic Principles

Optics is the area of physics that involves the study of electromagnetic waves in the visible-light spectrum, between 380 and 700 nm. Optics principles also apply to lasers, which are used in industry and medicine. Each laser has a specific wavelength. There are lasers that use wavelengths in the 100–400 nm range, others that use a wavelength in the visible spectrum, and some that use wavelengths in the infrared spectrum (greater than 700 nm).

Light behaves as both a wave and a particle. This duality has resulted in the division of optics into physical optics, which describes the wave properties of light; geometric optics, which uses rays to model light behavior; and quantum optics, which deals with the particle properties of light. Optics uses these theories to describe the behavior of light in the form of refraction, reflection, interference, polarization, and diffraction.

When light and matter interact, photons are absorbed or released. Photons are a specific amount of energy described as the sum of Planck's constant, h (6.626 × 10⁻³⁴) ,and the wavelength of the light. The formula to describe the energy of photons is E = hf. Photons have a constant speed in a vacuum. The speed of light is c = 2.998 × 10⁸. The constant speed of light in a vacuum is an important concept in astronomy. The speed of light is used in the measurement of astronomic distances in the unit of light-years.

Background and History

Optics dates back to ancient times. The three-thousand-year-old Nimrud lens is crafted from natural crystal, and it may have been used for magnification or to start fires. Early academics such as Euclid in 300 BCE theorized that rays came out of the eyes in order to produce vision. Greek astronomer Ptolemy later described angles in refraction. In the thirteenth century, English philosopher Roger Bacon suggested that the speed of light was constant and that lenses might be used to correct defective vision.

By the seventeenth century, telescopes and microscopes were being developed by scientists such as Hans Lippershey, Johannes Kepler, and Galileo Galilei. During this time, Dutch astronomer Willebrord Snellius formulated the law of refraction to describe the behavior of light traveling between different media, such as from air to water. This is known as Snell's law, or the Snell-Descartes law, although it was previously described in 984 by Persian physicist Ibn Sahl.

Sir Isaac Newton was one of the most famous scientists to put forward the particle theory of light. Dutch scientist Christiaan Huygens was a contemporary of Newton's who advocated the wave theory of light. This debate between wave theory and particle theory continued into the nineteenth century. French physicist Augustin-Jean Fresnel was influential in the acceptance of the wave theory through his experiments in interference and diffraction.

The wave-particle debate continued into the next century. The wave theory of light described many optical phenomena; however, some findings, such as the emission of electrons when light strikes metal, can be explained only using a particle theory. In the early twentieth century, German physicists Max Planck and Albert Einstein described the energy released when light strikes matter as photons with the development of the formula E = hv, which states that the photon energy equals the sum of the wavelength and Planck's constant.

In the early twenty-first century, it is generally accepted that both the wave and the particle theories are correct in describing optical events. For some optical situations, light behaves as a wave, and for others, the particle theory is needed to explain the situation. Quantum physics tries to explain the wave-particle duality, and it is possible that future work will unify the wave and particle theories of light.

How It Works

Physical Optics. Physical optics is the science of understanding the physical properties of light. Light behaves as both a particle and a wave. According to the wave theory, light waves behave similarly to waves in water. As light moves through the air the electric field increases, decreases, and then reverses direction. Light waves generate an electric field perpendicular to the direction the light is traveling and a magnetic field that is perpendicular both to the direction the light is traveling and to the electric field.

Interference and coherence refer to the interactions between light rays. Both interference and coherence are often discussed in the context of a single wavelength or a narrow band of wavelengths from a light source. Interference can result either in an increased intensity of light or a reduction of intensity to zero. The optical phenomenon of interference is used in the creation of antireflective films.

Coherence occurs when light is passed through a narrow slit. This produces waves that are in phase with the waves exactly lined up or waves that are out of phase but have a constant relationship with one another. Coherence is an important element to the light emitted by lasers and allows for improved focusing properties necessary to laser applications.

Polarization involves passing light waves through a filter that allows only wavelengths of a certain orientation to pass. For example, polarized sunglasses allow only vertical rays to pass and stop the horizontal rays, such as light reflected from water or pavement. In this way, polarized sunglasses can reduce glare.

Diffraction causes light waves to change direction as light encounters a small opening or obstruction. Diffraction becomes a problem for optical systems of less than 2.5 millimeters (mm) for visible light. Telescopes overcome the diffraction effect by using a larger aperture, however, for very large-diameter telescopes the resolution is then limited due to atmospheric conditions. Space telescopes such as the Hubble are unaffected by these conditions as they are operating in a vacuum.

Scattering occurs when light rays encounter irregularities in their path, such as dust in the air. The increased scattering of blue light due to particles in the air is responsible for the blue color of the sky.

Illumination is the quantitative measurement of light. The watt is the measurement unit of light power. Light can also be measured in terms of its luminance as it encounters the eye. Units of luminance include the lumen, the candela, and the now-obsolete apostilb.

The photoelectric effect that supports the particle theory of light was discovered by German physicist Heinrich Rudolph Hertz in 1887 and later by Albert Einstein. When light waves hit a metallic surface, electrons are emitted. This effect is used in the generation of solar power.

Geometric Optics. Geometric optics describes optical behavior in the form of rays. In most ordinary situations, the ray can accurately describe the movement of light as it travels through various media such as glass or air and as it is reflected from a surface such as a mirror.

Geometric optics can describe the basics of photography. The simplest way to make an image of an object is to use a pinhole to produce an inverted image. When lenses and mirrors are added to the pinhole a refined image can be produced.

Reflection and refraction are two optical phenomena in which geometric optics applies. Reflections from plane (flat) mirrors, convex mirrors, and concave mirrors can all be described using ray diagrams. A plane mirror creates a virtual image behind the mirror. The image is considered virtual because the light is not coming from the image but only appears to because of the direction of the reflected rays. A convex mirror can create a real image in front of the lens or a virtual image behind the lens depending on where the object is located. If an object is past the focal point of the convex mirror then the image is real and located in front of the mirror. If the object is between the focal point and the convex mirror then the image is virtual and located behind the mirror. A convex lens will create a virtual image. Geometric optics involves ray diagrams that will allow the determination of image size (magnification or minification), location of the image, and if it is real or virtual.

Refraction of light happens when light passes between two different substances such as air and glass or air and water. Snell's law expresses refraction of light as a mathematical formula. One form of Snell's law is: n_i sin θ_i = n_t sin θ_t where n_i is the refractive index of the incident medium, θ_i is the angle of incidence, n_t is the refractive index of the refracted medium, and θ_t is the angle of transmission. This formula, along with its variations, can be used to describe light behavior in nature and in various applications such as manufacturing corrective lenses. Refraction also occurs as light travels from the air into the eye and as it moves through the various structures inside the eye to produce vision.

Magnification or minification can be a product of refraction and reflection. Geometric optics can be applied to both microscopes and telescopes, which use lenses and mirrors for magnification and minification.

Quantum Optics. Quantum optics is a division of physics that comes from the application of mathematical models of quantum mechanics to the dual wave and particle nature of light. This area of optics has applications in meteorology, telecommunications, and other industries.

Applications and Products

Vision and Vision Science. There is a vast network of health-care professionals and industries that study and measure vision and vision problems as well as correct vision. Optometrists measure vision and refractive errors in order to prescribe corrective spectacles and contact lenses. Ophthalmologists are medical doctors who specialize in eye health and vision care. Some ophthalmologists specialize in vision-correction surgery, which uses lasers to reduce the need for glasses or contact lenses. In order to perform vision-correction surgeries there are a number of optical instruments, including wave-front mapping analyzers, that may be used.

The industries that support optometry and ophthalmology practices include laser manufacturers, optical diagnostic instruments manufacturers, and lens manufacturers. Lenses are used for diagnosis of vision problems as well as for vision correction.

Development of new lens technology in academic institutions and industry is ongoing, including multifocal lens implants and other vision-correction technologies.

Research. Many areas of research, including astronomy and medicine, use optical instruments and optics theory in the investigation of natural phenomena. In astronomy, distances between planets and galaxies are measured using the characteristics of light traveling through space and expressed as light-years. Meteorological optics is a branch of atmospheric physics that uses optics theory to investigate atmospheric events. Both telescopes and microscopes are optimized using optical principles. Many branches of medical research use optical instruments in the investigations of biological systems.

Communication. The Internet has become the backbone of modern society. Enabling this backbone are the thousands of kilometers of fiber optic cables running underneath the oceans, connecting continents. Fiber-optic communication is immune to electromagnetic interference and electrical noise, making it suitable for long-distance communication, especially between continents. A fiber-optic communication network consists of a laser at the transmitter end, an optical fiber, optical signal amplification and repeating equipment, and a photodiode at the receiver end.

Medicine. Lasers have become commonplace in medicine, from skin-resurfacing and vision-correction procedures to the use of carbon-dioxide lasers in general surgery. Fiber-optics have also found a critical place in endoscopy and biomedical sensor applications to further advance minimally invasive surgeries (MIS).

Industry. As noted above, there is an industry sector that is dedicated to the manufacture and development of vision-correction and diagnostic lenses and tools. Optics is an important part of the telecommunications industry, which uses fiber optics to transmit images and information. Photography, from the manufacture of cameras and lenses to their use by photographers, involves applied optics. Lasers are also used for precision manufacturing of a variety of products.

Fault Detection. Specialized optical fiber cables embedded in bridges and tunnels are used as a low-cost alternative to detect structural fatigue by utilizing the optical behavior of optical fiber material.

Careers and Course Work

A career in an optics field can be as varied as the applications. An interest in optics might lead to a career in physics, astronomy, meteorology, vision care, photography, or communication engineering. Depending on the specific position desired, the training may range from a high school diploma and on the job training to a university degree and postgraduate work.

Optics involves a combination of math and physics. An understanding of human eye anatomy is also essential for a career in vision care. For all of optics-related fields it is important to have a strong background in high school mathematics. For occupations in allied health care such as opticians or ophthalmic technicians, a high school diploma and technical training is required post high school. Photographers may pursue formal training through a university or art school or might develop skills through experience or an apprenticeship.

Many careers in physics, astronomy, communication, and meteorology require at least a bachelor's degree and most require a master's or doctoral degree. University course work in these fields includes mathematics, physics, basics of network communication, and basic electronics. To become an optometrist a bachelor's degree plus a doctor of optometry degree is required. An ophthalmologist will need a bachelor's degree, a degree in medicine, and residency training.

Social Context and Future Prospects

The advancements in optics theory and application have changed the fabric of life in industrialized countries, from the way people communicate to how the universe is understood. It is almost impossible to imagine what future advances will occur in optics, since the last fifty years has brought profound changes in the fields of photography, medicine, astronomy, manufacturing, and a number of other fields.

As wireless technology advances it seems possible that this technology may replace some of the millions of miles of fiber-optic telecommunications cables that currently exist. Because of their reliability, fiber optics will continue to be used for the foreseeable future. Advancements in laser technology will enable the generation of more focused and narrower light beams, further improving reliability. Existing lasers will continue to be to optimized, and most likely new lasers will be developed.

Refinements in optical systems will aid in research in a variety of fields. For example, oceanographers already apply optics theory to the study of low-light organisms and to the development of techniques for conducting research in low light. Improved optical systems will likely have a positive impact on this and other research.

Quantum computers using photonic circuits are a possible future development in the field of optics. A quantum computer that takes advantage of the photoelectric effect may be able to increase the capacity of computation over conventional computers. Optics and photonics may also be applied to chemical sensing, imaging through adverse atmospheric conditions, and solid-state lighting.

Some scientists have commented that the wave and particle theories of light are perhaps a temporary solution to the true understanding of light behavior. The area of quantum optics is dedicated to furthering the understanding of this duality of light. It is possible that in the future a more unified theory will lead to applications of optics and the use of light energy in ways that have not yet been imagined.

Bibliography

Azar, Dimitri T. Clinical Optics, 2014–2015. San Francisco: Amer. Acad. of Ophthalmology, 2014. Print. Basic and Clinical Science Course 3.

Jacoby, Mitch. "As Telecom Demands Grow, Optical Fibers Will Need to Level Up." C&EN, 16 Mar. 2020, cen.acs.org/materials/photonics/telecom-demands-grow-optical-fibers/98/i10. Accessed 9 Jun. 2021.

Mendez, Alexis. "Medical Applications of Fiber-Optics: Optical Fiber Sees Growth as Medical Sensors." Laser Focus World, 1 Jan. 2011, www.laserfocusworld.com/fiber-optics/article/16562291/medical-applications-of-fiberoptics-optical-fiber-sees-growth-as-medical-sensors. Accessed 8 Jun. 2021.

Meschede, Dieter. Optics, Light and Lasers: The Practical Approach to Modern Aspects of Photonics and Laser Physics. 2nd ed. Weinheim: Wiley, 2007. Print.

Pedrotti, Frank L., Leno M. Pedrotti, and Leno S. Pedrotti. 3d ed. Introduction to Optics. 3rd ed. Upper Saddle River: Prentice, 2007. Print.

Siciliano, Antonio. Optics: Problems and Solutions. Singapore: World Scientific, 2006. Print.

Tipler, Paul A., and Gene Mosca. Physics for Scientists and Engineers. 6th ed. New York: Freeman, 2008. Print.

Wolfe, William J. Optics Made Clear: The Nature of Light and How We Use It. Bellingham: SPIE, 2007. Print.