Optical physics

Optical physics is the study of light, its fundamental properties, how it interacts with matter, and the instruments used to measure or apply its interactions. Light refers to electromagnetic radiation; within optical physics, this includes ultraviolet, visible, and infrared light, not electromagnetic radiation at higher (i.e., x-rays and gamma rays) or lower frequencies (i.e., microwaves and radio waves). Optical physics studies classical optical phenomena, such as reflection, refraction, diffraction, and interference, as well as the quantum mechanical properties of individual packets of light known as photons. The study of optical physics can generally be separated into three branches:

1. Geometrical optics—the study of light as rays
2. Physical optics—the study of light as waves
3. Quantum optics—the study of light as particles

Optical physics forms the basis for a number of optical devices, including microscopes, telescopes, cameras, lasers, and optical fiber, with applications in a wide range of fields, including astronomy, biotechnology, defense, meteorology, photonics, and medical instruments.

Ancient Greek and Arabic civilizations had some knowledge of the properties of light. The first known treatise on the subject is Euclid's textbook, called Optics (around 300 BC). His work described light as visual rays that travel in straight lines with the various angles formed by these defining the perspective and the size of an object from an observer's position. For example, demonstrating the geometry behind why nearer objects appear larger than distant objects and measuring the height of distant objects from their shadows or reflected images. An extensive survey of optical phenomena was published by mathematician and astronomer Ptolemy (2nd century AD). The only surviving form of Ptolemy's treatise is an incomplete Latin translation from the twelfth century based on a now-lost Arabic translation. The text covers geometric optics, catoptrics, and experimental areas related to binocular vision and general principles.

The next major contribution to the field of optics was made by Arabic mathematician and physicist Ibn al-Haytham (born 965, died 1039), who published theories on refraction, reflection, binocular vision, focussing with lenses, the rainbow, parabolic and spherical mirrors, spherical aberration, atmospheric refraction, and the apparent increase in the size of planetary bodies when they are near the horizon. He also was the first to offer an accurate theory of vision, stating that light comes from the object seen to the eye.

In his 1690 Treatise on Light (Traité de la lumière), Dutch mathematician and physicist Christiaan Huygens provided a mechanical explanation of reflection and refraction and offered a theory on the nature of light relating it to wave motion. In 1704, Isaac Newton published Opticks, a book detailing refraction, dispersion, diffraction, polarization, and a theoretical description of light as moving particles. Newton's views, particularly the theory of light as particles, became the dominant opinion in scientific circles for over a century, overtaking Huygen's wave model.

During the early 1800s, English physician and physicist Thomas Young, studying light interference, produced experimental results that could only be explained if light consisted of waves. In 1801, he performed the famous "double-slit experiment," using a pinhole in a window shutter and a mirror to shine a horizontal beam of light across the room. A small paper card broke the pinhole beam into two separate beams producing an interference pattern.

Diagram demonstrating Young's double slit experiment.

Light passing through the two slits in the paper card would diffract, interacting with each other via constructive and destructive interference and creating a pattern of light and dark regions or fringes. This interference pattern was projected onto a screen where measurements could be made to determine the wavelength of the light.

Interference pattern produced by Young's double slit experiment.

Young's findings were corroborated by the mathematical analysis of French physicist Augustin-Jean Fresnel and led to the resurrection of the wave theory of light. This theory informed the work of Scottish mathematician James Clerk Maxwell, whose electromagnetic theory of light was published in 1864. Maxwell's work showed light and other forms of radiant energy propagate in the form of electromagnetic waves, disturbances generated by the oscillation or acceleration of an electric charge, and characterized by the temporal and spatial relations associated with wave motion.

At the turn of the twentieth century, quantum theory described light as photons with specific packets or quanta of energy. In 1900, German physicist Max Planck presented his quantum hypothesis to the German Physical Society. His model of black body radiation retained classical properties except with the quantized interaction of light with matter. In 1905, Einstein combined Planck’s blackbody quantum hypothesis with statistical mechanics to conclude that light must be quantized. In 1909, Einstein authored a paper studying energy fluctuations in blackbody radiation. This paper was the first time the wave-particle duality of light was suggested. Einstein concluded the paper with the following statement:

the next stage of the development of theoretical physics will bring us a theory of light which can be regarded as a kind of fusion of the wave theory and the emission theory ... a profound change in our views of the nature and constitution of light is indispensable.

The framework for a wave-particle theory of light was introduced by English Physicist Paul Dirac in 1927, after further development of quantum theory.

A branch of optics where light is described by rays. These light rays are conceived as geometrical lines originating from sources, extending through media, and revealed by detectors. Early optics research used geometry to model this view of light where it is postulated to travel along rays – line segments that remain straight in free space but may change direction or even curve when encountering matter. Two laws dictate what happens when light encounters a material surface:

1. The law of reflection—first stated by Euclid around 300 BC, states that light encountering a flat reflective surface will bounce off such that the angle of incidence of a ray is equal to the angle of reflection
2. The law of refraction—first experimentally determined by Willebrord Snell in 1621, explains how light rays change direction when passing across planar boundaries from one material to another

These two laws allow us to determine the behavior of optical devices, such as telescopes and microscopes. Tracing the paths of different rays (known as "ray tracing") as they pass through optical systems, it is possible to determine how images form, their relative orientation, and the magnification produced. Even with more advanced descriptions of light, ray tracing remains a valuable use of geometrical optics today. A simple illustration of it in action is shown below for the example of a clear glass lens. Rays incoming from the left are refracted twice by the lens, once on entry and once on exit, and the net result is the accumulation of all rays at a focal point on the right.

A simple demonstration of ray tracing for a clear glass lens refracting light to a focal point.

The path of light rays is related to the refractive index n of the media, defined as the ratio between the speed of light in a vacuum and the given medium. Assuming a constant refractive index allows for simplified ray equations where light always travels in a straight line within each medium. The basic properties of optical imaging systems are described by the first-order approximation of these ray equations.

Geometrical optics is based on the short-wavelength approximation of electromagnetic theory. It is defined in terms of a series of rules that can be derived from Maxwell's equations in a consistent approximation scheme.

Physical optics studies the wave properties of light, including the following:

• Interference—the ability of a wave to interfere with itself, creating localized regions where the field is alternately extremely bright and extremely dark
• Diffraction—the ability of waves to ‘bend’ around corners and spread after passing through an aperture
• Polarization— properties of light related to its transverse nature

Diagram showing the light pattern at the focal point of a lens.

The difference between physical and geometrical optics can be demonstrated by the ray tracing diagram above. If all of the rays on the lens intersect at the focal point, the density would be infinite and therefore infinitely bright. This cannot be possible and is instead explained by the wave properties of light. Placing a black screen at the plane of the focal point would produce the image shown here, with a small central bright spot surrounded by fainter rings caused by interference.

Quantum optics is a field of research applying quantum phenomena to light and its interactions with matter. One of the main goals is to understand the quantum nature of information and to learn how to formulate, manipulate, and process it using physical systems that operate via quantum mechanical principles.

Returning to the focal point example, physical optics would suggest the focal spot pattern remains fixed, only changing in intensity depending on the brightness of the source. When considering individual photons of light, a different picture emerges within quantum optics. Below a certain threshold of brightness, we detect localized spots of light that build up to make the interference pattern described by physical optics.

Quantum optics effect on the focal point of light from a lens.

Quantum optics considers both the wave and particle properties of light depending on the circumstances by which it is measured.