Optics is the branch of physics that studies light, its propagation, its interaction with matter, and the instruments used to produce, control, and detect it. Its central problems include explaining reflection and refraction, describing wave phenomena, and understanding image formation. Optical science connects classical electromagnetic theory with quantum descriptions of radiation, while optical engineering applies these principles to imaging, measurement, and communication. (openstax.org)
Scope and physical foundations
Optics principally concerns visible light and neighboring infrared and ultraviolet regions of the electromagnetic spectrum, although its methods also apply to other wavelengths. In classical theory, light is electromagnetic radiation governed by Maxwell’s equations. A plane light wave in free space has electric and magnetic fields perpendicular to each other and to its direction of travel. The theory relates optical propagation to the electrical and magnetic properties of materials. (ocw.mit.edu)
Three complementary descriptions organize the subject. Geometrical optics represents propagation by rays; wave optics describes amplitude, phase, and the superposition of fields; quantum optics treats radiation and its interactions using quantum mechanics. These descriptions have different domains of usefulness rather than constituting mutually exclusive accounts. Rays provide a practical approximation for many imaging systems, whereas diffraction and photon-level measurements require more detailed models. (ocw.mit.edu)
Geometrical optics and image formation
Geometrical optics is most effective when relevant optical dimensions are large compared with the wavelength. Rays travel along straight paths in a homogeneous medium. At a smooth reflecting surface, reflection makes the angle of reflection equal to the angle of incidence, measured from the surface normal. Refraction changes a ray’s direction when it crosses between media with different refractive indices. (pressbooks.online.ucf.edu)
For a transparent medium, the refractive index is (n=c/v), where (c) is the speed of light in vacuum and (v) is its phase velocity in the medium. Snell’s law expresses refraction as
[ n_1\sin\theta_1=n_2\sin\theta_2. ]
Both angles are measured from the normal. When light travels toward a lower-index medium, incidence beyond the critical angle produces total internal reflection, a principle used to guide light in optical fibers. (openstax.org)
A lens redirects rays to form real or virtual images. In the thin-lens, small-angle approximation, object distance (d_o), image distance (d_i), and focal length (f) obey (1/f=1/d_o+1/d_i), with signs assigned according to the adopted convention. Real images arise where rays converge; virtual images arise where their backward extensions appear to meet. Practical optical design also considers apertures, magnification, and aberrations—departures from ideal image formation. (openstax.org)
Wave optics
Wave optics explains effects that ray tracing alone cannot capture. Interference occurs when overlapping fields reinforce or cancel according to their relative phases. In a double-slit experiment, these variations produce alternating bright and dark fringes. Stable fringes depend on suitable optical coherence, meaning sufficiently consistent phase relationships between the contributing fields. (openstax.org)
Diffraction describes the spreading and redistribution of light associated with apertures, obstacles, and wave propagation. A finite slit produces a diffraction pattern even without a second slit because contributions from different parts of its opening interfere. Finite apertures therefore limit the resolving ability of optical instruments: an ideal optical system does not turn a point source into a perfectly sharp mathematical point. (openstax.org)
Polarization characterizes the behavior of the electric-field direction. Light may be linearly, circularly, or elliptically polarized, while unpolarized light lacks a single stable polarization state. Polarizers selectively transmit particular components. Polarization also changes through reflection, scattering, and passage through birefringent materials, and is used in glare reduction and liquid-crystal displays. (openstax.org)
Quantum and nonlinear optics
Quantum optics studies phenomena involving the quantized electromagnetic field. A photon of frequency (\nu) carries energy (E=h\nu), where (h) is the Planck constant. The photoelectric effect illustrates the importance of this description: light can eject electrons from a metal only above a material-dependent threshold frequency in the ordinary single-photon regime. Albert Einstein explained this effect using light quanta in 1905. (openstax.org)
Nonlinear optics concerns material responses that are not simply proportional to the applied optical field. These responses enable frequency conversion, parametric amplification, and intensity-dependent changes in propagation. A laser supplies controlled optical radiation for many such experiments. Quantum optical systems also investigate correlations and quantum entanglement, including their use in communication. (ocw.mit.edu)
Historical development and applications
The historical study of optics included both geometrical descriptions of light and theories of vision. In the eleventh century, Ibn al-Haytham developed an influential treatment of vision and optical geometry in his seven-volume Book of Optics. Later wave experiments, including Young’s double-slit experiment, established interference as a central optical phenomenon. Electromagnetic theory subsequently connected light with electric and magnetic fields, while quantum theory addressed aspects of emission and absorption unexplained by classical models. (meetings-archive.aps.org)
Optical instruments extend observation and support quantitative measurement. Cameras and microscopes form images, while telescopes collect light from distant objects. Interferometry uses phase-sensitive interference patterns to measure optical path differences. Spectroscopy separates or analyzes radiation by wavelength; diffraction gratings provide one means of doing so. Optical communication transmits information through guided light, and computational imaging combines optical systems with numerical processing to obtain or interpret images. (assets.openstax.org)