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Nonlinear optics studies the response of matter to light when the induced electric polarization is not proportional to the driving electric field. An intense laser can make these additional terms observable. The striking consequence is that a material can emit light at a frequency absent from the incident beam: second-harmonic generation doubles it. This extends a s usable optical spectrum without requiring a second lasing transition.

Franken and colleagues frequency-doubling experiment established an optical harmonic in 1961. Nicolaas S theoretical work developed the description of interacting light waves in nonlinear matter. His 1981 lecture connects a nonlinear constitutive relation with Maxwell's equations and explains compensation of optical dispersion by birefringence.^[ref]

A Lippmann plate records colour as a depth-dependent optical structure in a photographic emulsion. A standing wave writes a spacing; wave interference during later illumination selects the reflected wavelengths. This makes a photographic plate a spatial record of an optical spectrum. The striking idea is that information about colour can live in geometry.

Gabriel Lippmann presented his colour process in 1891 and received the 1908 Nobel Prize in Physics. His career belongs principally to physics: he held chairs in mathematical and experimental physics.^{[ref] [ref]}

Phosphorescence is delayed luminescence sustained by long-lived excited states after excitation has ceased. In a common molecular mechanism, population reaches a triplet state and returns to a singlet ground state through a weak, spin-changing transition. Long-lived emission in a solid can also involve trapped charge carriers, so a slow decay alone does not uniquely establish the microscopic mechanism.

A useful experiment shutters the excitation and records emitted intensity versus time with a photodetector. Scattered excitation should disappear with the shutter, whereas delayed emission persists. The measured decay can depend on temperature, quenching and multiple states rather than following one universal lifetime.

Optical nonreciprocity means that exchanging source and detector paths need not restore the corresponding transmission when external conditions, including the magnetic field, are held fixed. The Faraday effect gives a linear polarization rotation with the same laboratory sense on both traversals; see Faraday rotation on a round trip.

A maser uses stimulated emission to amplify microwaves. With population inversion and feedback it can sustain a coherent wave field as an oscillator. The name abbreviates "microwave amplification by stimulated emission of radiation".^[ref]

Why make molecules supply an oscillator? Their energy levels offer a reproducible frequency reference, while stimulated emission lets successive molecules reinforce one coherent wave field. In his 1964 lecture, Charles Hard Townes traced his motivation to microwave spectroscopy: resolving molecular structure required useful coherent sources.^[ref] The interesting balance is between molecular amplification and escaping energy: even a beam with population inversion fails to oscillate if cavity losses win. This is the microwave counterpart of laser threshold.

Additive color produces a color stimulus by superposing light from different sources. For mutually incoherent sources, their spectral intensities add, and the combined optical spectrum drives the observer's receptors. Red and green display primaries can therefore give a yellow appearance without emitting only yellow-wavelength light.

Project two colored beams onto the same diffuse screen and compare their overlap with the separately illuminated regions. Holding the surroundings and adaptation fixed makes the comparison interpretable. The attainable colors depend on the primary optical spectra and their allowed powers, which define a device gamut; merely naming three primaries does not specify every stimulus they can produce.

Light emission from excited matter that is not simply its thermal radiation at the local temperature. Fluorescence is one form of luminescence.

The ability of an imaging or measurement system to distinguish nearby spatial features. A resolution criterion must specify the features and measurement conditions: diffraction, noise and sampling can all limit the distinction. A small pixel spacing alone does not establish high spatial resolution; electron microscope resolution illustrates the role of wavelength and numerical aperture.

Ellipsometry measures the change in polarization of an electromagnetic wave upon specular reflection to infer material properties. For a simple planar sample, the complex reflection-coefficient ratio is written $\rho =r_p/r_s=\tan \Psi e^{i\Delta }$: $\Psi$ describes a relative amplitude and $\Delta$ a relative wave phase between the two incident polarizations.

A source, polarization-control optics, sample at a known incidence angle and analyzer with a photodetector form a basic instrument. Repeating measurements across wavelength and angle constrains refractive index and film thickness through a model of the layered sample. The fitted thickness is not read directly from $\Psi$: roughness, layer structure and correlated model parameters can produce ambiguous fits.

Microscopy makes small structures observable by using an optical microscope, an electron microscope, or a scanned probe such as an atomic force microscope. Each instrument measures a different interaction with the specimen: transmitted light, scattered electrons, or the force on a tip. Consequently two images of the same object need not show the same features.

A useful measurement distinguishes magnification, optical resolution and image contrast. Enlarging a blurred image changes its displayed size without recovering the missing detail. For an optical microscope, imaging small separated objects and recording how their images merge tests optical resolution; changing the illumination while holding the specimen fixed tests whether a feature comes from specimen structure or the imaging method.

This section concerns specialists in optical instruments.

Color vision distinguishes distributions of visible light across wavelengths through the different spectral sensitivities of visual receptors. A perceived colour need not correspond to one wavelength: different optical spectra can produce the same receptor responses. Consequently the broad spectral filtering of thin-film interference need not produce pure monochromatic colours.

Online text: opendatastructures.org/ods-python/5_2_LinearHashTable_Linear_.html

Section of Open Data Structures: An Introduction by Pat Morin, whose whole-book author is Pat Morin. The online Python edition is undated; 2013 in the parent record is the print-book publication year.

Cited passages: §5.2, opening description of direct slot storage and consecutive modular probing; three entry types and counters n and q; find, add, remove and correctness paragraphs distinguishing del from nil; resize and resetting q to n.

Institutional entry by International Union of Crystallography. Last edited 8 May 2018; original publication date unestablished.

Institutional entry by International Union of Crystallography. Last edited 17 November 2017; original publication date unestablished.

Entry in Online Dictionary of Crystallography by International Union of Crystallography. Last edited 24 February 2019; no original publication date is established.

Online text: dictionary.iucr.org/Reflection_conditions

Cited passages: Definition; Table 1: no centering condition for primitive cells; unmixed indices for all-face-centered cells.

Entry in Online Dictionary of Crystallography by International Union of Crystallography. Last edited 20 November 2017; no original publication date is established.

Online text: dictionary.iucr.org/Structure_factor

Cited passages: Definition: sum over unit-cell atoms, amplitudes and phases; Derivation: primitive one-scatterer model and addition of companion atoms.

Entry in Online Dictionary of Crystallography by International Union of Crystallography. Last edited 9 November 2017; no original publication date is established.

Online text: dictionary.iucr.org/Bragg%27s_law

Cited passages: Definition: interplanar spacing, glancing angle, wavelength and order; Direct derivation and Order of the reflection inspected for consistency; the worked examples and drawings were independently composed.

Entry in Online Dictionary of Crystallography by International Union of Crystallography. Institutional reference issued and hosted by the International Union of Crystallography; no individual byline is assigned. Last edited 11 December 2017; no original publication date is established.

Online text: dictionary.iucr.org/Miller_indices

Cited passages: Definition, Direct space, plane intercepts and primitive versus centered indexing; Definition, Reciprocal space, normal direction and inverse spacing. The entry uses reciprocal-length basis vectors; an angular reciprocal convention introduces a factor of $2\pi$.