= Ultraviolet
{wiki}
<Ultraviolet> is <electromagnetic radiation> with <wavelengths> shorter than the violet end of ordinarily visible <light>. The CIE convention uses $100$–$400\,\mathrm{nm}$, subdivided into UV-A at $315$–$400\,\mathrm{nm}$, UV-B at $280$–$315\,\mathrm{nm}$ and UV-C at $100$–$280\,\mathrm{nm}$. These divisions name intervals; they do not mark universal changes in every material's response. The boundary with visible <light> is not sharp, and other fields use near-, far-, vacuum- and extreme-<ultraviolet> divisions. https://cie.co.at/eilvterm/17-21-008[CIE, International Lighting Vocabulary (2020), ultraviolet radiation: ranges and boundary caveats]{ref}. For example, NASA's Solar Dynamics Observatory observes extreme-<ultraviolet> bands below $100\,\mathrm{nm}$; the CIE interval is therefore not the full extent of astronomical <ultraviolet> usage. https://sdo.gsfc.nasa.gov/data/channels.php[NASA, Solar Dynamics Observatory HMI and AIA channels: the Atmospheric Imaging Assembly's ultraviolet and extreme-ultraviolet bands]{ref}.
The CIE <ultraviolet> interval corresponds to <frequencies> about $7.49\times10^{14}$–$3.00\times10^{15}\,\mathrm{Hz}$ and <photon> <energies> $4.97\times10^{-19}$–$1.99\times10^{-18}\,\mathrm J$, using <vacuum> <wavelength> and $E=hc/\lambda$ with <Planck constant> $h$ and <speed of light> $c$. Shorter <wavelength> means larger <energy> per <photon>; larger <irradiance> means more incident <power> per <area>. These are different comparisons. At the same incident <power>, a $254\,\mathrm{nm}$ <ultraviolet> beam has fewer <photons> per second than a $940\,\mathrm{nm}$ <infrared> beam, even though each <ultraviolet> <photon> has more <energy>.
A common <ultraviolet> mechanism is <photon absorption> promoting an <electron> from a <ground state> to an <excited state> of a <chemical atom> or <molecule>. By contrast, many <infrared> absorption bands involve stretching or bending <chemical bonds>. For either mechanism, both the <energy> difference and the coupling to the incident <electromagnetic field> matter. One concrete example is the $217\,\mathrm{nm}$ absorption discussed for <butadiene>: an <electron> moves between <molecular orbitals>, and the position of the absorption reveals their <energy> separation. <Ultraviolet> absorption can therefore probe <molecular orbitals>, while <infrared spectroscopy> probes many molecular motions. https://openstax.org/books/organic-chemistry/pages/14-7-ultraviolet-spectroscopy[John McMurry, Organic Chemistry, §14.7: ultraviolet excitation and the 217 nm butadiene example]{ref}.
Subsequent <chemical reactions> depend on the absorbing <chemical substance> and its environment; the label <ultraviolet> alone does not predict a <reaction rate>. At sufficiently short <wavelength>, a <photon> can cause <photoionization> if its <energy> exceeds the relevant <ionization energy>. There is no single <wavelength> that separates all ionizing from all nonionizing behavior for every <chemical substance>. Likewise, eventual heating after <photon absorption> is possible in both <ultraviolet> and <infrared>. The distinction is useful because different <optical spectra> expose different processes, not because the two ranges obey different laws of <electromagnetism>.
Ritter's 1801 observation used a chemical readout rather than a <thermometer>. In Caltech/IPAC's account, sunlight passed through a glass <optical prism> and <silver chloride> was placed in different parts of the <optical spectrum>. Darkening was weak toward red, stronger toward violet, and pronounced beyond the visible violet edge. The measured evidence was a change in the <chemical substance>'s appearance at a spatial location with no visible color, establishing radiation beyond the eye's ordinary range. It was not a calibrated measurement of a universal <ultraviolet> <optical spectrum>. https://coolcosmos.ipac.caltech.edu/page/lesson_ritter_experiment[Caltech/IPAC Cool Cosmos, The Ritter Experiment (2022): the historical silver-chloride observation]{ref}.
A controlled modern interpretation of such a chemical test would compare equal exposure durations and matched fresh samples, keep an unexposed sample in darkness, monitor <temperature>, and distinguish scattered visible <light> from the intended short-<wavelength> beam with characterized filtering. These are proposed controls, not an assertion that Ritter performed them. Unequal sample preparation or heating can change a <reaction rate>. The <optical prism>, <terrestrial atmosphere> and sample response restrict which <wavelengths> the test detects. Darkening is a detector-weighted response, so its spatial maximum is not automatically the maximum of incoming <radiant flux> or of <photon> <energy>.
Thomas Larason and Jeanne Houston's NIST report documents a quantitative <ultraviolet> detector comparison. An argon source feeds a double-<diffraction grating> <spectrometer>; a shutter and mirrors deliver the selected band to a working-standard or test <photodiode>, successively at the same position. A <beam splitter> supplies a monitor <photodiode>. Amplifiers and voltmeters record detector and monitor simultaneously; shutter-closed readings provide dark baselines. The $200$–$500\,\mathrm{nm}$ scan, typically with $4\,\mathrm{nm}$ bandpass, measures <spectral responsivity>. Monitor normalization suppresses source drift; known <spectral lines> calibrate <wavelength>. The report assesses <temperature> effects and unwanted <light> outside the band, weighted by detector response. The latter need not cancel between different <spectral responsivities>. <Spectroradiometric Detector Measurements: Ultraviolet, Visible, and Near-Infrared Detectors for Spectral Power by Thomas C. Larason and others>[Thomas C. Larason and Jeanne M. Houston, NIST SP 250-41 (2008), §§3.2, 4.2 and 7.2: apparatus, readouts and controls]{ref}.
This determines electrical response without visual perception. Bandpass and spatial nonuniformity limit the calibration; aging and changed <temperature> can affect later use. The scan includes visible <light> and <ultraviolet>; no response below $200\,\mathrm{nm}$ follows from it.
For a narrow beam, the idealized substitution calculation is
$$
R_{\mathrm{test}}(\lambda)=R_{\mathrm{std}}(\lambda)
\frac{I_{\mathrm{test}}/M_{\mathrm{test}}}{I_{\mathrm{std}}/M_{\mathrm{std}}}.
$$
Here $R$ is <spectral responsivity>; $I$ and $M$ are dark-subtracted detector and monitor <electric currents>. This follows by writing $I=RP$ for each detector and using the monitor to track changes in <radiant flux>. It assumes linear response, unchanged beam splitting and collection geometry, and a sufficiently narrow band. For an illustrative standard with $R_{\mathrm{std}}=0.15\,\mathrm{A\,W^{-1}}$, ratios $I_{\mathrm{std}}/M_{\mathrm{std}}=2.0$ and $I_{\mathrm{test}}/M_{\mathrm{test}}=1.6$ give $R_{\mathrm{test}}=0.12\,\mathrm{A\,W^{-1}}$. These invented arithmetic inputs illustrate the method; they are not NIST measurements.
An independent <photon>-counting model connects <spectral responsivity> to <quantum efficiency>. If $\eta(\lambda)$ is the number of collected <electrons> per incident <photon>, there is no internal multiplication, and $q$ is the <elementary charge>, then
$$
\dot N=\frac{P\lambda}{hc},\qquad I=q\eta\dot N,\qquad
R(\lambda)=\frac{I}{P}=\eta(\lambda)\frac{q\lambda}{hc}.
$$
At illustrative constant $\eta=0.60$ and $P=1\,\mathrm{\mu W}$, this gives about $0.123\,\mathrm{\mu A}$ at $254\,\mathrm{nm}$ and $0.455\,\mathrm{\mu A}$ at $940\,\mathrm{nm}$. The <ultraviolet> readout is smaller in this model because there are fewer incident <photons>, not because their individual <energies> are smaller. Real <quantum efficiency> depends on <wavelength>, windows, absorption and carrier collection; a detector's <band gap> and construction determine its usable range. Constant $\eta$ across these two <wavelengths> is an analytical comparison, not a claimed specification of a real <photodiode>.
\Image[/infrared-ultraviolet-energy-detector-model.png]
{title=<Photon> <energy> across conventional <ultraviolet> and <infrared> ranges, with a schematic detector-substitution experiment}
The curve is the calculated relation $E=hc/\lambda$, and the shaded ranges use the stated CIE conventions. The lower diagram abstracts the detector comparison rather than reproducing an instrument drawing. Together with the <infrared> heat-balance illustration, it shows why a calibrated detector response, rather than what looks bright to an observer, is the bridge between invisible radiation and a quantitative measurement.
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