Photon Energy (eV)
Definition
Photon energy expressed in electron volts; UV photons carry 3-12 eV (chemically reactive), visible 1.8-3.1 eV, mid-IR 0.05-0.5 eV (vibrational excitation only).
- UV range
- 3 to 12 eV, chemically reactive
- Visible range
- 1.8 to 3.1 eV
- Mid-IR range
- 0.05 to 0.5 eV, vibrational excitation only
- Unit
- Electron volt (eV)
Common questions
Why is UV described as chemically reactive but mid-IR is not?+
UV photons carry enough energy to break or rearrange chemical bonds and excite electrons, which is why UV exposure can degrade evidence such as dyes or biological stains. Mid-IR photons only have enough energy to make bonds vibrate more, which is why IR is used for non-destructive spectroscopic identification instead.
How is this energy scale used when choosing an alternate light source at a scene?+
Examiners pick a wavelength, and therefore a photon energy, that will excite fluorescence in the target evidence such as body fluids or fibres without an energy so high that it risks damaging the sample, balancing sensitivity against preservation.
Related terms
- Chromophore
- The molecular subunit responsible for a dye's colour, typically a conjugated double-bond system; chemical erasure destroys the chromophore while leaving the decolourised...
- Ionising Radiation
- Electromagnetic radiation whose photon energy exceeds the ionisation potential of atoms (typically above ~10 eV); includes gamma rays and X-rays. UV below...
- Mid-Infrared (Mid-IR)
- The 2500-25000 nm (400-4000 cm^-1) spectral region where functional-group vibrational absorptions serve as chemical fingerprints; the operating region of FTIR spectroscopy.
- Photon
- A quantum of electromagnetic radiation; a discrete energy packet with energy E = hν where h is Planck's constant and ν is...
- Planck-Einstein Equation
- E = hν = hc/λ; relates photon energy to frequency and wavelength. Foundation of all spectral energy calculations.
- Refractive Index
- A physical property of glass (and other transparent materials) that describes how much light bends when passing through it. Forensic glass comparison...
- Soret Band
- The intense porphyrin absorption band of haemoglobin at 415 nm; the spectral basis for bloodstain contrast enhancement.
- UV-a
- The ultraviolet band from 315 to 400 nm; the primary band used for UV-reflectance forensic imaging of bruises, GSR, and bite marks...
- UV-C
- Ultraviolet radiation in the 100-280 nm band; the 254 nm germicidal line excites biological-fluid fluorescence in forensic screening.
- Wave-Particle Duality
- The quantum-mechanical property of electromagnetic radiation: it behaves as a wave (diffraction, interference, polarisation) and as a stream of photons (photoelectric effect,...
- Wavenumber (Cm^-1)
- Reciprocal of wavelength in centimetres, the standard unit for mid-infrared spectra; 1 cm^-1 = 10^7 / λ(nm).