spectroscopy

Spectroscopy is the study of the interaction between matter and electromagnetic radiation. It analyzes how light is absorbed, emitted, or scattered by atoms and molecules and how these processes produce spectra—plots of intensity as a function of wavelength, frequency, or energy. The central principle is that energy transitions between discrete levels generate characteristic features that can identify substances and reveal information about their concentration, structure, and environment.

Techniques are categorized by the type of interaction and spectral region. Absorption and emission spectroscopy examine how a sample takes up or releases energy. Common methods include UV-Visible spectroscopy for electronic transitions, infrared spectroscopy for vibrational modes, Raman spectroscopy for inelastic scattering, fluorescence spectroscopy for emissive processes, and nuclear magnetic resonance for spin-based transitions. X-ray spectroscopy probes core-level transitions, while mass spectrometry is often used in tandem with spectroscopic detectors.

Instrumentation typically combines a light source, a sample, a dispersive or interferometric element, and a detector. A spectrum is obtained by dispersing light with a prism or grating or by interferometry, then recording intensity with a photodetector or camera. Modern spectrometers may use Fourier-transform analysis for rapid, high-resolution data and rely on calibrated standards to determine concentration, identity, or structural information. Applications span chemistry, materials science, biology, astronomy, environmental monitoring, and medicine.

Historically, spectroscopy emerged from the study of solar and flame spectra in the early 19th century. Fraunhofer first observed dark lines in the solar spectrum, and Bunsen and Kirchhoff linked spectral features to chemical composition, laying the foundations of analytical spectroscopy and quantum theory. Today, spectroscopy remains a central technique in scientific measurement and diagnostics.