Spectrometers read what stars and metals are made of from their light
Every chemical element leaves its own pattern of bright or dark lines when light passes through or comes off it. A spectrometer spreads that light out by wavelength so the pattern can be read, which is how astronomers can name the ingredients of a star without ever touching it.
The earliest spectrometers simply split light into a row of colours. Modern optical versions plot how bright the light is at each wavelength or frequency, separating the colours either by bending them through a prism or by scattering them off a diffraction grating. The light under study might form an unbroken rainbow, a set of bright emission lines, or a rainbow crossed by dark absorption lines. Calibrate the instrument to measure the actual power of the incoming light and it becomes a spectroradiometer.
Metalworkers use a harsher version. In spark discharge spectrometry, a high voltage fires a spark at a metal surface, turning a little of it into plasma. The glowing particles and ions give off radiation at telltale wavelengths, and photomultiplier tubes pick it up, giving the alloy's makeup with great precision.
Not every spectrometer deals in light. Protons, electrons and many nuclei behave like tiny magnets, so in a strong external field their response reveals their chemical surroundings; that idea underlies nuclear magnetic resonance, while unpaired electrons give electron paramagnetic resonance. Mass spectrometers sort gas-phase ions by their mass-to-charge ratio to identify which chemicals are present and in what amounts. Time-of-flight machines clock particles between two detectors to get their speed.
The simplest magnetic design, the semicircular spectrometer credited to J. K. Danysz, bends charged particles passing a slit into half-circles whose radius depends on momentum. Particles of the same momentum land at almost one spot, so varying the field lets a counter there measure alpha or beta particle energies. Across all these instruments runs one trade-off: with mechanical slits, sharper resolution usually means a dimmer signal.
Source: Spectrometer
