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🔬Modern Optics

🔬modern optics review

15.1 Principles of spectroscopy

3 min readLast Updated on July 22, 2024

Spectroscopy is all about how light and matter interact. It's a powerful tool that helps us understand the world around us by looking at how atoms and molecules absorb, emit, or scatter light.

In modern optics, spectroscopy is used for everything from identifying materials to medical imaging. It's based on the idea that atoms and molecules have specific energy levels, and transitions between these levels create unique spectral fingerprints.

Principles of Spectroscopy

Principles of spectroscopic techniques

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  • Spectroscopy studies the interaction between electromagnetic radiation and matter
    • Analyzes absorption, emission, or scattering of light by atoms, molecules, or materials
  • Fundamental principles of spectroscopy:
    • Atoms and molecules have quantized energy levels
    • Transitions between energy levels result in absorption or emission of photons with specific energies
    • Photon energy is related to frequency by E=hνE = hν, where hh is Planck's constant and νν is frequency
  • Applications in modern optics:
    • Characterizes and identifies materials
    • Analyzes chemicals and senses their presence
    • Monitors environmental conditions
    • Enables biomedical diagnostics and imaging (MRI, fluorescence microscopy)

Electromagnetic radiation-matter interactions

  • Absorption occurs when a photon's energy matches the energy difference between two states in an atom or molecule
    • Photon is absorbed, exciting the atom or molecule to a higher energy state
  • Emission:
    • Spontaneous emission: Excited atom or molecule spontaneously decays to lower energy state, releasing a photon with energy equal to the difference between states
    • Stimulated emission: Incoming photon stimulates excited atom or molecule to decay to lower energy state, releasing a second photon with same energy, phase, and direction as stimulating photon (lasers)
  • Scattering:
    • Elastic scattering (Rayleigh scattering): Photons scattered by atoms or molecules without change in energy (blue sky)
    • Inelastic scattering (Raman scattering): Photons interact with molecular vibrations or phonons, resulting in scattered photons with slightly different energies than incident photons

Energy levels and spectral transitions

  • Atoms and molecules have discrete, quantized energy levels
  • Transitions between energy levels occur when atom or molecule absorbs or emits photon with energy equal to difference between levels
  • Photon energy is related to wavelength by E=hc/λE = hc/λ, where hh is Planck's constant, cc is speed of light, and λλ is wavelength
  • Spectra observed in spectroscopic techniques:
    • Absorption spectra: Dark lines or bands corresponding to wavelengths of light absorbed by sample
    • Emission spectra: Bright lines or bands corresponding to wavelengths of light emitted by sample (neon signs, fireworks)
  • Intensity and position of spectral lines or bands provide information about composition, structure, and properties of sample

Types of spectroscopy

  • Electronic spectroscopy involves transitions between electronic energy levels in atoms or molecules
    • Examples: UV-visible spectroscopy, X-ray spectroscopy, photoelectron spectroscopy
  • Vibrational spectroscopy involves transitions between vibrational energy levels in molecules
    • Examples: Infrared (IR) spectroscopy, Raman spectroscopy
  • Rotational spectroscopy involves transitions between rotational energy levels in molecules
    • Examples: Microwave spectroscopy, far-infrared spectroscopy
  • Nuclear spectroscopy involves transitions between nuclear energy levels
    • Examples: Nuclear magnetic resonance (NMR) spectroscopy, Mössbauer spectroscopy

Spectroscopic Techniques and Applications

Principles of spectroscopic techniques

  • Spectroscopic techniques based on interaction between electromagnetic radiation and matter
    • Different techniques utilize different regions of electromagnetic spectrum (radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays)
  • Key applications of spectroscopic techniques in modern optics:
    • Determines atomic and molecular structures
    • Analyzes chemical composition quantitatively and qualitatively
    • Characterizes surfaces and interfaces
    • Monitors chemical reactions and processes in real-time
    • Enables remote sensing and imaging (satellite imagery, astronomical spectroscopy)


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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.