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🔬Modern Optics

🔬modern optics review

13.1 Absorption and emission processes

3 min readLast Updated on July 22, 2024

Light absorption and emission are fundamental processes in optics. When a photon interacts with matter, it can be absorbed, causing an electron to jump to a higher energy state. This process is crucial in various applications, from solar cells to photosynthesis.

Energy levels play a key role in these transitions. Electrons move between discrete energy states during absorption and emission, following specific rules. These processes are affected by factors like material composition, temperature, and impurities, leading to unique absorption and emission spectra for different materials.

Absorption and Emission Processes

Principles of light absorption

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  • Absorption occurs when a photon interacts with a material and transfers its energy to an electron
  • Electron transitions from a lower energy state to a higher energy state absorbing the photon's energy
  • Photon energy must precisely match the energy difference between the two states described by the equation ΔE=hf\Delta E = hf where hh is Planck's constant and ff is the photon frequency
  • Examples of absorption include solar cells converting sunlight into electricity and plants using chlorophyll to absorb light for photosynthesis

Energy levels in optical transitions

  • Energy levels are discrete energy states that electrons can occupy in a material determined by the material's atomic or molecular structure
  • Transitions involve electrons moving between energy levels during absorption and emission processes
    • Absorption transitions occur when an electron moves from a lower to a higher energy state by absorbing a photon (ultraviolet light absorbed by DNA)
    • Emission transitions occur when an electron moves from a higher to a lower energy state releasing a photon (LED emitting light)
  • Transition probabilities depend on the material's electronic structure and selection rules governing allowed transitions (atomic selection rules)
  • Conservation of energy dictates that the total energy is conserved during absorption and emission with the photon energy equaling the energy difference between electron states

Factors affecting absorption spectra

  • Material composition determines the available energy levels and transitions resulting in unique absorption and emission spectra for different materials (dyes, pigments)
  • Temperature affects the population of higher energy states influencing the intensity and shape of emission spectra (blackbody radiation)
  • Dopants and impurities introduce additional energy levels within the material's bandgap modifying the absorption and emission spectra of the host material (doped semiconductors)
  • Strain and defects alter the local electronic structure and energy levels causing shifts or broadening of absorption and emission peaks (strained quantum wells)

Applications of absorption and emission

  • Optical filters exploit the absorption properties of materials to selectively transmit or block certain wavelengths
    1. Bandpass filters transmit a narrow range of wavelengths while blocking others (color filters)
    2. Longpass filters transmit wavelengths above a cutoff while blocking shorter wavelengths (IR filters)
    3. Shortpass filters transmit wavelengths below a cutoff while blocking longer wavelengths (UV filters)
  • Light sources utilize the emission properties of materials to generate light
    • Light-emitting diodes (LEDs) emit light through electroluminescence in semiconductors (LED displays)
    • Lasers produce coherent light through stimulated emission in a gain medium (laser pointers, fiber optics)
    • Fluorescent lamps emit light through the fluorescence of phosphors excited by UV emission from mercury vapor (CFL bulbs)
  • Spectroscopy analyzes the absorption and emission spectra of materials providing information about composition, structure, and energy levels
    • Material characterization identifies elements and compounds based on their spectral fingerprints (Raman spectroscopy)
    • Chemical analysis detects and quantifies substances by their absorption or emission spectra (atomic absorption spectroscopy)
    • Environmental monitoring uses spectroscopy to measure pollutants and trace gases in the atmosphere (LIDAR)


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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.