Lecture Notes: Absorption and Emission Spectra

Absorption Spectra

• Definition: When a continuous spectrum of light (all wavelengths) is shown on an atom, some wavelengths are absorbed.
• Observation: Post-light exposure, the spectrum shows black bands where energies are missing.
  • Example: Missing energies corresponding to certain wavelengths like purple and yellow light.
• Process:
  • The absorbed energy excites electrons to a higher energy level, further from the nucleus.
  • Terminology: Spectra should be singularized to 'spectrum' when discussing absorption or emission individually.

Emission Spectra

• Process:
  • If atoms are heated or a voltage is applied, electrons may be excited to higher energy levels.
  • Electrons circulate at higher energy levels briefly before dropping back to lower levels.
  • As electrons fall back, they release photons.
• Outcome: Collected photons produce a line spectrum, synonymous with emission spectrum.
• IB Context: Considered the inverse of absorption spectra - lines represent light, whereas absorption shows missing light.

Historical Context

• Solar Spectra:
  • Early observations of the Sun's spectra revealed extra, unaccounted lines.
  • A bright yellow-orange line initially attributed to sodium.
  • Green lines speculated to be new elements like Caronia or Naboo Liam.
• Scientific Development:
  • With advanced science, these lines were attributed to O2+ and Fe13+, but this was unsatisfactory.
  • Resulted in the discovery of helium (He), first identified on the Sun before Earth.

Common Misunderstandings

• Absorption and Emission Reversal:
  • Query: When electrons return to lower levels, shouldn't they emit the absorbed light, leaving no spectral change?
  • Clarification: Emitted light is random in direction; thus, absorption spectra still exhibit gaps.