Emission and Absorption Spectra

Main ideas / concepts

• Absorption vs. emission spectra are distinct “fingerprints” of elements/atoms, determined by which wavelengths (colors) of light they interact with.
• In the 1860s, scientists discovered that each element emits a unique set of light colors when excited. This set is called an emission spectrum.
• Earlier work (from Isaac Newton) showed that white light through a prism separates into a continuous rainbow of colors.
• When that continuous spectrum is tested with different elements:
  • Instead of all rainbow colors appearing, only certain colors appear for each element.
  • Each atom emits only specific wavelengths, producing discrete spectral lines.
• When white light passes through a gas:
  • The gas absorbs specific colors.
  • The result is an absorption spectrum, observed as missing lines where absorption occurs.
• A key explanation (attributed to Boh[r], typically Niels Bohr) is:
  • Light color corresponds to energy: higher-energy light (toward purple/UV) can drive larger electron transitions than lower-energy light (toward red/IR).
  • Electron transitions between quantized energy levels explain why only certain wavelengths are absorbed or emitted.

Methodology / “how it works” (step-by-step)

Emission spectrum concept

1. Assume an electron starts in a lower energy level (the first energy level in the example).
2. Shine light on the atom:
   • If the light has the exact energy needed, the electron absorbs that light.
   • The electron jumps to a higher energy level.
   • The specific required color matches the energy gap between the two levels.
3. Absorption spectrum observation:
   • In the absorption spectrum, wavelengths that electrons absorb appear as missing lines (e.g., black bars/lines absent).
   • Examples described:
     • Ultraviolet (UV) light is absorbed → the corresponding absorption line is missing.
     • Blue light is also absorbed → another missing line appears.
     • A transition to a higher level might correspond to infrared (IR) energy, which may be less visible in the example due to the small energy involved (as stated).

Emission spectrum generation

1. After the electron is excited to a higher energy level, it can fall back down to a lower level.
2. When it falls:
   • It releases energy.
   • That released energy appears as emitted light with a specific color/wavelength.
3. Meaning of the emitted color:
   • The emitted color indicates the energy difference between the energy levels.
   • By measuring the emitted spectral lines, you can infer the spacing of energy levels in the atom.

Link between absorption and emission

• Absorption:
  • Electron moves up
  • Specific colors are absorbed
  • Shown as black/missing lines in the absorption spectrum
• Emission:
  • Electron moves down
  • Specific colors are emitted
  • Shown as distinct lines/bars in the emission spectrum

Speakers / sources featured

• Isaac Newton: prism-based separation of white light into a rainbow.
• Boh[r] (referenced as Niels Bohr): linked to reconciling spectra with atomic behavior and quantized energy levels.