Lecture 6: Discrete Spectra of Atoms

Three types of Spectra

  • Continuous spectrum
  • Discrete emission spectra
  • Discrete absorption spectra
E.g. blackbody spectrum from a heated source Emission by heated transparent gas Stars


Kirchhoff's Laws

  • Continuous (or blackbody) spectrum occurs when we look directly at a hot opague source.
  • Emission line spectrum occurs when we look at a transparent gas which is being excited by the blackbody source, but we can't see the source directly.
  • Absorption line spectrum occurs when we look at a cool layer of gas which blocks a blackbody source and absorbs the blackbody photons.

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Atomic Spectra: Fingerprints of Atoms

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Astronomy: A Hydrogen Emission Nebula

  • A hot star sits inside this cloud of gas (called a nebula)
  • UV photons emitted by the star excite the atoms in the gas.
  • The electrons jump back to the ground state, emitting light with the colours typical for that atom.
  • In this case, the gas is mostly Hydrogen, so most of the light emitted is the red Balmer alpha light and the nebula glows pinkish-red.
  • Since Hydrogen is the most common element in the Universe, most emission nebulae are red.

Astronomy: The Sun's Absorption Spectrum

The Sun's spectrum is an absorption line spectrum. The spectral lines give us the chemical composition of the Sun's atmosphere. (Mainly Hydrogen)

Some fun links to research-quality Solar spectra:
Interactive Solar Spectrum
Complete Solar Spectrum from 380 to 870 nm

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Hydrogen's Spectrum

  • Lyman Series
    • When an electron jumps from an excited state to the ground state n=1 an Ultraviolet photon is emitted.
    • These ultraviolet spectral lines are called the Lyman series . and denoted Ly&alpha, Ly&beta, Ly&gamma ...
  • Balmer Series
    • When an electron jumps from an excited state to the n=2 state a Visible photon is emitted.
    • The jump down to n=2 is called Balmer series, with the lines H&alpha, H&beta, H&gamma ...
    • The Balmer alpha photon is red (wavelength = 656 nm)

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Let us practice some computations

We need the following constants
c=3.00 x 108 m    h=6.63 x 10-34 J.s     kB=1.38 x 10-23 J/K     R = 1.10 x 107 m-1     1 eV = 1.60 x 10-19 J

and formulas

Ly-alpha line with longest wavelength ?

Minimum energy to excite hydrogen atom ?

Which jump between close levels is in visible range ?

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Hydrogen's Spectrum Balmer lines

NameInitial StateFinal StateColour Wavelength
Balmer alpha32red656 nm
Balmer beta42green-blue486 nm
Balmer gamma52blue434 nm
Balmer delta62violet410 nm

Interactive Solar Spectrum: can you find Balmer lines ?

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Doppler Shift

Waves in water, air ...

  • Stationary source produces waves with constant wavelength in all directions.
  • If the source is moving then the observer sees that:
    • wavelength is shorter in the direction which the source moves ;
    • wavelength is longer in the direction opposite to the source motion .
Useful animations 1, 2

Electromagnetic waves (light):

Doppler effect is extremely useful:


Next lecture: Optics and Telescopes
Read Chapter 6