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Saturn from Cassini Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30

Lecture 25: Measuring Light:

Key Ideas:

Every atom, ion, and molecule has a unique spectral signature.

Absorption and Emission of Photons


Looking inside the Atom

Electrons cannot orbit just anywhere around a nucleus: The details are dictated by quantum mechanics.

Hydrogen: The Simplest Atom

An atom of Hydrogen (1H) consists of: The energy levels in the 1H atom look schematically like this:
Schematic Energy Level diagram for Hydrogen
(Click on the image to view at full scale [Size: 8Kb])

First orbital: Ground State (n=1)

Higher orbitals: Excited States (n=2,3,...)

Emission & Absorption Lines

Emission Lines:

When an electron jumps from a higher to a lower energy orbital, a single photon is emitted with exactly the energy difference between orbitals. No more, no less.

Formation of Hydrogen Emission Lines (Balmer Series)
(Click on the image to view at full scale [Size: 19Kb])

Electrons can get into the excited states by either

Absorption Lines

When an electron absorbs a photon with exactly the energy needed to jump from a lower to a higher orbital.

Formation of Hydrogen Absorption Lines (Balmer Series)
(Click on the image to view at full scale [Size: 51Kb])

Absorption is very specific:
The excited states decay by emitting photons in random directions.


If an atom or molecule absorbs enough energy from a photon or a collision, an electron can be ejected. Similarly, you can also add extra electrons: Ions differ from their parent neutral atoms or molecules:

Fingerprinting Matter

Other atoms have more electrons, and hence more complex electron orbital structures.

Every element has its own, distinctive spectral signature.


Molecules are more complex still:

Results in very complex spectra:

Molecules mainly produce strong lines at infrared, microwave, and radio wavelengths.

The Importance of Spectroscopy

From the emission or absorption lines in an object's spectrum, we can learn: These data give us a nearly complete picture of the physical conditions in the object.

Spectroscopy is one of the most important tools of the astronomer.


Want to explore spectroscopy? You can build a simple hand-held spectrograph using common household items, plus a specially-purchased (but inexpensive) diffraction grating:
Build Yourself a Simple Hand-Held Spectrograph

Return to [ Unit 4 Index | Astronomy 161 Main Page ]
Updated: 2007 October 19
Copyright Richard W. Pogge, All Rights Reserved.