Earth, Our Environment - Class Notes
Chapter 2, Earth: Origin and Composition - 03
Information and Light

We can estimate the composition of interstellar clouds, galaxies, stars, and planets by studying the light they emit, absorb, or reflect. Light is a wave, and different colors of light have different wavelengths

Light is one of many familiar electromagnetic waves:


Typical Wavelength


many miles


feet to inches


tens of thousands of atoms

visible light

four thousand to eight thousand atoms


hundreds of atoms

X ray

one atoms

gamma ray

the size of a nucleus

[Study Figure 2.3 on page 22 of the text book]


Spectral "Fingerprints"

Sources of light emit light at characteristic frequencies. We can use an instrument such as a prism to separate light into it's native frequency constituents.

Electrons inhabit energy states that we call "shells". The observed spectral patterns arise from the "movement" of electrons between quantized electron energy "states". In a burning star, electrons are constantly absorbing and emitting energy and mak ing transitions from one energy shell to another.

A result of the jump that decreases and electron's energy state is the emission of a photon (a "particle" of light) with a specific frequency. Different elements have different electron configurations, thus the transition of electron energy releases photons of characteristic frequency, producing in effect a spec tral fingerprint of the element.

If we observe and split the light emanating from stars, galaxies, etc., we find a line spectrum, because only certain frequencies are permitted (a consequence explained by a fascinating scientific theory called Quantum Mechanics).

[Study Figure 2.5 on page 23 of the text]

Because the process works backwards as well (an electron can absorb a photon with a specific frequency and go to a higher energy state) we can observe two types of spectra: absorption and emission.

Red Shifts and the Expanding Universe

We are now in a position to examine an important observation in support of the Big Bang Hypothesis - red shifts in the spectra of galaxies.

[Study Figure 2.6 on page 24 of the text book]


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Prepared by: Charles J. Ammon
February 1997