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Summary: ASTR 1120 Section 1 (3 credit hours): Spring 2006
SUMMARY OF KEY CONCEPTS: WEEK #4
Lecture #7 textbook Chapter 5 (4th
edition) or Chapter 6 (3rd
edition) on `Light'
The wave / particle properties of light are connected via a famous formula of quantum theory:
Photon energy = Planck's constant x wave frequency
...written as E = hf. This means that high frequency (short wavelength) waves correspond to
photons with high energy e.g. X-ray photons have more energy than visible light photons.
The electromagnetic spectrum goes (in order of increasing wavelength) from gamma-rays, X-
rays, ultraviolet, visible light, infra-red, to radio waves. These are all basically the same
phenomena, but practically we use different techniques to detect photons in different parts of
the spectrum. The short wavelength radiation (X-rays and gamma-rays), in particular, is
blocked by the atmosphere.
A spectrum is a graph of the intensity of radiation at different wavelengths. There are two basic
types of spectrum you need to be aware of line spectra and thermal spectra. Line spectra come
from `transparent' sources (e.g. diffuse clouds of gas in space), whereas thermal spectra are
emitted by `opaque' objects (ones you can't see through like planets or stars).
Line spectra arise due to transitions of electrons between a set of energy levels within atoms.
Due to energy conservation, when an electron makes a transition between energy levels, a
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