The Electromagnetic Spectrum and Observing for Educators by ghkgkyyt


									       YAAYS course:

The Electromagnetic Spectrum
 and Observing for Educators

        February 19, 2008
           Rich Kron
from last week:

visible light, radio, infrared, X-rays etc. are all electromagnetic
radiation, differing only by energy

electromagnetic radiation can be considered to be a wave (with a
certain wavelength), or a particle (photon)

energy can be measured by frequency or by wavelength
almost everything we know about the Universe is decoded from
the light we receive from distant objects

Earth’s atmosphere transmits at some wavelengths and absorbs at

a spectrum (“rainbow”) is a representation of the number of
photons something emits according to the energy of each photon

there are different types of spectra
               Types of Spectra

✦ emission

✦ continuous

✦ absorption

      Discovering the Universe   Chapter 4
                        Types of Spectra

    alternate titles:

    How Light is Created and Absorbed

    Properties of Glowing Objects

by measuring spectra, we can tell a lot about how
something radiates (how it “works”)
Bells and violin strings are
examples of systems that oscillate
at some particular frequency
(also overtones, harmonics,

You can add energy to these
systems if the energy is tuned to
that frequency.

You can also get energy out
(e.g. moving air).
The Tacoma Narrows Bridge is another example
of a system with a particular frequency of
oscillation - energy was supplied by wind (1940).

Bridges are now designed so they don’t do this.
Rather, the energy goes into numerous small
vibrations that eventually turn into heat.
Lick Observatory, Mt.
Hamilton, CA

note the large fork of the
telescope mounting - can it
vibrate like a tuning fork?
Like a violin string, atoms can absorb energy
(photons) only at particular frequencies, and can
emit energy (photons) only at particular

The emission or absorption of light corresponds
to the transition of the atom between one state
or energy level and another.
The spectrum of hydrogen shows energy only at
specific wavelengths (“lines”).

Ephoton = Eatom(initial state) - Eatom(final state)

The structure of the spectrum of an atom -
hydrogen, helium, argon, etc. - is related to the
structure of the atom itself.
                                             Niels Bohr
                                            1885 - 1962

  Johann Balmer
   1825 - 1898
Swiss math teacher

  hydrogen spectrum:
λ = const. × n2 / (n2 - 4)
                             Carlsberg brewery, Copenhagen
a low-density gas with energy
put into it (for example, via
electrical spark) emits light
with a line spectrum.

                                Gustav Kirchhoff, Robert Bunsen
               Examples of line emission

neon light         hot clouds of interstellar gas
street light       upper atmosphere of the Sun
lightning          low-density gas near a black hole
spectrum of
lightning on
spectrum of hot interstellar gas: emission lines
 spectrum of hot dense gas
near black hole: emission lines
all dense objects that are
hotter than their
surroundings radiate

the spectrum of the
radiation is continuous
(no “features”)
           Examples of continuous emission

light bulb filament       high-density gas near a black hole
hot lava                 interstellar dust particles
people                    the Universe
gas spirals in
towards black hole,
heats up to high
radiates X-rays, UV,
visible light
infrared image of interstellar dust particles in Cygnus
all-sky map - the Universe as revealed
 by a telescope tuned to microwaves
Why does low-density hot gas emit a
 line spectrum, while high-density hot
material emits a continuous spectrum?
the third kind of spectrum is called an
absorption spectrum: light is observed at
all wavelengths except specific ones

lower layers in the Sun are denser and hotter,
upper layers are less dense and transparent

light from the lower layers (continuous
spectrum) is absorbed by the cooler gas
through which it passes
Spectrum of daylight = spectrum of Sun
spectrum of Sun-like star
transparency of Earth’s atmosphere

 visible                infrared
“A” is the naked core of the evolving
 star that created the Ring Nebula
            Summary: Types of Spectra

✦ hot, low-density gas emits a line spectrum

lots of diagnostics: atom, ion, isotope; pressure;
temperature; etc.

✦ hot, dense material (solid, liquid, dense gas)
emits a continuous spectrum

for a perfect radiator (black body), get only temperature

✦ astronomical sources are very often one of
these types

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