Visible Light

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CHAPTER 4 Visible Light and Other
Electromagnetic Radiation
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WHAT DO YOU THINK?

• How hot is a red hot object?
• What color is the Sun?
• How can we determine the age of space debris
  found on Earth?

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• You will discover
• the origins of electromagnetic radiation
• the structure of atoms
• that stars with different surface temperatures
  emit different intensities of electromagnetic
  radiation
• that astronomers can determine the chemical
  compositions of stars and interstellar clouds by
  studying the wavelengths of electromagnetic
  radiation that they absorb or emit
• how to tell whether an object in space is moving
  toward or away from Earth

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BLACKBODY RADIATION

• A heated iron poker will begin to glow emitting
  photons. This is different from a burning
  process because no chemical change is involved.
  The amount and wavelength of the radiation
  changes with temperature.
• As the object heats up, it gets brighter,
  emitting more photons of all colors
  (wavelengths).
• The brightest color (most intense wavelength) of
  the radiation changes with temperature.

AS THE TEMPERATURE RISES, THE POKER BECOMES
BRIGHTER AND GLOWS ORANGE
WHEN FIRST HEATED THE POKER GLOWS DIMMLY AND IS
RED
AT HIGHER TEMPERATURES THE POKER BECOMES EVEN
BRIGHTER AND GLOWS YELLOW
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Shown is a plot of intensity versus wavelength
for blackbodies at different temperatures. At
higher temperatures the most intense wavelengths
are shorter. Since the observed color depends on
these emitted wavelengths, blackbodies at
different temperatures will appear different
colors.
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Stellar surfaces emit light that is close to an
ideal blackbody. We can estimate the surface
temperature of a star by examining the intensity
of emitted light across a wide range of
wavelengths.
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A spectroscope is used to examine the wavelengths
of light emitted from a source
When a chemical is burned, the light produced is
made of only specific wavelengths. Different
chemical elements have their own series of
wavelengths.
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Elements are arranged in order of increasing
number of protons (atomic number) and their
properties. The elements in each column have
similar chemical properties.
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The combination of lines from a stellar spectrum
allow us to determine which chemicals are present
and in what quantities.
For example, by matching the spectrum of iron to
the absorption lines from the Sun, we see that
there is iron present in the Suns atmosphere.
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A grating spectrograph separates light from a
telescope into different colors by passing it
through a grating of tiny parallel grooves.
Peacock feathers are natural gratings.
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THE SPECTRUM OF HYDROGEN GAS
ABSORPTION SPECTRUM Signature wavelengths appear
as dark lines on an otherwise continuous
rainbow. Lines appear as dips in the intensity
versus wavelength graph.
EMISSION SPECTRUM Signature wavelengths appear as
bright lines on an otherwise black
background. Lines appear as peaks in the
intensity versus wavelength graph.
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Different types of spectra are produced depending
on how a how blackbody and a cloud of gases are
observed
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ATOMIC STRUCTURE
At the center of an atom is a dense nucleus which
contains positively-charged particles, called
protons, and particles with no charge, called
neutrons. This nucleus is surrounded by a cloud
of negatively-charged particles called electrons.
Most of the mass of an atom is contained within
its nucleus. The neutron and proton in the
nucleus are each about 1800 times more massive
than the electrons in the surrounding
cloud. However, most of the space of an atom is
occupied by the electron cloud. The nucleus of
an atom is 100,000 times smaller than the atom
itself. The remaining space is filled by the
electron cloud.
NUCLEUS TINY AND MASSIVE CENTER CONTAINING
PROTONS AND NEUTRONS
THE ELECTRON CLOUD EXTENDS FAR FROM THE NUCLEUS
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The number of protons contained in the nucleus,
called the atomic number, determines the element
of the atom. Elements containing the same number
of protons in their nucleus (and are thus the
same chemical element) but have different numbers
of neutrons are called isotopes. Because two
protons of like charge repel each other, there
must be another force which holds the nucleus of
an atom together. We call this force the strong
nuclear force, and it is the strongest of the
four fundamental forces. However, it only has a
range of effect inside the atomic nuclei.
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The electrons in an atom can only exist in
certain allowed orbits with specific energies.
The lines seen from the chemicals are made when
an electron moves from one energy level to
another.
When an electron moves from a lower energy level
to a higher one, a photon is absorbed. When an
electron moves from a higher energy level to a
lower energy one, a photon is emitted. The
energy of the photon, and thus its wavelength,
are determined by the energy difference between
the two energy levels.
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The visible light portion of the spectrum of a
hydrogen atom shows the photons representing
transitions to and from the n 2 energy level to
a higher energy level, forming a series of lines,
the Balmer Series. This spectrograph shows the
hydrogen Balmer absorption lines from 13-40.
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Emitted photons sent out in all directions will
cause the gas surrounding a star to glow
different colors, depending on which gases are
abundant.
HYDROGEN RICH CLOUDS GLOW RED.
OXYGEN RICH CLOUDS GLOW GREEN.
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Radial Velocity
The proper motion of a star is its motion
perpendicular to our line of sight across the
celestial sphere. This is so small that it can
only be measured for the closest stars.
The radial velocity of a star is its motion along
our line of sight either toward or away from us.
Using the spectrum, we can measure this for
nearly every object in space.
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Recall that the wavelength of light, and
therefore the wavelength of the photons that
light contains, is slightly shifted when the
source is traveling toward or away from the
observerthe Doppler Effect.
Stars moving toward us show spectral lines that
are shifted to blue. Stars moving away from us
show spectral lines that are shifted to red. The
amount of the shift increases with the radial
speed.
The Balmer series lines from the spectrum of the
star Vega are all shifted toward the blue side by
the same amount. From this we determine that
Vega is moving toward us (blueshift) with a speed
of 14km/s (determined from the amount of the
shift).
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We can examine the proper motion of nearby stars
over long periods of time. This picture is made
from three overlapping photographs taken over a
four-year period. The three dots in a row are
Barnards Star seen moving over the four-year
period.
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WHAT DID YOU THINK?

• How hot is a red hot object?
• Of all objects that glow from heat stored or
  generated inside them, those that glow red are
  the coolest.
• What color is the Sun?
• The Sun emits all wavelengths of electromagnetic
  radiation, with blue-green most intense.
• How can we determine the age of space debris
  found on Earth?
• We measure how much long-lived radioactive
  elements have decayed in the object.

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Key Terms
absorption line absorption line spectrum atomic
number blackbody blackbody curve continuous
spectrum diffraction grating element emission
line emission line spectrum energy flux excited
state ground state ion ionization isotope Kirchhof
fs laws luminosity
quantum mechanics radial velocity radioactive spec
tral analysis spectrograph Stefan-Boltzmann
law strong nuclear force transition (of an
electron) virtual particles Wiens
law molecule nucleus (of an atom) periodic
table Plancks law proper motion