Nature of Light
Wave Properties
• Light is a selfpropagating electromagnetic wave
– A time-varying electric
field makes a magnetic
field
– A time-varying
magnetic field makes
an electric field
•Wavelength (or frequency) are
related to energy
•Wave amplitude  brightness
•Angle of field lines  polarization
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Nature of Light
Particle Properties
• Photons have energy,
but no mass.
• Photon flux 
brightness
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Nature of Light
Properties
Short wavelength, λ
Long wavelength, λ
High Frequency, ν
High Photon Energy
Blue
Low Frequency, ν
Low Photon Energy
Red
Small Amplitude - Faint
Low Photon Flux
Faint
Large Amplitude - Bright
High Photon Flux
Bright3
What is a Spectrum
• Brightness or Intensity as a function of Energy
• May equivalently shown be shown as a function of
wavelength or frequency
• Describes the energy distribution of the observed light, or
• Describes how much of the observed light has what energy
value
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Wien’s Law
• Wien’s law allows
astronomers to determine
the temperature of a star.
• The wavelength at which
a star is brightest is
related to its temperature
• Hotter objects radiate
more strongly at shorter
wavelengths
• Blue has a shorter
•Objects can emit radiation at many
different wavelengths.
wavelength than red, so
•The wavelength at which a star is brightest hotter objects look bluer.
is related to its temperature.
•This is Wien’s Law
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When can you use Wien’s Law?
•
•
•
•
Only for objects that emit light not for those that reflect light
Light emitted by hot, solid objects obey Wien’s Law
Can not use with gases unless they are of a high density
The Sun and other stars obey Wien’s Law since the gases they
are composed of remain at a high density (at least up to the
outermost layers of the star).
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Why Learn about Atomic Structure?
• Knowing the structure
of atoms tells us about
their
– chemical properties
– light-emitting
properties
– light-absorbing
properties
An example of absorption spectra
from many different types of stars.
• From this information
we can learn about
galaxies, stars, planets,
asteroids, based on the
light they emit or
reflect.
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Atomic Structure
• An atom is composed of a dense core
called a nucleus and surrounding this
nucleus one or more negatively
charged electrons.
• The nucleus is composed of positively
charged protons and neutral neutrons.
• The electron is also 1,800 times
lighter than a proton. Protons and
neutrons however weigh about the
same.
• The electric force of attraction
between the positive protons and
negative electrons keeps the electrons
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bound to the nucleus.
Atomic Structure
• An atom is mostly empty space because the electron
moves around the nucleus at such a great distance. If
the proton were 1 cm wide a hydrogen atom would be
larger than a football field!
• A chemical element is determined based on how
many protons the nucleus contains (Hydrogen has 1,
Carbon has 6, Oxygen has 8 protons).
• When two atoms with the same number of protons
have different numbers of neutrons the two atoms are
isotopes of one another (Carbon has 6 protons but
can have 6, 7, or 8 neutrons).
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Ions
• An atom normally has the same
number of electrons as protons.
With the same number of
positive and negative charges,
an atom normally has no net
charge.
• If an atom loses or gains one or
more electrons it has become
ionized. With too few electrons,
the atom has a net positive
charge, too many electrons and
it has a net negative charge.
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Quantum Structure of an Atom
• An electron does not orbit a nucleus like a
planet orbits the Sun.
• For a given atom there are only a select few
orbits that an electron can occupy.
• This means that the orbits are quantized.
• Electrons may shift between these quantum
levels with either the emission or absorption
of a photon of electromagnetic radiation.
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Energy Level Transitions
• If an electron absorbs a photon
of light it can shift to a higher
quantum level
– Atom (or ion) gains energy
• If an electron emits a photon of
light it can shift to a lower
quantum level
– Atom (or ion) loses energy
• The energy and wavelength of
the photon in both cases
depends on the energy
difference between the two
quantum levels.
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Spectra and Atomic Structure
• Each type of atom has a
unique set of wavelengths
of light that it can absorb
and emit
– Hydrogen emits red light
at 656 nm, blue light at
486 nm and other lines
– Hydrogen will also absorb
only red light at 656 nm,
blue light at 486 and a few
other lines
• We use this to identify the
atoms present by studying
the spectrum of an object
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Emission or Absorption Spectra
• The brightness of an
emission line or the
darkness of an
absorption line
indicates how many
atoms are absorbing
or emitting that
color.
• The number of atoms
absorbing or
emitting depends on
the number present
and on the
temperature of the
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gas.
Types of Spectra
• Continuous Spectra from hot, dense or
solid objects.
• Emission Line Spectra
- from hot, tenuous
(thin) gas.
• Absorption Line
Spectra - from cold,
tenuous gas through
which light from a hot,
dense object passes.
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Types of Spectra
• Continuous Spectra atoms/ions are closely
packed-outer electron
orbits are distorted-more
(any/all) energy transitions
are allowed.
• Emission Line Spectra electron drops to lower
energy level and emits a
photon.
• Absorption Line Spectra –
atom (or ion) absorbs a
photon; electron is raised
to a higher energy state.
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Continuous and Absorption
Spectra
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Emission Spectrum
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Types of Spectra
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Doppler Shift
• If a source of light is
moving towards or
away from an observer
its spectral lines are
shifted based on the
speed and direction
• The faster the object is
moving the greater the
shift
• Shifts to longer
wavelengths means
object is moving away.
Shorter , coming
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closer.
Doppler Shift
• Radar guns used by
police to catch speeders
use Doppler shift to
determine speed.
• Astronomers refer to an
increase in wavelength
(object moving away) as
a redshift. A decrease in
wavelength (object
moving closer) is called a
blueshift.
• This technique is also
used to search for planets
around other stars.
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Redshift / Blueshift
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The Motion of Stars
This star is moving away from
Earth. The light from this star will
appear redshifted.
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Light and Atoms
Summary 1
• Wien’s Law
– hot objects emit most of their light at short
wavelength’s (high energy)
– cool objects emit most of their light at long
wavelengths (low energy)
• Blue stars are hot – red stars are cool.
• Atoms in thin gases emit or absorb radiation at
discrete wavelengths (energies)
– the wavelengths of the spectral lines depend on the
types of atomic or ionic elements present in the gas
– the strength of the spectral lines corresponds to the
temperature and density of the gas.
• Hot solid objects or hot dense gases (stars) emit
continuous spectra.
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Light and Atoms
Summary 2
• Different atoms emit/absorb light at different
wavelengths
• Observe in as many wavelengths (or energy
bands) as possible to fully understand a
physical system.
• A picture is worth a thousand words, a
spectrum is worth a thousand pictures.
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Light and Atoms
Summary 3
• Spectral lines from a moving source will
move to shorter wavelengths (blueshift) if
the source moves towards the observer, or
move to longer wavelengths (redshifts) if the
source moves away from the observer.
• The velocity of an object towards
(blueshifted) or away from (redshifted) an
observer can be determined by measuring
the wavelengths of observed object’s spectral
lines relative to a fixed reference spectrum.
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Light and Atoms
Summary 4
• Light (especially spectra) gives us
– Temperature
– Chemical Composition
• What elements are present and their relative
abundances
– Density
– Line of sight velocity
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