Astro110-01
Lecture 14
Light and Matter
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What are the building blocks of
matter?
Or the parts of an atom
Electron
cloud
Nucleus
Atom
proton neutron
mass (proton) ≅ mass (neutron) ≅ 2000 x mass(e)
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Atomic Terminology
• Atomic Number
– number of protons in nucleus
• Atomic Mass Number
– number of protons + neutrons
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Atomic Terminology (cont’d)
• Isotope:
– same # of protons but different # of neutrons (4He, 3He)
• Molecules:
- two or more atoms (H2O, CO2)
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How do light and matter
interact?
• Emission
• Absorption
• Transmission:
– Transparent objects transmit light.
– Opaque objects block (absorb) light.
• Reflection
– Scattering = random reflection
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Reflection and Scattering
Mirror reflects light
in a particular
direction
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Movie screen scatters light in
all directions
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Interaction of Light with Matter
Interaction between light and matter determines
the appearance of everything around us.
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Probing Matter through Light:
The spectrum
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Different ways of visualizing a spectrum
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Three Types of Spectra:
Kirchhof’s Laws
Continuous
Emission
Absorption
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Spectra of astrophysical objects
are usually combinations of these
three basic types
Continuous Spectrum
Emission Line Spectrum
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Absorption Line Spectrum
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Continuous Spectrum
• The spectrum of a common (incandescent)
light bulb spans all visible wavelengths,
without interruption.
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Emission Line Spectrum
• A thin or low-density cloud of gas emits light only at
specific wavelengths that depend on its composition
and temperature, producing a spectrum with bright
emission lines.
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Absorption Line Spectrum
• A cloud of gas between us and a light bulb can
absorb light of specific wavelengths, leaving dark
absorption lines in the spectrum.
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How does light tell us what
things are made of?
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Recall the parts of an atom
Electron
cloud
Nucleus
Atom
proton neutron
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Chemical Fingerprints
 Each type of atom has a unique spectral
fingerprint
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Production of Emission Lines
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Chemical Fingerprints
Energy Levels
Each type of atom
has a unique set
of energy levels.
Energy levels of hydrogen
E=hxf=h/λ
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Each transition
corresponds to a
unique photon
energy and
frequency (or
wavelength).
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Transitions yielding emission
lines
• Downward
transitions
produce a unique
pattern of
emission lines.
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Transitions yielding absorption
lines
• Because those
atoms can
absorb photons
with those same
energies, upward
transitions
produce a pattern
of absorption
lines at the same
wavelengths.
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Production of Absorption Lines
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 Chemical fingerprints in a
spectrum indicate
which kinds of atoms are present
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Example: the Solar Spectrum
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How does light tell us the
temperatures of planets and
stars?
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Thermal Radiation
• Nearly all large or dense objects emit thermal
radiation, including stars, planets, and you.
• An object’s thermal radiation spectrum
depends on only one property: its
temperature.
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Properties of Thermal Radiation
1. Hotter objects emit more light at all frequencies
per unit area.
2. Hotter objects emit photons with a higher
average energy.
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Wien’s Law
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λ max = 2.9 106 / T (K)
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