Light

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Light Matter
Early scientists described 4 ways in which light
will interact with matter.

• Matter can
• emit light (Emission)
• A stove heating element will glow red when hot
• You saw hydrogen emit discrete colors of light
  when energized
• absorb light (Absorption)
• A black cat looks black because its fur absorbs
  all frequencies of visible light
• transmit light (Transmission)
• A transparent object such as glass or clear
  plastic lets light pass through
• reflect or scatter light (Reflection/Scattering)
• A smooth mirrored surface reflects all colors of
  light in a general direction
• A rough surface will reflect light in random
  directions (scatter)

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Illustrating Kirchhoffs Rules

• All three of Kirchhoffs empirical rules of
  spectral analysis are illustrated.
• A continuous spectrum produced by the hot
  compressed gas (a).
• A low density cloud of gas absorbs certain
  wavelengths but allows others to pass unaffected
  (b).
• These elements become heated and retransmit the
  absorbed light in random directions to produce an
  emission spectra (c).

Note that the frequencies missing from the
spectrum of (b) are the frequencies present in
the spectrum of (c).
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The Modern AtomAtomic Terminology

• Atomic Number of protons in nucleus
• Atomic Mass Number of protons neutrons

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Atomic Terminology

• Isotope same of protons but different of
  neutrons (4He, 3He)

• Molecules consist of two or more atoms (H2O,
  CO2)
• Ions atoms that have a net charge
• (have more or less electrons than protons)

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At least for hydrogen, Bohrs atomic model
supports the observations of Kirchhoff and the
calculations of Balmer
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Wiens Law

• In essence, the higher the frequency of the
  dominate radiation of the light emitted (NOT
  REFLECTED) by a black body, the higher its
  temperature.
• Therefore objects that emit BLUE are hotter (more
  energetic) than objects that emit RED.

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Stefan-Boltzmann Law

• Hotter objects emit more light energy per area,
  at all wavelengths, than cooler objects do.
• I sT4
• Intensity is directly proportional to the
  fourth power of object temperature

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The graph at right shows four curves that are
representative of the radiation spectra of the
following stars (from top to bottom) Rigel,
Sirius, Capella, and Betelguese.
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Measuring the Shift
Stationary
Moving Away
Away Faster
Moving Toward
Toward Faster

• We generally measure the Doppler effect from
  shifts in the wavelengths of spectral lines.

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The Sun is broken up into six layers From
innermost to outermost they are Core
Radiation Zone Convection Zone
Photosphere Chromosphere Corona
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Temperature Profile of the Outer Layers Note that
the temperature surrounding the Sun INCREASES as
you move away from the photosphere! A sharp
increase in temperature occurs between the
Chromosphere and the Corona over a small region
known as the transition region. Current models
suggest that intense magnetic fields outside of
the photosphere give rise to increased particle
energies in the corona.
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Solar Winds The Solar Winds can be seen in the
Corona as streaks of high energy charged
particles such as electrons and protons racing
away from the Sun. The Sun blows out about 2 x
1017 kg of material each year through solar wind
(thats 200,000,000,000,000,000
kg)!!! That may seem like an unbelievable
amount of mass loss, but compared to the mass of
the Sun (2 x 1030 kg or 2 000 000 000 000 000 000
000 000 000 000 kg), the Sun can keep this up for
billions of years!
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The Zeeman Effect When light passes through an
intense magnetic field, spectral lines are
split into multiple lines. This effect is used
to identify regions of strong magnetic field on
the surface of the Sun.
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• Take note that Sunspots, Prominences, Solar
  Flares, and CMEs are all a result of intense
  magnetic activity.

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The proton-proton chain reaction When these
particles are brought together they begin a
reaction known as the p-p chain reaction.
The diagram shows 4 steps of the PP chain
reaction.
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Step 1
Two protons come together resulting in a
neutron-proton pair and the generation of a
positron (a positively charged electron) and a
neutrino, ?.
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Step 2
The positron combines with a neighboring electron
and they destroy each other to release GAMMA
PHOTONS. All of the mass of the two particles is
converted into radiative energy (light).
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Step 3
The newly formed neutron-proton pair will join
with a neighboring proton to form a helium
nucleus isotope (3He) and a gamma photon.
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Step 4
Two 3He combine to form a helium nucleus and a
pair of protons.
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Mass loss is the source of the Suns Energy Take
note of the difference in mass before and after
the reactions. This tiny difference yields
extreme energies (remember, Emc2)!
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Hydrostatic Equilibrium

• The Sun maintains its size due to a condition of
  Hydrostatic Equilibrium.
• The forces of gravity directed inward are
  countered by an outward force due to internal
  thermal pressure.

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Solar ThermostatNegative Feedback

• The rate of fusion reactions in the Suns core is
  maintained by a natural negative feedback process

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