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Chapter 2 (My Take)
It’s what we see
(or don’t see for that matter)
that counts
Astro 101
(including the scientific method)
in a nutshell….
Chapter 2
Light and Matter
Units of Chapter 2
Information from the Skies
Waves in What?
The Electromagnetic Spectrum
Thermal Radiation
Spectroscopy
The Formation of Spectral Lines
The Doppler Effect
Summary of Chapter 2
2.1 Information from the Skies
Electromagnetic radiation: Transmission of
energy through space without physical
connection through varying electric and
magnetic fields
Example:
Light
Wave motion: Transmission of energy without
the physical transport of material
Example: Water wave
Water just
moves up
and down.
Wave
travels and
can
transmit
energy.
Frequency: Number of wave crests that pass a
given point per second
Period: Time between passage of successive
crests
Relationship:
Period = 1 / Frequency
Wavelength: Distance between successive
crests
Velocity: Speed at which crests move
Relationship:
Velocity = Wavelength / Period
2.2 Waves in What?
Diffraction: The
bending of a wave
around an obstacle
Interference: The
sum of two waves;
may be larger or
smaller than the
original waves
Water waves,
sound waves,
and so on,
travel in a
medium (water,
air, …).
Electromagnetic
waves need no
medium.
Created by
accelerating
charged
particles
Magnetic and electric fields are inextricably
intertwined.
A magnetic
field, such as
the Earth’s
shown here,
exerts a force
on a moving
charged
particle.
Electromagnetic waves: Oscillating electric and
magnetic fields; changing electric field creates
magnetic field, and vice versa
2.3 The Electromagnetic Spectrum
Different colors of light are distinguished by their
frequency and wavelength.
The visible spectrum is only a small part of the
total electromagnetic spectrum.
Different parts of the full electromagnetic
spectrum have different names, but there is
no limit on possible wavelengths.
Note that the atmosphere is only transparent at a
few wavelengths – the visible, the near infrared,
and the part of the radio spectrum with frequencies
higher than the AM band. This means that our
atmosphere is absorbing a lot of the
electromagnetic radiation impinging on it, and also
that astronomy at other wavelengths must be done
above the atmosphere.
Also note that the horizontal scale is logarithmic –
each tick is a factor of 10 smaller or larger than the
next one. This allows the display of the longest and
shortest wavelengths on the same plot.
2.4 Thermal Radiation
Blackbody spectrum: Radiation emitted by an
object depending only on its temperature
More Precisely 2-1: The Kelvin
Temperature Scale
Kelvin temperature
scale:
• All thermal motion
ceases at 0 K.
• Water freezes at
273 K and boils at
373 K.
Radiation laws:
1. Peak wavelength
is inversely
proportional to
temperature.
Radiation laws:
2. Total energy emitted is proportional to fourth
power of temperature.
Compared to optical photons
A.
B.
C.
D.
E.
Radio photons have longer wavelength.
X-ray photons have a larger frequency.
Infrared photons have a smaller energy.
All of the above.
None of the above.
Which of the following has
the LONGEST wavelength?
A. Red light.
B. Blue light.
C. Green light.
D. Infrared light.
If Star A is hotter than Star B, and Star A is
emitting most of its light at a wavelength
corresponding to yellow light, which of the
following statements is true?
A. Star B will emit most of its light at a wavelength
longer than yellow.
B. Star B will emit most of its light at a wavelength
shorter than yellow.
C. Star B will emit most of its light at the same
wavelength as Star A.
D. More information is required to answer this
question
The energy of a photon is
A. proportional to the wavelength
and inversely proportional
to the frequency.
B. proportional to the wavelength
and proportional to the
frequency.
C. inversely proportional to the
wavelength and inversely
proportional to the frequency.
D. inversely proportional to the
wavelength and proportional
to the frequency.
Electromagnetic radiation will be
created by any charged particle
that
A. accelerates (changes speed or
direction).
B. moves in a straight line at constant
speed.
C. remains at rest.
D. is subjected to a gravitational field.
2.5 Spectroscopy
Spectroscope: Splits light into component
colors
Emission lines: Single frequencies emitted by
particular atoms
Emission spectrum can be used to identify
elements.
Absorption spectrum: If a continuous spectrum
passes through a cool gas, atoms of the gas will
absorb the same frequencies they emit.
Absorption spectrum of the Sun
Kirchhoff’s laws:
• Luminous solid, liquid, or dense gas
produces continuous spectrum.
• Low-density hot gas produces emission
spectrum.
• Continuous spectrum incident on cool, thin
gas produces absorption spectrum.
Kirchhoff’s laws illustrated
2.6 The Formation of Spectral Lines
Existence of spectral lines required new model of
atom, so that only certain amounts of energy
could be emitted or absorbed.
Bohr model had certain allowed orbits for
electron.
Emission energies correspond to energy
differences between allowed levels.
Modern model has electron “cloud” rather than
orbit.
Atomic excitation
leads to emission.
(a) Direct decay
(b) Cascade
Absorption spectrum:
Created when atoms
absorb photons of right
energy for excitation
Multielectron atoms:
Much more complicated
spectra, many more
possible states
Ionization changes
energy levels.
Molecular spectra are much more complex
than atomic spectra, even for hydrogen.
(a) Molecular hydrogen
(b) Atomic hydrogen
Spectral lines unique to each type of
atom are caused by
A. each atom having a unique set of
protons.
B. the unique sets of electron orbits.
C. the neutron-electron interaction being
unique for each atom.
D. each type of photon emitted by the
atom being unique.
E. none of the above; spectral lines are
not unique to each type of atom.
If an electron moves from a lower energy
level to the next higher energy level,
then
A.
B.
C.
D.
the atom has become excited.
the atom has become ionized.
the atom's light will be blue shifted.
the atom's light will be red shifted.
In general, the observed spectra
of stars appear as what kind of
spectrum?
A.
B.
C.
D.
Absorption.
Continuous.
Emission.
Nonthermal.
If you view the light from an opaque,
hot gas through a spectroscope, you
would expect to see what kind of
spectrum?
A.
B.
C.
D.
Continuous.
Emission line.
Absorption line.
Combination of emission line and
continuous.
When an atom is excited, then it has:
A. more electrons than protons.
B. the same number of electrons as
protons.
C. one or more electrons stripped off.
D. one or more electrons move to higher
energy levels.
When you see a spectrum with
absorption lines in it, you can infer
that:
A. the light passed through ionized atoms.
B. electrons moved up in energy levels to
absorb the light.
C. electrons moved down in energy levels
to absorb the light.
D. all the atoms were in excited states.
2.7 The Doppler Effect
If one is moving toward a source of radiation, the
wavelengths seem shorter; if moving away, they
seem longer.
Relationship between frequency and speed:
Depends
only on the
relative
motion of
source and
observer
The Doppler effect shifts an object’s entire
spectrum either toward the red or toward the
blue.
If a star is moving towards us,
its speed can be determined by
measuring it’s
A.
B.
C.
D.
red shift.
Doppler shift.
parallax shift.
average photon speed.
The Doppler effect
A. is a measure of a star's velocity in
space.
B. is a shift of the star's spectrum
which depends on its velocity in
the line of sight.
C. is a shift in the stars apparent
position with respect to
background stars.
D. is a shift in the star's spectrum
which depends on its temperature.
If a yellow spectral line is
Doppler shifted toward the blue
end of the spectrum:
A. the source is moving away from
you.
B. the source is moving toward
you.
C. the distance between you and
the source is increasing.
D. the distance between you and
the source is decreasing.
Summary of Chapter 2
• Wave: period, wavelength, amplitude
• Electromagnetic waves created by
accelerating charges
• Visible spectrum is different wavelengths of
light.
• Entire electromagnetic spectrum:
• includes radio waves, infrared, visible
light, ultraviolet, X-rays, gamma rays
• can tell the temperature of an object by
measuring its blackbody radiation
Summary of Chapter 2, cont.
• Spectroscope splits light beam into component
frequencies.
• Continuous spectrum is emitted by solid,
liquid, and dense gas.
• Hot gas has characteristic emission spectrum.
• Continuous spectrum incident on cool, thin
gas gives characteristic absorption spectrum.
Summary of Chapter 2, cont.
• Spectra can be explained using atomic
models, with electrons occupying specific
orbitals.
• Emission and absorption lines result from
transitions between orbitals.
• Doppler effect can change perceived frequency of
radiation.
• Doppler effect depends on relative speed of source
and observer.