Astro 1051 Homework
9th Ed
Chapters 3, 4, 

Assigned:
  Ch. 3 R&D 2,4-6,8-10,12-14
  Ch. 4 R&D 1,6,9   (also shown 8-12,15)

Ch. 3
 R&D
1. What is motion wave?    [SKIP, bad question]
1. What is a wave?    [Old version]
A: A disturbance in a medium or field that travels carrying
energy and information.

2. What is the relationship between wavelength, wave frequency,
and wave velocity?
A: The golden rule of waves relates all three: v = w*f, where
v is velocity, w is wavelength and f is the frequency.
Basically, there is an inverse relationship between wavelength
and frequency when wave velocity is held fixed.  So, an increase
in wavelength is accompanied by a decrease in frequency.

[3. What are diffraction and interference, and how do they relate
to wave motion?    [SKIP]
A: Diffraction is an interaction between waves an edge of
an object or a hole/slit in an object.  The wave is observed
to "bend" around corners in a way which is inconsistent with
a particle model.  For example, being able to hear someone
around a corner means that the sound is diffracting around the 
edges of the doorway.  Interference is the way two or more
waves can arrive at the same point and produce an amplitude
the is the sum of the waves. Sometimes two waves can cancel
which again is only a wavelike phenomenon and not a particle
phenomenon. ]

4. Why is light referred to as an electromagnetic wave?
A:  Because it is electric and magnetic fields that 
change as the wave passes.

5. What effect does a positive charge have on a nearby
negatively charged particle?
A: A positive (+) charge attracts a negative (-) charge,
meaning that it exerts a force on that particle which,
unless countered by another force, will pull the negative
particle towards it.

6. What's so special about c?
A: c is the speed of light in a vacuum.  It is the fastest
speed there is.  All forms of E-M radiation travel at the
speed c in a vacuum.  (In a medium, the speeds vary a little
depending on wavelength.) It is also special in that a given
beam of light will have the same speed as measured in
any inertial reference frame. This is a postulate of the special
theory of relativity.

9. If Earth were completely blanketed with clouds and
we couldn't see the sky, could we learn about the realm
beyond the clouds?  What forms of radiation might be
received?
A:  Radio waves would be our only wave of learning about
space - radio astronomy can be done on cloudy days.

[ 10. Name the colors that make up white light. What is it about
these colors that causes us to perceive them differently?   [SKIP]

A: Red, Orange, Yellow, Green, Blue, (Indigo), Violet = ROYGBIV.
These are in order of decreasing wavelength and increasing 
frequency.  The different cone cells of our eyes differ in
their sensitivity to different colors presumably because
different colors correspond to different photon energies.  ]

11. What do radio waves, infrared radiation, visible light,
ultraviolet radiation, X-rays, and gamma rays have in common?
How do they differ?
A: All the types of E-M radiation consist of varying electric
and magnetic fields.  They also all travel at the speed
of light, c, in a vacuum.  They differ in their frequencies
and wavelengths.  They also differ in how well they
transmit through matter.

12. What is a blackbody?  What are the main characteristics
 of the radiation it emits?
A: A blackbody is an idealized object that absorbs all radiation
falling on it.  It also emits radiation with a particular
continuous spectrum (the "Planck" or "blackbody" spectrum) 
which has a single peak wavelenth that depends only on the temperature
of the blackbody.  (The higher the temperature, the
shorter the wavelength of peak radiation.)


13. In terms of its blackbody curve, describe what
happens as a red-hot glowing coal cools.
A: The rate of energy emission in the form of E-M waves
will decrease as the coal cools down.  Also, the
wavelength of maximum intensity will increase
as it cools.

14. What does Wien's law reveal about stars in the sky?
A: that stars with a bluish color have higher surface
temperatures than red stars.

15. How do astronomers use the Doppler effect to determine
the velocities of astronomical objects? What are some possible
limitations of this approach?
A: They measure the spectrum of the object and look for
the wavelengths of features in the spectrum (like emission
or absorption lines or the peak of the continuum).  If
these features are at higher wavelengths than their rest
values, the object is moving away from us, if their wavelengths
are lower than the rest values, the object is approaching us.
A limitation of the doppler effect is that it only tells us
about the component of the velocity along the line of sight,
not across the line of sight.

[Old 10: In what parts of the electromagnetic spectrum is the
atmosphere transparent enough for ground-based astronomy?
A: Light from space makes it all the way to the ground
if it is in the visible range (400-800 nm), or in certain
parts of the radio and infrared ranges.  Gamma rays,
x-rays, and most UV is blocked by the atmosphere.

Multiple Choice
1. A
2. C
3. B
4. B
5. D
6. A
7. B
8. D
9. A
10. B

  Ch. 4 R&D 1,6,9  
1. What is an absorption spectrum? An emission spectrum?
How are they related?
An absorption spectrum is created when the light from a source
of a continuous spectrum (e.g., a blackbody) shines through a 
gas. It looks like the continuous spectrum, but with sudden
drops in intensity at specific wavelengths.  The dips in
intensity are called absorption lines. (If the spectrum is
displayed as a horizontal band of light, the abs. lines are dark
vertical bands.)
An emission spectrum has very little continuum (mostly dark)
but with sudden spikes in intensity at specific wavelengths.
It can be created by observing an excited gas with no light
source behind it.
The two are related because an emission spectrum and absorption
spectrum can be created by the SAME gas depending on the
line of sight to the gas.  The wavelengths that have spikes
of intensity (emission) are the same as the wavelengths with dips in
intensity (absorption).  For both spectra, the wavelengths
of the lines depend on the elements present in the gas (e.g.,
hydrogen or sodium).

6. What is an atom?  In what ways does the Bohr model of
atomic structure differ from the modern view?
An atom is the smallest possible piece of an element.
Smaller subdivisions are sub-atomic particles like electrons,
protons and neutrons.  The Bohr model differs from the
modern view in that it still visualizes the electrons as
moving in circular orbitals rather than blurry electron
clouds or probability distributions.

9. What is the normal condition for atoms?  What is an excited atom? 
What are orbitals?
Normally, the number of electrons in an atom equals the number of protons 
in the nucleus, and the electrons are in their lowest energy level, 
the “ground state.” When an atom is excited, an electron absorbs 
energy from an outside source and moves to a higher energy orbital. 
The precisely-defined energy states or energy levels are referred 
to as orbitals. They are the regions surrounding the nucleus that 
can be occupied by electrons.


--------------------  Start of extra answers for Ch. 4

2. Describe the basic components of a simple spectroscope.
The basic structure of a spectroscope consists of a slit, 
a disperser, and an eyepiece or screen. The slit turns the light 
into a narrow beam. The disperser (prism or diffraction grating) 
spreads the beam of light out into its various wavelengths or colors. 
The eyepiece or screen then allows the spectrum to be observed and 
photographed.

5. In the particle description of light, what is color?
Different colors correspond to different photon energies.
The energy of a photon particle is directly proportional to frequency, 
so red light has the lowest frequency and energy per photon in the 
visible spectrum, and violet has the highest energy per photon.


8. What does it mean to say that a physical quantity is quantized?
When a physical quantity is quantized it means that the quantity 
can take on only specific discrete values rather than having an 
infinite number of possible values.  ('Quantized' is almost the 
opposite of 'continuous'.)

10. Why do excited atoms absorb and reemit radiation at characteristic
frequencies?
In order for a photon to be absorbed by an electron, it must have an 
energy that is precisely equal to the energy difference between the 
energy level occupied by the electron and a higher energy level. When 
the electron absorbs the photon, it moves immediately to the higher 
energy level. Very quickly thereafter the electron moves back down to a 
lower energy level by emitting a photon. The emitted photon must have 
an amount of energy equal to the energy difference between the two 
levels. These discrete amounts of photon energy translate into 
particular frequencies of radiation.

11. How are absorption and emission lines produced in a stellar
spectrum?  What information might absorption lines in the spectrum 
of a star reveal about a cloud of cool gas lying between us and 
the star?
  The hot, dense core of a star produces a continuous spectrum. 
However, this light passes through a cooler layer of gas that 
comprises the outer atmosphere of the star. Specific wavelengths are 
absorbed by the atoms of this gas and the resulting spectrum appears 
as an absorption spectrum, a continuous spectrum with specific 
wavelengths missing. Since different elements absorb different 
types of light, the wavelengths that are missing can tell us what 
elements are found in the outer layers of the star. Emission lines 
are not normally found in a stellar spectrum, unless the gases in the 
outer layers of the star are unusually hot. However, in some cases, a hot, 
low density layer can form or can be found in clouds of gas between 
stars and those clouds give off an emission spectrum. In this case, 
the wavelengths of the emission lines can tell

12. Why might spectral lines of an element in a star's spectrum
be weak, even though that element is abundant in the star?
  The absorption lines in a star could be weak because that element is 
scarce in that star. However, it is possible that the element could be 
abundant – even the majority of the star’s composition – and not produce 
strong lines, if the electrons of most of the atoms of that element 
are not in a condition to absorb light. For example, the H-alpha absorption 
line of hydrogen results from electrons jumping from the second to the 
third atomic orbital. If a star’s outer atmosphere is rather cool, 
relatively few atoms have electrons that have absorbed enough energy 
to jump to the second excited state or higher; most are in the ground 
state. Hence, the second to third level transition occurs rarely, and 
the wavelength of light corresponding to the H-alpha absorption line is 
rarely absorbed. A weak spectral line results. If the star is very hot, 
then the electrons of most the hydrogen atoms may have too much energy, 
and may have left the atoms entirely. Such stars are mostly hydrogen, 
but the ionized hydrogen has no electrons to absorb light. Only a few 
are capable of absorbing, and so the lines of hydrogen are again weak.

15. List three properties of a star that can be determined from
observations of its spectrum.
  Many stellar properties can be deduced from the spectrum: Radial 
velocity of the star, elemental abundances, temperature, rate of 
rotation, presence of turbulence, strength of magnetic field, and 
atmospheric pressure.

Ch. 4 Mult Choice
1. C 
2. C 
3. D 
4. C 
5. B 
6. B 
7. B 
8. B 
9. B 
10. D 

--------------------------------  End of extra answers