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```Chapter 2
Light and Matter
Chapter 2
Part One
Ideas in Chapter 2
Information from the Skies
Waves in What?
The Electromagnetic Spectrum
Spectroscopy
The Formation of Spectral Lines
The Doppler Effect
Question 1
Which of these is
NOT a form of
electromagnetic
a) gamma rays
b) infrared
c) sound
d) visible light
Question 1
Which of these is
NOT a form of
electromagnetic
a) gamma rays
b) infrared
c) sound
d) visible light
Sound comes from
pressure waves; all
others are types of
different wavelengths.
2.1 Information from the Skies
energy through space without physical
connection through varying electric and
magnetic fields
Example: Light
2.1 Information from the Skies
Wave motion: Transmission of energy without
the physical transport of material
2.1 Information from the Skies
Example: Water wave
Water just moves
up and down.
Wave travels and
can transmit
energy.
2.1 Information from the Skies
Frequency: Number of wave crests that pass a
given point per second units of Hertz (Hz)
Period: Time between passage of successive
crests
Relationship: Period = 1 / Frequency
Wave with frequency of 1000 Hz
Period = 1/1000 Hz
0.001 s or 1 ms
2.1 Information from the Skies
Wavelength: Distance between successive
crests
Velocity: Speed at which crests move
Relationship:
Velocity = Wavelength / Period
1 m/s = 1 m / 1 s
3x108 m/s = 660 nm / 2.2 x10-15 s
What is the Frequency?
3x108 m/s = 660 nm / 2.2 x10-15 s
Period = 1 / Frequency
Frequency = 1 / period
1 / 2.2x10-15 s = 4.54x1014 Hz
Question 2
The distance between
successive wave crests
defines the ________ of a
wave.
a) wavelength
b) frequency
c) period
d) amplitude
e) energy
Question 2
The distance between
successive wave crests
defines the ________ of a
wave.
a) wavelength
b) frequency
c) period
d) amplitude
e) energy
Light can range
from shortwavelength
gamma rays to
long-wavelength
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
2.2 Waves in What?
Water waves, sound
waves, and so on,
travel in a medium
(water, air, …).
Electromagnetic
waves need no
medium.
Created by
accelerating
charged particles
2.2 Waves in What?
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.
2.2 Waves in What?
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.
2.3 The Electromagnetic Spectrum
Different parts
of the full
electromagnetic
spectrum have
different names,
but there is no
limit on possible
wavelengths.
2.3 The Electromagnetic Spectrum
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.
Question 3
The frequency at
which a star’s
intensity is greatest
depends directly on its
b) mass.
c) magnetic field.
d) temperature.
e) direction of motion.
Question 3
The frequency at
which a star’s
intensity is greatest
depends directly on its
Wien’s Law means that
hotter stars produce
much more highfrequency light.
b) mass.
c) magnetic field.
d) temperature.
e) direction of motion.
Blackbody spectrum: Radiation emitted by an
object depending only on its temperature
The Kelvin Temperature Scale
Kelvin temperature
scale:
• All thermal motion
ceases at 0 K.
• Water freezes at
273 K and boils at
373 K.
1. Peak
wavelength is
inversely
proportional to
temperature.
The higher the
temperature the
shorter the
wavelength
Temperature vs. Wavelength
2. Total energy emitted is proportional to fourth
power of temperature.
Question 4
Rigel appears as a bright
bluish star, whereas
Betelgeuse appears as a
bright reddish star.
Rigel is ______ Betelgeuse.
Betelgeuse
The constellation ORION
Rigel
a) cooler than
b) the same temperature as
c) older than
d) hotter than
e) more massive than
Question 4
Rigel appears as a bright
bluish star, whereas
Betelgeuse appears as a
bright reddish star.
Rigel is ______ Betelgeuse.
a) cooler than
b) the same temperature as
c) older than
d) hotter than
e) more massive than
Betelgeuse
The constellation ORION
Rigel
Hotter stars
look bluer in
color; cooler
stars look
redder.
Chapter 2
Part Two
First exam on September 14th, 2011
Mastering Astronomy
50 unregistered students
4:00 PM to 5:30 PM
Class ID MAMILLER18823
i>clickers
20 unregistered clickers
50 unregistered students
Great clicker shortage of 2011
Supernova SN2011fe
Brightness of a Billion Suns
Coming to a Galaxy near you!
Look at the Big Dipper with a small
telescope or binoculars
Spectroscopy
Spectroscope: Splits light into component
colors
Spectroscopy
Emission lines: Single frequencies emitted by
particular atoms
Spectroscopy
Emission spectrum can be used to identify
elements.
Spectroscopy
Absorption spectrum: If a continuous spectrum
passes through a cool gas, atoms of the gas will
absorb the same frequencies they emit.
Spectroscopy
Absorption spectrum of the Sun
Spectroscopy
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.
Spectroscopy
Kirchhoff’s laws
illustrated
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.
Bohr Model
The Formation of Spectral Lines
Emission energies correspond to energy
differences between allowed levels.
Modern model has electron “cloud” rather than
orbit.
Question 1
The wavelengths of
emission lines
produced by an
element
a) depend on its temperature.
b) are identical to its absorption lines.
c) depend on its density.
d) are different than its absorption lines.
e) depend on its intensity.
Question 1
The wavelengths of
emission lines
produced by an
element
a) depend on its temperature.
b) are identical to its absorption lines.
c) depend on its density.
d) are different than its absorption lines.
e) depend on its intensity.
Elements absorb or emit the same wavelengths
of light based on their electron energy levels.
Excited State 1
Excited State 2
Excitation
Excitation
Question 2
Which of the following
has a fundamentally
different nature than
the other four?
a) proton
b) electron
c) neutron
d) atomic nucleus
e) photon
Question 2
Which of the following
has a fundamentally
different nature than
the other four?
a) proton
b) electron
c) neutron
d) atomic nucleus
e) photon
Photons are packages of light
energy.
Protons, neutrons, & electrons
are particles of matter within an
atomic nucleus.
The Formation of Spectral Lines
Atomic excitation
(a) Direct decay
The Formation of Spectral Lines
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.
The Formation of Spectral Lines
Molecular spectra are much more complex
than atomic spectra, even for hydrogen.
(a) Molecular hydrogen
(b) Atomic hydrogen
Question 3
If a light source is
approaching you,
you will observe
a) its spectral lines are redshifted.
b) the light is much brighter.
c) its spectral lines are shorter in wavelength.
d) the amplitude of its waves has increased.
e) its photons have increased in speed.
Question 3
If a light source is
approaching you,
you will observe
a) its spectral lines are redshifted.
b) the light is much brighter.
c) its spectral lines are shorter in wavelength.
d) the amplitude of its waves has increased.
e) its photons have increased in speed.
The Doppler Shift explains that wavelengths from
sources approaching us are blueshifted.
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:
The Doppler Effect
Depends only on
the relative
motion of source
and observer
The Doppler Effect
The Doppler effect shifts an object’s entire
spectrum either toward the red or toward the
blue.
Question 4
Analyzing a star’s
spectral lines can tell
EXCEPT
a) its composition.
b) its surface temperature.
c) its transverse (side-toside) motion.
d) its rotation.
e) its density.
Question 4
Analyzing a star’s
spectral lines can tell
EXCEPT
a) its composition.
b) its surface temperature.
c) its transverse (side-toside) motion.
d) its rotation.
e) its density.
Only motion toward
or away from us
influences a star’s
spectral lines.
Spectra can also tell
magnetic field.
Question 5
from space reach the
surface of Earth?
b) X rays & ultraviolet light
c) infrared & gamma rays
d) visible light & radio waves
e) visible & ultraviolet light
Question 5
from space reach the
surface of Earth?
b) X rays & ultraviolet light
c) infrared & gamma rays
d) visible light & radio waves
e) visible & ultraviolet light
Earth’s atmosphere
and visible light to
reach the ground.
Equations and Symbols
•
•
•
•
•
Wavelength = l (Lambda) meters (m)
Velocity = n (Nu)
meters/second
Frequency = f
Hz or 1/s
Period = seconds
s
Speed of light = c
3x108 m/s
Basic Equations
n=lf
f=n/l
l = n /f
m/s = m x 1/s
1/s = (m/s) / m
m = (m/s) / 1/s
Example
• Radio signals travel at the speed of light.
• What is the wavelength of a radio signal
at 1MHz? (M = million)
l = n /f
m = (m/s) / 1/s
A.
B.
C.
D.
1m
3m
100 m
300 m
D. 300 m
 l = n /f
• M = (m/s) / 1/s
• 300 m = 3x108 m/s / 1x106/s
Exam 1 Example Questions
If the Moon appears half lit, and is
phase is
A
B
C
D
E
waxing crescent.
waning crescent.
full.
third quarter.
first quarter.
The fact that the Earth has moved along its
orbit in the time it took to rotate once is the
reason for
A
B
C
D
E
the difference between solar and sidereal
time.
precession.
Earth's 23.5-degree tilt.
seasons.
the position of the Celestial Equator.
Completed
Latitude and right ascension are coordinate
systems used to find objects on the Celestial
Sphere.
A True
B False
A planet whose distance from the Sun is 3
A.U. would have an orbital period of how
many Earth-years?
A
B
C
D
E
3
Square root 27
81
9
Square root of 9
p2 = a3
p2 = 33
p2 = 27
p = square root 27
Consider this diagram. Which statement is true?
A
The amplitude is 6 and the wavelength is 4.
B
The amplitude is 8 and the wavelength is 12.
C
The amplitude is 4 and the wavelength is 12.
D
The amplitude is 8 and the wavelength is 6.
E
The amplitude is 4 and the wavelength is 6.
Completed