# lecture6

```Astronomy Picture of the Day
Light
Chapter 5
What is Light?
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Newton
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Prism shows white light contains all colors
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Maxwell
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Theory of electricity and magnetism
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Light is electromagnetic waves
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Produced by wiggling electrons
Quantum Mechanics
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Light is both: particle and wave
Waves
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Wavelength ( l )
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Distance between crests (or troughs)
Frequency ( f )
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How often it repeats (wiggles up and down)
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Measured in Hertz (Hz)
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number of times per sec
Waves
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Speed c = 3 x 108 m/s
c = lf
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Wavelength inversely related to frequency
l=c/f
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high frequency = short wavelength
–
low frequency = long wavelength
Particles as Waves
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“Wave Packet”
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particle/photon = localized wave
Properties of Light
Color
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Depends on frequency
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blue = high frequency = short wavelength
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red = low frequency = long wavelength
Carries energy (heat)
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Photon energy
–
E=hf
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high frequency = high energy = blue
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low frequency = low energy = red
h = Planck’s constant
Red light has ____ than blue light.
A. larger frequency, energy, and wavelength
B. smaller frequency, energy, and wavelength
C. larger frequency and energy, but smaller wavelength
D. smaller frequency and energy, but larger wavelength
Which of the following travels fastest?
A.
B.
C.
D.
E.
}
infrared (heat) waves
microwaves
blue light waves
none of the above
All are
types of
light!
All types of light travel at the same speed the “speed of light”, c
CPS Question
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The color of visible light is determined by its ____.
A) brightness
B) amplitude
C) speed
D) wavelength
CPS Question
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If the wavelength of light increases, the frequency
must ____.
A) increase also
B) decrease
C) remain unchanged
CPS Question
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The bending of light that occurs when moving
between media of different densities is called ___.
A) reflection
B) refraction
C) diffraction
D) distortion
Propagation of Light
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Photons travel in straight lines
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energy spread over larger area at larger distances
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produces 1/r2 decrease in brightness
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Double distance - brightness decreases by 4
If a 100-watt light bulb is placed 10 feet away from
you, and an identical 100-watt light bulb is placed
100 feet away from you, which will appear brighter?
A. The closer one
B. The farther one
C. They will appear the same brightness
How much fainter will the far one appear
compared to the close one?
A.
B.
C.
D.
Twice as faint
10 times fainter
100 times fainter
1000 times fainter
~ 1/r2
The Electromagnetic Spectrum
1 nm = 10 -9 m , 1 Angstrom = 10 -10 m
c = lf
Electromagnetic
Spectrum
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Visible light:
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red, orange, yellow, green, blue,
indigo, violet (ROYGBIV)
Invisible Light:
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Ultraviolet = bluer than blue
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Infrared = redder than red
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Other wavelengths:
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Short: X-rays, gamma-rays
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Atoms (and their electrons) vibrate
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Bigger objects produce more light
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Higher temperature = stronger vibration
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Hotter objects emit more light
Perfect absorber is black
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Absorbed light (energy) heats object
Temperature increases until
emitted energy = absorbed energy
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Thermal radiation emitted by most objects similar
What does the spectrum of an astronomical object's
Many objects (e.g. stars) have roughly a &quot;Black-body&quot;
spectrum:
Brightness
Frequency
also known as the Planck spectrum or Planck curve.
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Luminosity, L
L = energy emitted per second
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Luminosity for a spherical object (a star)
L = 4pR2 s T4
Stefan-Boltzmann Law
R = radius (size) of star; T = temperature
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double size, luminosity increases by 2x2 = 4
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double temperature; luminosity increases by 2x2x2x2
= 16
Spectroscopy and Atoms
How do you make a spectrum?
Refraction of light
When you bend light, bending angle depends on wavelength, or color.
Questions
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How is temperature related to the amount of
How is temperature related to the color of the
object?
(Blackbody Demo)
The wavelength of peak emission tells us the temperature of
the object!
&quot;cold&quot; dust
&quot;cool&quot; star
Sun
&quot;hot&quot; stars
frequency increases,
wavelength decreases
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Color
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Wavelength where most light emitted
lmax = 3 x 106 / T
T in Kelvin; lmax in nanometers (1 nm=10-9m)
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Cool stars are red
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Hot stars are blue
Color indicates temperature!
As T
As T
, Wavelength
, Wavelength
, Color = redder
, Color = bluer
Wien’s Law
The graph above shows blackbody spectra for three
different stars. Which of the stars is at the highest
temperature?
Because peak energy emission
A. Star A
occurs at shortest wavelength
B. Star B
Doppler Shift
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Originally discovered using sound waves
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Moving object
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emits light with slightly different color
Frequency (pitch) of approaching object is higher
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Blueshift
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Wavelength shorter (shifted blueward)
Frequency (pitch) of receeding object is lower
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Redshift
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Wavelength longer (shifted redward)
video
Video
Doppler Shift
Redshift
Blueshift
Spectroscopy
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Prism separates light into different colors
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Continuous spectrum
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contains all colors
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Example: blackbody spectrum
Spectroscopy
Absorption
Line
Spectrum
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Some
colors are
missing
(discrete
lines)
Solar
Spectrum
N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF
Spectroscopy
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Emission Line spectrum
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Only certain colors are present (discrete lines)
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Spectrum for each element unique (like fingerprints)
Pattern of lines is a fingerprint of the element
For a given element, emission and absorption lines occur at the same
wavelengths.
Helium discovered in Sun’s spectrum before being found on Earth!
Sodium emission and absorption spectra
Spectrum of the Sun
• Absorption spectrum
• What causes
emission/absorption
of light at specific
wavelengths?
Interactive
Video 1, 2, 3
Types of Spectra
1. &quot;Continuous&quot; spectrum
2. &quot;Emission&quot; spectrum
3. &quot;Absorption” Spectrum
video
The Particle Nature of Light
Light interacts with matter as individual packets of
energy, called photons.
c
photon energy is proportional to frequency:
1
E f (or E 
l
example: ultraviolet
photons are more harmful
than visible photons.
Model Atom
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Nucleus
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contains protons and neutrons
number of protons = element
hydrogen
(1 proton = hydrogen, 2 protons = helium, etc.)
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number of neutrons about same as protons
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Isotope = different number of neutrons
helium
Isotopes of hydrogen
Model Atom
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Electrons orbit nucleus
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Number of electrons = number of protons
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Ionization = removing electrons
Only certain orbits are allowed
hydrogen
helium
The Nature of Atoms
The Bohr model of the Hydrogen atom:
electron
_
_
+
+
proton
&quot;ground state&quot;
an &quot;excited state&quot;
(Fair Analogy)
Atomic Absorption
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Atom absorbs photon energy
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electron “jumps” to higher energy orbit
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only certain discrete orbits are allowed
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Atom can absorb only discrete colors (energies)
When an atom absorbs a photon, it moves to a higher energy state briefly
When it jumps back to lower energy state, it emits photon(s) in a
random direction, conserving the total energy of the system!
Atomic Emission
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Electron “jumps” to a lower energy orbit
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Atom emits photon
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can emit only discrete colors
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same colors (wavelengths/energies) as absorption
Atomic Energy Levels
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Energy Levels
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Different for each element
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each element has unique set of absorption/emission lines
Other elements
Helium
neutron
Carbon
proton
Each element has its own allowed energy levels yielding a unique
spectral fingerprint.
Kirchoff’s Laws
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Continuous spectrum
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Emission line spectrum
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Produced by hot solid (or dense gas)
Produced by hot, low density gas
Absorption line spectrum
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Produced when continuous source is viewed through
cooler low density gas
Kirchoff’s Laws
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Absorption lines same wavelengths as emission lines
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Gas can only absorb and emit at certain discrete
frequencies/wavelengths/energies
video
If you analyze the light from a low density
object (such as a cloud of interstellar gas),
which type of spectrum do you see?
A. dark line absorption spectrum
B. bright line emission spectrum
C. continuous spectrum
Imagine that you observe the Sun while in
your space ship far above Earth’s atmosphere.
Which of the following spectra would you
observe by analyzing the sunlight?
A. dark line absorption spectrum
B. bright line emission spectrum
C. continuous spectrum
CPS Question
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Which ONE of these is constant for all forms of
A) amplitude
B) wavelength
C) frequency
D) speed
E) energy
CPS Question
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Which ONE is NOT a property of a blackbody?
A) It appears black, regardless of its temperature.
B) It emits radiation in a continuum of
wavelengths.
C) Its spectrum peaks at a wavelength determined
by its temperature.
D) The total energy that it radiates increases rapidly
with temperature.
CPS Question
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The Sun's observed spectrum is _____.
A) A continuum with no lines, like the rainbow.
B) A continuum with bright emission lines.
C) Only absorption lines on a black background.
D) Nearly a continuum with some absorption lines.
Ionization
Hydrogen
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+
+
_
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Helium
+
+
+
+
_
_
&quot;Ion&quot;
Absorbing a high energy photon and atomic collisions can both lead
to ionization.
Spectrum of the Sun
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Complicated objects
=&gt; many different
elements
Nearly continuous
absorption spectrum
What causes
emission/absorption
of light at certain
wavelengths?
Why emission lines?
hot cloud of gas
.
.
.
.
.
.
- Photon absorption/atomic collisions excite atoms
- Electron drops back to lower level
- Photons at specific frequencies emitted
Why absorption lines?
.
.
.
. .
.
cloud of gas
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Stellar Spectra
Sun's
'atmosphere'
specific wavelengths
Fusion
generates
continuous
spectrum
Star
absorbs
Kirchhoff's Laws
1. Continuous spectrum
2. Emission spectrum
3. Absorption spectrum
Question
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How does the pitch or tone of a sound wave
change when the source of the sound is moving
towards or away from you?
What about when you are moving towards or
away from the source?
Does this effect occur for all types of waves or
just for sound waves?
Doppler Shifted Atomic Spectra
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Why don’t we see the color of everyday objects
change as they move?
We've used spectra to find planets around other stars!
(Ch. 4)
• Star wobbling causes Doppler shift of its absorption lines.
• Only gives information about velocity along line of sight!
```