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Lecture Outline
Light
ELECTROMAGNETIC SPECTRUM
The Speed of Light is Finite
• Light moves at 300,000 km/s in a vacuum.
• First measured by Rømer when observing
Jupiter’s moons.
• Speed is slower in other materials, e.g., glass.
But, what is light?
• In the 17th Century, Isaac Newton argued that
light was composed of little particles while
Christian Huygens suggested that light travels in
the form of waves.
• In the 19th and 20th Centuries Maxwell, Young,
Einstein and others were able to show that Light
behaves both like a particle and a wave
depending on how you observe it.
Thomas Young’s interference experiment
Scottish physicist James Clerk Maxwell showed
mathematically in the 1860s that light must be a
combination of electric and magnetic fields.
Light is an Electromagnetic Wave
• Light is a wave of combined electricity and
magnetism, called an electromagnetic wave.
• Changing electric and magnetic fields create a
self-sustaining electromagnetic wave.
In 1905 Einstein calculated the energy of a
particle of light (photon) and proposed the
photoelectric effect.
Ephoton = hc/hf
h = Planck’s constant
c = speed of light
 = wavelength
f = frequency
photo
n
Which means:
e-
Long  = Low f
= Low E
Short  = High f
= High E
Waves
• Wavelength ():
length between crests.
• Amplitude: height
• Frequency (f): number
of waves that pass by
each second
• Period (P): Time to
complete one cycle
Wavelength and Frequency
© 2007 W.W. Norton & Company, Inc.
21st CENTURY ASTRONOMY, 2/e
11
Wavelength, Frequency, and Speed
• A long wavelength
means low frequency.
• A short wavelength
means high frequency.
• The speed of light, c,
is constant.
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Electromagnetic Spectrum
← Visible Light
There are other kinds of
electromagnetic waves
that we can’t see.
•Radio
•Microwave
•Infrared
•Visible
•Ultraviolet
•X-ray
•Gamma Ray
The Electromagnetic Spectrum
• Visible light is just one small part of the
electromagnetic spectrum:
Gamma rays X-rays UV Visible IR Microwave Radio

increasing
f
decreasing
Where Does Light Come From?
• Light comes from atoms in an excited state.
• Energy is added to it, an electron jumps up to
a higher energy level (or shell)
• When it comes back down out of the excited
state, it emits light.
• Exists first in ground state.
Emission/Absorption
• Different elements emit different
wavelengths (i.e. different colors)
• You can identify an element based on this
spectrum.
Electron Configuration
• Quantum Number = Shell Number = Energy
Level
• Notice in the last column: En = n2E1
Hydrogen’s First Four Principal
Energy Levels
These Do Not Go In Order
Examples
Which of the following would be true
about comparing gamma rays and
radio waves?
A. The radio waves would have a lower energy and would travel
slower than gamma rays.
B. The gamma rays would have a shorter wavelength and a lower
energy than radio waves.
C. The radio waves would have a longer wavelength and travel the
same speed as gamma rays.
D. The gamma rays would have a higher energy and would travel faster
than radio waves.
E. The radio waves would have a shorter wavelength and higher energy
than gamma rays.
On the FM dial, the frequencies run
from 88 – 107 MHz (million cycles
per second).
On the AM dial, the range is 570 –
1600 kHz (thousand cycles per
second).
Which statements are true?
A. FM photons have shorter wavelengths than
AM.
B. FM photons have higher energies than AM.
C. FM photons travel at the same speed as AM.
D. All the above statements are true.
Lecture Tutorial –
Electromagnetic Spectrum of Light (p. 45)
• Work with a partner!
• Read the instructions and questions
carefully.
• Discuss the concepts and your answers with
one another. Take time to understand it
now!!!!
• Come to a consensus answer you both agree
on.
• If you get stuck or are not sure of your
answer, ask another group.
Light and Matter
• Almost all knowledge
of the Universe
beyond Earth comes
from light.
• Light can tell us about
objects in space:
temperature,
composition, speeds,
and more.
Light and Matter
Today we will discuss how light can
give us speed of an object.
• Almost all knowledge
of the Universe
beyond Earth comes
from light.
• Light can tell us about
objects in space:
temperature,
composition, speeds,
and more.
The Doppler Effect
• Definition: “The change in wavelength of
radiation (light) due to the relative motion
between the source and the observer along
the line of sight.”
Astronomers use the Doppler Effect to learn
about the radial (along the line of sight) motions
of stars, and other astronomical objects.
Real Life Examples of Doppler
Effect
• Doppler Radar (for weather)
• Airplane radar system
• Submarine radar system
– Ok, anything with radar
• Radar gun, used by the police
The Doppler Effect
• Definition: “The change in wavelength of
radiation (light) due to the relative motion
between the source and the observer
along the line of sight.”
Doppler Effect
• When something which is giving off light moves
towards or away from you, the wavelength of the
emitted light is changed or shifted
V=0
Doppler Effect
When the source of light is moving away from the
observer the wavelength of the emitted light will
appear to increase. We call this a “redshift”.
Doppler Effect
When the source of light is moving towards the
observer the wavelength of the emitted light will
appear to decrease. We call this a “blueshift”.
The Doppler Effect
Definition: “The change in wavelength of
radiation due to relative motion between the
source and the observer along the line of
sight.”
Doppler Effect
“Along the line of sight” means the Doppler
Effect happens only if the object which is
emitting light is moving towards you or away
from you.
An object moving “side to side” or perpendicular,
relative to your line of sight, will not experience a
Doppler Effect.
Astronomy Application
V=0
The Doppler Effect
1. Light emitted from an object moving towards
you will have its wavelength shortened.
BLUESHIFT
2. Light emitted from an object moving away from
you will have its wavelength lengthened.
REDSHIFT
3. Light emitted from an object moving
perpendicular to your line-of-sight will not
change its wavelength.
Measuring Radial Velocity
The amount of shift of the lines in the spectrum
(measure from where they should be to where they
are) tells us how fast the object is moving along our
line of sight (radial velocity)
If the lines are shifted to smaller wavelengths = blueshift
= moving towards us
If the lines are shifted to larger wavelengths = redshift =
moving away from us
 = redshift or blueshift
change in wavelength
0 = source wavelength
 = vr
0
c
vr = radial velocity of
source
c = speed of light
A bright star is moving toward Earth. If you
were to look at the spectrum of this star, what
would it look like?
A. an absorption spectrum that is redshifted
relative to an unmoving star
B. an emission spectrum that is redshifted relative
to an unmoving star
C. an absorption spectrum that is blueshifted
relative to an unmoving star
D. an emission spectrum that is blueshifted
relative to an unmoving star
Lecture Tutorial –
Doppler Shift Parts I & II (p. 73)
• Work with a partner!
• Read the instructions and questions
carefully.
• Discuss the concepts and your answers with
one another. Take time to understand it
now!!!!
• Come to a consensus answer you both agree
on.
• If you get stuck or are not sure of your
answer, ask another group.