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Questions of the Day
 What is the electromagnetic spectrum?
 How do energy and wavelength change along
the electromagnetic spectrum?
 What is a thermal spectrum?
 Describe the relationship between temp,
energy and color.
 How do electrons in an atom change their
energy?
 How are emission/ absorption produced: In
the atom? In space?
Astronomers face a challenge that few
other scientists do. The objects that
astronomers study are so staggeringly far
away that traditional experimentation is
impossible. Thus, astronomers rely almost
exclusively on observation to gather data.
An understanding of electromagnetic
radiation and what information it can
convey is therefore essential to the study
of astronomy.
Virtually all we know about
the Universe beyond Earth’s
atmosphere has been gleaned
from the analysis of
electromagnetic radiation
received from objects far
away.
Our understanding
depends completely on
our ability to decipher
the steady stream of
data reaching us from
space.
When most people hear
“astronomy” a picture comes
to mind… like
a total solar eclipse,
the moon
Halley’s Comet
a Hubble image…
It is the study of
spectroscopy – not
photography – that has
unraveled the mysteries
of the universe…
Scientists use
spectroscopy to analyze
light and reveal how
matter absorbs and
emits radiation.
Electromagnetic radiation: Transfer of
energy through space without physical
connection (no medium) through
varying electric and magnetic fields
Example:
Light
The Facts of Light
Light has energy
Emission of light = radiation
Light is both a PARTICLE
and a WAVE
Light as a particle
Light sometimes acts like a
massless particle called a PHOTON
Goes in a straight line unless
something interacts with it
Is discrete: individual photons can
be counted, like ping pong balls or
anything else.
Light also acts like a Wave
Like an ocean wave, waves of light have
a size, energy, direction, etc.
Water
Light
We can mush both views together
• We’ll talk about emission and absorption of
individual photons.
• But, we’ll often assign wave-like qualities to
the photon:
1. Wavelength
2. Frequency
3. Velocity
4. Energy
Wavelength
The distance over which the wave
pattern repeats is the wavelength. (crest
to crest)
“lambda”
l is the symbol for wavelength
Wavelength and Color
The color of light depends upon its
wavelength.
Short wavelengths are
Blue
• Long wavelengths are
Red
•
Measured in
“nanometers” (10-9 m)
or in
“angstroms” (10-10 m)
We see only a tiny range of
possible wavelengths!
Size of
atomic
nucleus!
Size of
Mt.
Everest!
We only think of visible
wavelengths as being “light”, but a
lot of familiar things involve light of
different wavelengths!
Wavelength
Complex colors can
be built from a
superposition of
wavelengths (i.e.
light of different
colors).
PS. A prism
bends light of
different
wavelengths by
different angles,
spreading the
“spectrum”.
A “spectrum” is a map of how much
light is emitted at each wavelength
Graph of brightness versus
wavelength(i.e. color) of
the spectrum
Picture of what you’d see if the light were passed through
a prism or “diffraction grating”
Frequency
The rate at which peaks pass a given
location is the frequency.
Measured in
“Hertz”, which
has units of
1/second. One
crest in one
second is 1 Hertz,
Two crests in one
second is 2 Hertz,
etc.
n (greek “nu”) is the symbol for frequency
The Speed of Light
The speed of light is constant.
The speed of light is closely related to its
wavelength and its frequency.
distance between peaks
c
time between peaks
OR
You always know
the speed of
light…
l
c
 ln
n
…so if you know the
frequency, you can
calculate the
wavelength, or visa
versa
The energy of a photon depends entirely on
its wavelength (or frequency):
Ephoton  n  1l
(Energy is “proportional” to frequency, and “inversely
proportional” to wavelength)
Energy is
greater
when…..
FREQUENCY
WAVELENGTH
Energy and Wavelength
TV: Mostly harmless
(depending on the show)
X-rays: There’s a reason
you have to wear that
lead apron!
Quick Summary:
BLUE
RED
 SHORT Wavelength
 LONG Wavelength
 HIGH Frequency
 LOW Frequency
HIGH ENERGY!
LOW ENERGY!
Quick Show of Hands:
Which has a longer wavelength?
Which has higher energy?
A. (blue)
B. (green)
C. (pink)
Ultraviolet light (UV)
Visible light
Infrared Light (IR)
So, … why do I care?
Most objects in the sky are too far to
go to-- but we can collect their light
and interpret it!
This is the foundation
of astronomy.
Complete
the EM
Review…
Two main ways astronomical
objects radiate light:
1. “Thermal radiation”
–
Important for a broad
spectrum of stars.
2. Movement of Excited
Atoms
–
Important for hot gas in
space. We will talk about
this type later with a lab…
THERMAL
RADIATION
Hot, so it emits light
Anything that has a temperature
above absolute zero emits
photons (light)!
BUT:
Sometimes these photons are at nonvisible wavelengths.
Thermal radiation from people!
Humans emit light in the Infrared.
Think about it: You can only see other people when there is light to reflect off
of them. We do not glow in the dark in visible light, only in the IR!
Rules of thermal radiation
If its hotter, its BLUER.
The atoms in hot things have more energy
per particle.
The photons associated with the higher
energy atoms will also have more energy per
photon bluer.
If its hotter, there’s MORE light emitted per
unit area (higher “surface brightness”).
 BRIGHTER at every wavelength
More energetic atoms can produce photons more
easily.
Stars are different
colors because they
emit different amounts
of radiation at each
wavelength.
Why are there No green
stars?
Our eyes’ color receptors,
called cones, receive 3
signals: red, blue, and
green. When light hits our
eyes, it triggers each type of
cone in a different amount
based on wavelength.
A star that peaks in the
green still emits red and
blue. So our cones
respond to all
wavelengths washing out
the green and causing
the star to appear white.
The thermal radiation distribution:
If its hotter, its BLUER.
If its hotter, there’s MORE of it  BRIGHTER
HIGH ENERGY
LOW ENERGY
An exception to the rules of thermal
radiation…
 Cooler objects can sometimes emit more light
overall.
 Decreasing temp means less light emitted per unit
area. An object can compensate by being
BIGGER.
 Lower “surface brightness”, but larger surface area.
Hot.
Cool.
Same total light emitted
Cool, but big.
Why does thermal radiation
matter for astronomy?
 You can tell a star’s Hot!
temperature just
by measuring its
Cool!
color!!!!
Cooler
stars!
Hotter
stars!
 You can tell the
temperature of stars
in other galaxies as
well!!!!
Thermal radiation can explain much of
this spectrum
Thermal radiation
at 6000K
Thermal radiation
at 225K
This is a spectrum of Mars! The 6000K radiation is
reflected light from the sun. The 225K radiation is
thermal emission from the planet.
Two main ways astronomical
objects radiate light:
1. “Thermal radiation”
–
Important for a broad
spectrum of stars.
2. Movement of Excited
Atoms
–
Important for hot gas in
space. We will talk about
this type later with a lab…
Complete
Thermal
Radiation…
But what is that other stuff?
Emission lines!
Extra light at very
specific wavelengths.
Absorption lines!
Light has been
removed at very
specific wavelengths.
These lines are signatures of
specific atoms and molecules!
 If we can understand which atoms produce
which lines, we can determine the elements
making up what we see in space!
 How’s it work?
 Specific wavelengths correspond to specific
energies.
Ephoton 1l  hl
 Atoms have characteristic energies.
“h” = Planck’s constant, which tells you how much
energy a photon of a given wavelength has
Virtual Spectroscopy Lab
http://jersey.uoregon.edu/vlab/ele
ments/Elements.html
Reminder: Atoms are made up of a
nucleus surrounded by an electron “cloud”
Nucleus
Electron
Cloud
Different electrons
have different
energy
But how do electrons gain or lose energy?
Electrons gain and lose
energy by absorbing or
emitting
a
PHOTON
Energy is conserved: The
photon’s energy equals the
change in the energy of the
electron.
Ugh, I’m so
low energy!!
Whee!
I’m excited!
The amount of energy an electron
can have is NOT ARBITRARY!
 We can represent electrons with
different energy as being in
ENERGY LEVELS, like rungs on a
ladder
 The energy of these levels are
specific to every different
element
ENERGY
Energy Levels
Electrons, like people,
like to live on the
This electron is in its
ground…
lowest possible energy
level, the “ground state”
ENERGY
Energy Levels
Absorbing a photon
makes an electron
climb the ladder.
ENERGY
Energy Levels
Emitting a photon
lets an electron step
down the ladder.
Electrons can jump multiple levels in a
single step if they get enough energy!
All of these
transitions are
possible!
The Origin of Emission & Absorption
Lines.
Photons with
just the right
energy needed
to change an
electron’s
energy level are
responsible for
emission lines
and absorption
lines.
The Origin of Emission & Absorption
Lines.
Absorption lines:
A photon is taken
out of the
spectrum because
the electron of an
intervening atom
absorbed it and
increased its energy
The Origin of Emission & Absorption
Lines.
Emission lines:
A photon is added
to the spectrum
because the
electron of an
excited atom lost
energy and emitted
it
Really high resolution spectrum of the Sun:
lots of absorption lines!
Why absorption?