Light and Other Forms of Radiation - PowerPoint

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The Electromagnetic
Spectrum, Light,
Astronomical Tools
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Light and Other Forms
The Electromagnetic Spectrum

In astronomy, we cannot perform experiments
with our objects (stars, galaxies, …).

The only way to investigate them is by analyzing
the light (and other radiation) which we observe
from them.
Light as a Wave                  0



c = 300,000 km/s
= 3*108 m/s

•   Light waves are characterized by a
wavelength and a frequency f.
•   f and  are related through

f = c/
Wavelengths and Colors                       0

Different colors of visible light correspond
to different wavelengths.
Dark Side of the Moon

• “There is no dark side really. It’s all dark.” -- Pink Floyd
Dark Side of the Moon
• What is wrong
with this picture?
• Front: Not all
primary colors
(eg, pink,
Front cover        Back cover
magenta), also
refraction angles
inconsistent
• Back: Spectrum
is Convergent – I
think done for
More accurate, from Richard Berg     art’s sake
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Light as a Wave
• Wavelengths of light are measured in units
of nanometers (nm) or angstrom (Å):

1 nm = 10-9 m
1 Å = 10-10 m = 0.1 nm

Visible light has wavelengths
between 4000 Å and 7000 Å
(= 400 – 700 nm).
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The Electromagnetic Spectrum

Wavelength

Frequency

High
Need satellites   flying air
to observe        planes or
satellites
Light as Particles                      0

• Light can also appear as particles, called
photons (explains, e.g., photoelectric effect).
• A photon has a specific energy E,
proportional to the frequency f:

E = h*f
h = 6.626x10-34 J*s
is the Planck constant.

The energy of a photon does not
depend on the intensity of the light!!!
Why is energy per photon so important?
• Real life example: Ultra-Violet light hitting
– Threshold for chemical damage set by energy
(wavelength) of photons
• Below threshold (long wavelengths) energy too
weak to cause chemical changes
• Above threshold (short wavelength) energy
photons can break apart DNA molecules
– Number of molecules damaged = number of
photons above threshold
– Very unlikely two photons can hit exactly
together to cause damage
Temperature and Heat
• Thermal energy is “kinetic energy” of
moving atoms and molecules
– Hot material energy has more energy
available which can be used for
•   Chemical reactions
•   Nuclear reactions (at very high temperature)
•   Escape of gasses from planetary atmospheres
•   Creation of light
– Collision bumps electron up to higher energy orbit
– It emits extra energy as light when it drops back down
to lower energy orbit
– (Reverse can happen in absorption of light)
Temperature Scales
• Want temperature scale with energy proportional to T
– Celsius scale is “arbitrary” (Fahrenheit even more so)
• 0o C = freezing point of water
• 100o C = boiling point of water
– By experiment, available energy = 0 at “Absolute Zero” = –
273oC (-459.7oF)
– Define “Kelvin” scale with same step size as Celsius, but 0K = -
273oC = Absolute Zero

• Use Kelvin Scale for most astronomy work
– Available energy is proportional to T, making equations simple
(really! OK, simpler)
– 273K = freezing point of water
– 373K = boiling point of water
– 300K approximately room temperature
• Hot objects glow (emit light) as seen in PREDATOR, SSC Video, etc.
– Heat (and collisions) in material causes electrons to jump to high energy
orbits, and as electrons drop back down, some of energy is emitted as
light.

• Reason for name “Black Body Radiation”
– In a “solid” body the close packing of the atoms means than the electron
orbits are complicated, and virtually all energy orbits are allowed. So all
wavelengths of light can be emitted or absorbed. A black material is one
which readily absorbs all wavelengths of light. These turn out to be the
same materials which also readily emit all wavelengths when hot.

• The hotter the material the more energy it emits as light
– As you heat up a filament or branding iron, it glows brighter and brighter

• The hotter the material the more readily it emits high energy (blue)
photons
– As you heat up a filament or branding iron, it first glows dull red, then
bright red, then orange, then if you continue, yellow, and eventually blue
Planck and other Formulae
• Planck formula gives intensity of
light at each wavelength
– It is complicated. We’ll use two
simpler formulae which can be
derived from it.

• Wien’s law tells us what wavelength

 3, nmK
has maximum intensity
000 ,
K
3000
, 000 m
 Max
T   T

• Stefan-Boltzmann law tells us total

 

E4    2 K
 10 4
-8
T where
5.67
J/(m
s)

From our text: Horizons, by Seeds
Example of Wien’s law
• What is wavelength at which you
glow?
– Room T = 300 K so

3 3 10
, m

K
000 m
T

,
000
Max
K

K
300

m

– This wavelength is about 20 times
longer than what your eye can see.
Thermal camera operates at 7-14 μm.

• What is temperature of the sun –
which has maximum intensity at
roughly 0.5 m?
,
3m K000
,
000 m
3 K             

T   
0.m
Max5
K
000
,
6
    
From our text: Horizons, by Seeds
Kirchoff’s laws

• Hot solids emit continuous spectra

• Hot gasses try to do this, but can only
emit discrete wavelengths

• Cold gasses try to absorb these same
discrete wavelengths
Atoms – Electron Configuration
•   Molecules: Multiple atoms sharing/exchanging electrons (H 2O, CH4)

•   Ions:                   Single atoms where one or more electrons have
escaped (H+)

•   Binding energy: Energy needed to let electron escape

•   Permitted “orbits” or energy levels
–   From quantum mechanics, only certain “orbits” are allowed         From our text: Horizons, by Seeds
–   Ground State: Atom with electron in lowest energy orbit
–   Excited State: Atom with at least one atom in a higher energy orbit
–   Transition:         As electron jumps from one energy level orbit to
another,
atom must release/absorb energy different, usually as
light.

•   Because only certain orbits are allowed, only certain energy jumps are
allowed, and atoms can absorb or emit only certain energies
(wavelengths) of light.

•   In complicated molecules or “solids” many transitions are allowed
Hydrogen Lines
•   Energy absorbed/emitted depends on upper and lower levels
•   Higher energy levels are close together
•   Above a certain energy, electron can escape (ionization)

•   Series of lines named for bottom level
– To get absorption, lower level must be occupied
• Depends upon temperature of atoms
– To get emission, upper level must be occupied
• Can get down-ward cascade through many levels

n=3
n=2

From our text: Horizons, by Seeds
n=1
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Astronomical Telescopes
Often very large to gather
large amounts of light.

In order to observe
than visible light, very
different telescope
designs are needed.

The northern Gemini Telescope on Hawaii
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Refracting / Reflecting Telescopes
Refracting
Telescope:
Lens focuses
light onto the
focal plane
Focal length

Reflecting
Telescope:
Concave Mirror
focuses light
onto the focal
Focal length
plane
Almost all modern telescopes are reflecting telescopes.
Secondary Optics               0

In reflecting
telescopes:
Secondary
mirror, to re-
direct light path
towards back or
side of
incoming light
path.

Eyepiece: To
view and
enlarge the
small image
produced in
the focal
plane of the
primary
optics.

Refracting Telescopes
• Chromatic aberration:
Different wavelengths are
focused at different focal
lengths (prism effect).
Can be corrected, but not
eliminated by second lens
out of different material.

• Difficult and expensive
to produce: All surfaces
must be perfectly
shaped; glass must be
flawless; lens can only be
supported at the edges.
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The Powers of a Telescope:
Size does matter!

1. Light-gathering
power: Depends on the
surface area A of the
primary lens / mirror,   D
proportional to
diameter squared:

A =  (D/2)2
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The Powers of a Telescope (II)
2. Resolving power: Wave nature of light
=> The telescope aperture produces
fringe rings that set a limit to the
resolution of the telescope.
Astronomers can’t eliminate these
diffraction fringes, but the larger a
telescope is in diameter, the smaller the
diffraction fringes are. Thus the larger
the telescope, the better its resolving
power.
min = 1.22 (/D)
For optical wavelengths, this gives
min

min = 11.6 arcsec / D[cm]
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Seeing

Weather
conditions and
turbulence in the
atmosphere set
further limits to
the quality of
astronomical
images

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The Powers of a Telescope (III)
3. Magnifying Power = ability of the telescope
to make the image appear bigger.

A larger magnification does not improve the
resolving power of the telescope!
The Best Location for a Telescope                   0

Far away from civilization – to avoid light pollution
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The Best Location for a Telescope (II)

Paranal Observatory (ESO), Chile
http://en.wikipedia.org/wiki/Paranal_Observatory

On high mountain-tops – to avoid atmospheric
turbulence ( seeing) and other weather effects
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Secondary mirror

mirror: sturdy, heavy
to avoid distortions.
0

Telescopes (II)

The 4-m
Mayall
Telescope at
Kitt Peak
National
Observatory
(Arizona)
Advances in Modern Telescope Design                      0

Lighter mirrors with lighter support structures,
to be controlled dynamically by computers

Floppy mirror

Segmented mirror
0
Computer-controlled mirror support adjusts the mirror
surface (many times per second) to compensate for
distortions by atmospheric turbulence
Examples of Modern                            0

Telescope Design

The Very Large Telescope (VLT)

8.1-m mirror of the Gemini Telescopes
Interferometry                         0

Recall: Resolving power of a telescope depends on diameter D.

 Combine the signals from
several smaller telescopes to
simulate one big mirror 
Interferometry
CCD Imaging                             0

CCD = Charge-coupled device

• More sensitive than
photographic plates

• Data can be read directly
into computer memory,
allowing easy electronic
manipulations

False-color image to visualize
brightness contours
The Spectrograph                        0

Using a prism (or a grating), light can
be split up into different wavelengths
(colors!) to produce a spectrum.
Spectral lines in a
chemical composition and
other properties of the
observed object

Recall: Radio waves of  ~ 1 cm – 1 m also penetrate the
Earth’s atmosphere and can be observed from the ground.

Large dish focuses
waves onto a small

Amplified signals are
stored in computers
and converted into
images, spectra, etc.

Just as for optical
telescopes, the
resolving power of a
depends on the
diameter of the objective
lens or mirror min =
1.22 /D.
this is a big problem:
longer than visible light
The Very Large Array (VLA): 27
dishes are combined to simulate a
 Use interferometry to
large dish of 36 km in diameter.
improve resolution!
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The 100-m Green Bank
Telescope in Green Bank, West   The 300-m telescope in
Virginia.             Arecibo, Puerto Rico
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not visible at other wavelengths:

• Neutral hydrogen clouds (which don’t
emit any visible light), containing ~ 90 %
of all the atoms in the universe.

• Molecules (often located in dense
clouds, where visible light is
completely absorbed).

• Radio waves penetrate gas and
dust clouds, so we can observe
regions from which visible light is
heavily absorbed.
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Infrared Astronomy
Most infrared radiation is absorbed in the lower atmosphere.

However, from
high mountain
tops or high-
flying aircraft,
some infrared
still be
observed.

NASA infrared telescope on Mauna Kea, Hawaii
Infrared Telescopes

Spitzer Space Telescope

WIRO 2.3m
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Ultraviolet Astronomy
• Ultraviolet radiation with  < 290 nm is
completely absorbed in the ozone layer
of the atmosphere.
• Ultraviolet astronomy has to be done
from satellites.
• Several successful ultraviolet astronomy
satellites: IUE, EUVE, FUSE
• Ultraviolet radiation traces hot (tens of
thousands of degrees), moderately
ionized gas in the universe.
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NASA’s Great Observatories in Space (I)
The Hubble Space Telescope    • Launched in 1990;
maintained and
space shuttle service
missions throughout the
1990s and early 2000’s

• Avoids turbulence
in Earth’s
atmosphere

• Extends imaging
and spectroscopy to
(invisible) infrared
and ultraviolet
0

Hubble Space Telescope Images

Mars with its
polar ice cap

A dust-filled galaxy
Nebula around
an aging star
NASA’s Great Observatories in Space (II)     0

The Compton Gamma-Ray Observatory

Operated from
1991 to 2000

Observation of
high-energy
gamma-ray
emission, tracing
the most violent
processes in the
universe.
NASA’s Great Observatories in Space                       0

(III)
The Chandra X-ray Telescope
Launched in 1999 into a highly
eccentric orbit that takes it 1/3
of the way to the moon!
X-rays trace hot (million
degrees), highly ionized
gas in the universe.

Two colliding
Very hot gas
galaxies,
in a cluster
triggering a
of galaxies
burst of star
formation
Saturn
Chandra X-ray Observatory

Shuttle launched, highly eccentric orbit.
Grazing incidence mirrors – nested hyperboloids and paraboloids.
The Highest Tech Mirrors
Ever!

• Chandra is the first X-ray telescope to
have image as sharp as optical
NASA’s Great Observatories in Space (IV)         0

The Spitzer Space Telescope

Launched in 2003

Infrared light traces warm
dust in the universe.

The detector needs to be
cooled to -273 oC (-459 oF).
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Spitzer Space Telescope Images

A Comet
Warm dust in a
young spiral galaxy

Newborn stars that
would be hidden
from our view in
visible light
Spitzer Space Telescope

• Discovered by a Wyoming grad student and
professor. The “Cowboy Cluster” – an overlooked
Globular Cluster.
Kepler’s Supernova with all
three of NASA’s Great
Observatories
• Just 400 years ago:
(Oct. 9, 1604)
• Then a bright, naked eye
object (no telescopes)
• It’s still blowing up – now
14 light years wide and
expanding at 4 million
mph.
• There’s material there at
MANY temperatures, so
many wavelengths are
needed to understand it.
A Multiwavelength Look at Cygnus A

• A merger-product, and powerful radio galaxy.
The Future of Space-Based          0

Optical/Infrared Astronomy:

The James Webb Space Telescope
Terrestrial Planet Finder

A new VLT image of a
possible planet around
a brown dwarf star.

My Bet: Renamed after Carl Sagan. Will use both
interferometry and coronagraphs to image Earth-like planets.

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