Download 0506 - Astronomy

Light (Electromagnetic
Radiation) & Its Nature
Light (Electromagnetic Radiation) &
Its Nature
Light:
 also referred to as electromagnetic
radiation (EM radiation)
 form of energy that transverses
through space
 is the source of information about
the Universe
It’s a
Particle
It’s a
Wave
Light has a Wave Like Nature
Longitudal vs. Transverse wave:
http://paws.kettering.edu/~drussell/Dem
os/waves/wavemotion.html
Key Parameters of Light as a Wave
A - Amplitude –
Vertical height of
maxima or depth of
minima of a wave
proportional to intensity
/brightness of light
λ - Wavelength –
Distance between two
adjacent maxima
f - Frequency –
Number of maxima that
pass a certain point in a
second
Remember, light is a form of energy (E)
λ  f  E
λ  f  E
Light travels as a transverse wave
Transverse wave –
direction of vibration is
perpendicular to its
direction of travel.
Longitudinal Wave –
direction of vibration is
the same as its direction
of travel
No Light
Light has a Particle-Like Nature
Photoelectric effect
• Ejection of electrons from metal
surfaces by photon impact
• Minimum photon energy
(frequency) needed to
overcome electron binding PE
• Additional photon energy goes
into KE of ejected electron
• Intensity of light related to
number of photons, not energy
• Application: photovoltaic cells
Light is a stream of particles, called photons
E=hf
So Which is It?
Certain properties of light are best described by
thinking of it as a wave, while others are best
described by thinking of it as a stream of particles.
Both waves and particles transit energy through
space from one part of the universe to the next
Light interacts with matter
Interaction begins at surface and
depends on
Smoothness of surface
Nature of the material
Angle of incidence
Possible interactions
Reflection
Refraction
Absorption
Transmission
Transparent materials transmit light
Opaque materials do not allow
transmission of light
(reflect, absorb or combination)
The Electromagnetic Spectrum
Isaac Newton
colors seen
in a spider
web are
partially
due to
dispersion
The Electromagnetic Spectrum
Higher energy
Lower Energy
-15
10
m
gamma rays
X-rays
Ultra-violet
Visible
Infrared
Microwave
Radio waves
1000 km
Many wavelengths of
light outside of visible
Sound you’re hearing
represents....
}
.... 4-5 octaves above visible X-rays
}
}
}
... octave ...
of entire
“visible”
light
spectrum
....several
morebelow
octaves
below -... one octave
“visible”
microwave
radiation
infrared radiation
adopted from Prof. David Helfand at Columbia University
-15
10
m
gamma rays
X-rays
Ultra-violet
Visible
Infrared
Microwave
Radio waves
1000 km
•
•
Many wavelengths of
light outside of visible
Astronomers must
consider the full
EM spectrum
All information in Astronomy comes from collecting light using instruments
called telescopes
Location of Telescope Installations ?
Location of Telescope Installations ?
different wavelengths...
different considerations
The 100 inch (2.5 m)
Hooker telescope at Mount
Wilson Observatory near Los
Angeles, California.
Making use of
EM radiation
Reflected and Emitted Light
The Andromeda Galaxy at different Wavelengths:
Images from the Spitzer and Chandra space telescope web sites
Thermal “blackbody” radiation
•
The energy emitted per second by an object at different wavelengths
is called its spectrum
• An object emits a
thermal radiation spectrum due to its temperature
The temperature of an object determines what type of EM (light) it will emit.
Temperature: the quantity that tells how warm or cold an object is with respect
to some standard. It is a measure of the average kinetic energy of the
molecules or atoms in an object.
scales: Celsius (°C), Fahrenheit (°F), or Kelvin (K)
Comparison of the fahrenheit, celsius, and kelvin scales Credit: NASA
Converting between
F, C, K
T(°F) = 9/5 T(°C)+32°
T(°C) = 5/9 (T(°F)-32°)
T(°K)=T(°C)+273.15
Thermal “blackbody” radiation
•
The energy emitted per second by an object at different wavelengths
is called its spectrum
• An object emits a
thermal radiation spectrum due to its temperature
Hotter object (shorter λ) brighter
Cooler object (longer λ) dimmer
every object emits radiation that depends on its temperature:
 Cooler objects are redder than hotter objects
 Cooler objects are dimmer than hotter objects
Thermal Black Body Radiation:
As temperature increases, the glow color changes
from red to yellow to white to blue.
The temperature of a lava can
be estimated by observing its
color: lava flows at about 1,000
to 1,200 °C.
Thermal Black Body Radiation:
Black-body laws can be applied
to human beings. For example,
some of a person's energy is
radiated away in the form of
electromagnetic radiation, most
of which is infrared
Infrared Picture
What are we looking at?
Why does it appear this way?
Same picture, no humans.
Why is the spot in the middle brighter?
Thermal “blackbody” radiation
•
The energy emitted per second by an object at different wavelengths
is called its spectrum
• An object emits a
thermal radiation spectrum due to its temperature
Wein’s Law:
every object emits radiation that depends on its temperature:
 Cooler objects are redder than hotter objects
 Cooler objects are dimmer than hotter objects
Light transverses
electromagnetic energy
through space at
c = 3.0 108m/s
How long does it take light to travel one meter?
3.3 ns of “look-back” time
On the Moon
Time-delay
Light (Electromagnetic
Radiation) & Its Nature
Key Concepts for Week-3, Class-1:
(what You need to know, as You will be tested on this material):
 Dual nature of light: wave-like nature (double-slit experiment) & particle-like
nature (photoelectric effect experiment)
 Connection between wavelength, frequency and energy
 Distinction between transverse & longitudinal wave
 Phenomena: reflection, refraction, absorption, and transmission
 The span of EM radiation: radio-waves, microwaves, infrared light, visible
light, ultra-violet light, X-rays, gamma rays
 Thermal “blackbody-radiation” spectrum
 Temperature and its units (Fahrenheit, Celsius, Kelvin)
 Concept of look-back time
 Light year (ly) as a measure of distance
Visible Objects in the
Universe
The Hubble Ultra Deep Field
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.
Hubble Space Telescope
2.4m optical telescope
resides in orbit of Earth
The Hubble Ultra Deep Field
What objects do we see here?
Objects in a Visible Universe
The Universe is defined as the summation of all particles and energy that exist
and the space-time in/during which all events occur.
• Planets: an object that orbits a star, is large enough to have settled into
a round shape and dominates its orbital zone;
What is a Planet?
Conventional (past) definition: Planet is a body that orbits a
star, shines by reflecting the star’s light and is larger than an
asteroid.
What observation ignited the debate about the definition of
a planet?
 Observation of the vast population of objects in the
vicinity of Pluto (Kuiper Belt Objects = KBO);
 In particular, KBO Eris is larger than Pluto;
 If Pluto is a planet, not only Eris but also dozen of other
KBO objects will need to be considered a planet.
Dynamical effect presents a feature of clear
distinction between planets and other bodies
Key Feature: Planet is a body massive enough to dominate
its orbital zone by a) flinging smaller bodies away , b)
sweeping them up in direct collisions, or c) holding them in
stable orbits
Another way of stating the definition: a
body in the solar system that is more
massive than the total mass of all of the
other bodies in a similar orbit.
Proxy is µ= M(planet)/M(objects)
Earth is a pretty big rocky planet....
but not very big as planets go...
Objects in a Visible Universe
The Universe is defined as the summation of all particles and energy that exist
and the space-time in/during which all events occur.
• Planets: an object that orbits a star, is large enough to have settled into
a round shape and dominates its orbital zone;
• Stars: massive gaseous body in outer space, just like the Sun.
Unlike a planet, a star generates energy through nuclear fusion
and emits visible light;
Stars are in a dynamic balance between gravity and pressure
stars are point sources
cross-like spikes in image (diffraction spikes)
caused by strong + concentrated light
A sample of stars
stars
~ 109m
Super Nova: explosion of the star
One of the most energetic explosive
events known is a supernova. These
occur at the end of a star's lifetime,
when its nuclear fuel is exhausted and
it is no longer supported by the release
of nuclear energy.
and tiny compared to ordinary stars,
even smaller when compared to giant stars,
and invisible compared to supergiants
planets
~ 107m
Planet sizes are to scale,
but distance is not
http://www.youtube.com/watch?v=HEheh1BH34Q
Objects in a Visible Universe
The Universe is defined as the summation of all particles and energy that exist
and the space-time in/during which all events occur.
• Planets: an object that orbits a star, is large enough to have settled into
a round shape and dominates its orbital zone;
• Stars: massive gaseous body in outer space, just like the Sun.
Unlike a planet, a star generates energy through
nuclear fusion and therefore emits light;
http://janus.astro.umd.edu/SolarSystems/
Objects in a Visible Universe
The Universe is defined as the summation of all particles and energy that exist
and the space-time in/during which all events occur.
• Planets: an object that orbits a star, is large enough to have settled into
a round shape and dominates its orbital zone;
• Stars: massive gaseous body in outer space, just like the Sun.
Unlike a planet, a star generates energy through
nuclear fusion and therefore emits light;
http://janus.astro.umd.edu/SolarSystems/
• Galaxies: a large aggregate of stars (as well as other
materials such as gas, dust, and dark matter), held in
association by their mutual gravity, and relatively isolated from
other such aggregates.
Usually grouped into three main
types:
http://www.seasky.org/celestial-objects/stars.html
Spiral, Elliptical, and Irregular.
A sample of galaxies
galaxies
~ 1021m
Spiral galaxy
like our galaxy the Milky Way....
A sample of galaxies
Andromeda
speeding toward us at 500,000 km/sec!
will arrive in 4 billion years!
A sample of galaxies
Elliptical galaxy
Irregular galaxy
Group Activity
we are here
our cosmic address
The Hubble Ultra Deep Field
Describe what you see.
What are some of the interesting features?
The Hubble Ultra Deep Field
Look at the objects
Think about the time it took for “info” to arrive
Think about their colors; What can you tell about their temperature?
The Hubble Ultra Deep Field
Look at the objects
Think about the time it took for “info” to arrive
Think about their colors; What can you tell about their temperature?
Which way did the Hubble Space
Telescope point when taking the
Hubble Ultra Deep Field?
The Hubble Ultra Deep Field
Estimate how many galaxies are in this image.
The Hubble Ultra Deep Field
How many galaxies are there in the
visible Universe?
How can we use this image to figure out the
number of galaxies in the Universe?
The Hubble Ultra Deep Field
Assuming there are 100 billion galaxies in the
visible universe, what fraction of the sky is
covered by the HUDF image?
The Hubble Ultra Deep Field
How many planets are there in the visible Universe?
Is this really the only planet in the
only solar system
in the only galaxy that’s
comfortable for life?
http://www.youtube.com/watch?v=wJXSSYyIVqw&feature=related
67
The Hubble Ultra Deep Field
How do you read time in this image?
The Hubble Ultra Deep Field
13.7 billion years in one image
Objects in a Visible Universe
• Planets
• Stars
• Galaxies
only ~ 4% ordinary matter !
We are still in mostly “in the dark”…
What evidence do we have for dark matter?
present at ~ 23 %
What evidence do we have for dark energy?
present at ~ 73 %
STUDENTS:
NEXT WEEK PLEASE BRING LAPTOPS
(1 OR 2 PER GROUP)
& PRIOR TO COMING TO CLASS, UPLOAD THE
FOLLOWING WEBSITE INTO THE “CASH” MEMORY
http://burro.cwru.edu/JavaLab/GalCrashWeb/main.html
Light (Electromagnetic
Radiation) & Its Nature
Key Concepts for Week-3, Class-2:
(what You need to know, as You will be tested on this material):
 Definitions:
 Planets
 Stars
 Galaxies
 nuclear fusion reactions within stars