Transcript Powerpoint Draft for today

Electromagnetic Radiation
(How we get information about the cosmos)
Examples of electromagnetic radiation?
Light
Infrared
Ultraviolet
Microwaves
AM radio
FM radio
TV signals
Cell phone signals
X-rays
Clicker Question:
Which of the following is not an
electromagnetic wave:
A: radio waves
B: visible light
C: X-rays
D: sound waves
E: gamma-rays
Radiation travels as waves.
Waves carry information and energy.
Properties of a wave
wavelength (l)
crest
amplitude
trough
velocity (v)
l is a distance, so its units are m, cm, or mm, etc.
Also, v = λ ν
Period (T): time between crest (or trough) passages
Frequency (ν): rate of passage of crests (or troughs), n =
(units: Hertz or cycles/sec)
1
T
Radiation travels as Electromagnetic waves.
That is, waves of electric and magnetic fields traveling together.
Examples of objects with magnetic fields:
a magnet
the Earth
Examples of objects with electric fields:
protons
electrons
}
"charged" particles that
make up atoms.
Scottish physicist James Clerk Maxwell showed in 1865
that waves of electric and magnetic fields travel together =>
traveling “electromagnetic” waves.
The speed of all electromagnetic waves is the speed of light.
c = 3 x 10 8 m / s
or c = 3 x 10 5 km / s
light takes 8 minutes
Earth
Sun
c = λν
or, bigger λ means smaller ν
The Electromagnetic Spectrum
1 nm = 10 -9 m , 1 Angstrom = 10 -10 m
c= ln
Refraction of light
All waves bend when they pass through materials of different densities.
When you bend light, bending angle depends on wavelength, or color.
The "Inverse-Square" Law Applies to Radiation
Each square gets 1/4
of the light
Each square gets 1/9
of the light
apparent brightness α
1
D2
α means “is proportional to”. D is the distance
between source and observer.
Why is the Sun yellow? The Black-Body Spectrum
We form a "spectrum" by spreading out radiation according to
its wavelength (e.g. using a prism for light).
What does the spectrum of an astronomical object's radiation look like?
Many objects (e.g. stars) have roughly a "Black-body" spectrum:
• Asymmetric shape
Brightness
• Broad range of wavelengths
or frequencies
• Has a peak
Frequency
also known as the Planck spectrum.
Approximate black-body spectra of stars of different temperature
average star (Sun)
Brightness
cold dust
infrared visible UV
“cool" stars
very hot stars
infrared visible UV
frequency increases,
wavelength decreases
Laws Associated with the Black-body Spectrum
Wien's Law:
λmax energy α
1
T
(wavelength at which most energy is radiated is longer for cooler objects)
Stefan's Law:
Energy radiated per cm2 of area on surface every second α T 4
(T = temperature at surface)
1 cm2
The total energy radiated from entire surface every second is called the
luminosity. Thus
Luminosity = (energy radiated per cm2 per sec) x (area of surface in cm2)
For a sphere, area of surface is 4πR2, where R is the radius.
So
Luminosity α
R2 x T4
Clicker Question:
Compared to ultraviolet radiation, infrared
radiation has greater:
A: energy
B: amplitude
C: frequency
D: wavelength
Clicker Question:
The energy of a photon is proportional to its:
A: period
B: amplitude
C: frequency
D: wavelength
Clicker Question:
Compared to blue light, red light travels:
A: faster
B: slower
C: at the same speed
Clicker Question:
A star much colder than the sun would
appear:
A: red
B: yellow
C: blue
D: smaller
E: larger
Betelgeuse
Rigel
The Doppler Effect
Applies to all waves – not just radiation. The frequency or wavelength of a
wave depends on the relative motion of the source and the observer.
The Doppler Effect
Applies to all kinds of waves, not just radiation.
at rest
velocity v1
velocity v2
velocity v1
velocity v1
velocity v3
you encounter
more wavecrests
per second =>
higher frequency!
fewer wavecrests
per second =>
lower frequency!
Things that waves do
1. Refraction
Waves bend when they pass through material of different densities.
air
water
swimming pool
prism
air
glass
air
2. Diffraction
Waves bend when they go through a narrow gap or around a corner.
Clicker Question:
If a star is moving rapidly towards Earth
then its spectrum will be:
A: the same as if it were at rest
B: shifted to the blue
C: shifted to the red
D: much brighter than if it were at rest
E: much fainter than if it were at rest
Spectroscopy and Atoms
How do we know:
- Physical states of stars, e.g. temperature, density.
- Chemical make-up and ages of stars, galaxies
- Masses and orbits of stars, galaxies, extrasolar planets
- expansion of universe, acceleration of universe.
All rely on taking and understanding spectra: spreading out radiation
by wavelength.
Types of Spectra and Kirchhoff's
(1859) Laws
1. "Continuous" spectrum radiation over a broad range of
wavelengths (light: bright at every
color). Produced by a hot opaque
solid, liquid, or dense gas.
2. "Emission line" spectrum - bright
at specific wavelengths only.
Produced by a transparent hot gas.
3. Continuous spectrum with
"absorption lines": bright over a
broad range of wavelengths with a
few dark lines. Produced by a
transparent cool gas absorbing light
from a continuous spectrum source.
The pattern of lines is a fingerprint of the element (e.g. hydrogen,
neon) in the gas.
For a given element, emission and absorption lines occur at the same
wavelengths.
Sodium
The Particle Nature of Light
On microscopic scales (scale of atoms), light travels as
individual packets of energy, called photons. (Einstein 1905).
c
photon energy is proportional to radiation
frequency:
E  n (or E  1 )
l
example: ultraviolet
photons are more harmful
than visible photons.
The Nature of Atoms
The Bohr model of the Hydrogen atom (1913):
electron
_
_
+
+
proton
"ground state"
an "excited state"
Ground state is the lowest energy state. Atom must gain energy to
move to an excited state. It must absorb a photon or collide with
another atom.
But, only certain energies (or orbits) are allowed:
_
_
_
+
a few energy levels of H atom
The atom can only absorb photons with exactly the right
energy to boost the electron to one of its higher levels.
(photon energy α frequency)
When an atom absorbs a photon, it moves to a higher energy state briefly
When it jumps back to lower energy state, it emits a photon in a random direction
Other elements
Helium
neutron
Carbon
proton
Atoms have equal positive and negative charge. Each element has its own
allowed energy levels and thus its own spectrum. Number of protons
defines element. Isotopes of element have different number of neutrons.
Ionization
Hydrogen
_
+
Energetic UV
Photon
_
Helium
+
Energetic UV
Photon
+
_
"Ion"
Atom
Two atoms colliding can also lead to ionization. The hotter the gas,
the more ionized it gets.
So why do stars have absorption line spectra?
Simple case: let’s say these atoms
can only absorb green photons.
Get dark absorption line at green
part of spectrum.
.
.
.
..
.
. . . .
.
“atmosphere” (thousands
of K) has atoms and ions
with bound electrons
hot (millions of K), dense interior
has blackbody spectrum,
gas fully ionized
Stellar Spectra
Spectra of stars differ mainly due to atmospheric temperature (composition
differences also important).
“hot” star
“cool” star
Why emission lines?
hot cloud of gas
.
.
.
.
.
.
- Collisions excite atoms: an electron moves to a higher energy level
- Then electron drops back to lower level
- Photons at specific frequencies emitted.
We've used spectra to find planets around other stars.
Star wobbling due to gravity of planet causes small Doppler
shift of its absorption lines.
Amount of shift depends on velocity of wobble. Also know period of
wobble. This is enough to constrain the mass and orbit of the planet.
Now more than 300 extrasolar planets known. Here are the first few
discovered.
So why absorption lines?
.
. .
. cloud of gas
.
.
.
. .
.
.
The green photons (say) get absorbed by the atoms. They are emitted again in
random directions. Photons of other wavelengths go through. Get dark
absorption line at green part of spectrum.
Molecules
Two or more atoms joined together.
They occur in atmospheres of cooler stars,
cold clouds of gas, planets.
Examples
H2 = H + H
CO = C + O
CO2 = C + O + O
NH3 = N + H + H + H (ammonia)
CH4 = C + H + H + H + H (methane)
They have
- electron energy levels (like atoms)
- rotational energy levels
- vibrational energy levels
Molecule vibration and rotation
Types of Spectra
1. "Continuous" spectrum - radiation
over a broad range of wavelengths
(light: bright at every color).
2. "Emission line" spectrum - bright at
specific wavelengths only.
3. Continuous spectrum with "absorption
lines": bright over a broad range of
wavelengths with a few dark lines.
Kirchhoff's Laws (1859)
1. A hot, opaque solid, liquid
or dense gas produces a
continuous spectrum.
2. A transparent hot gas
produces an emission line
spectrum.
3. A transparent, cool gas
absorbs wavelengths from a
continuous spectrum,
producing an absorption line
spectrum.