Outline for Today

1
Properties of Light
2
Light as a Wave

• Light (or electromagnetic radiation), can be
  thought of as either a particle or a wave. As a
  wave, light has
• a wavelength, ? (distance between waves)
• a frequency, ? (number of waves passing you each
  second)
• a speed, c ? ? (this is always the same
  300,000 km/s)
• an energy, E h ? (where h is just a constant)
• Note that because the speed of light is a
  constant, ?, ?, and E are linked if you know
  one, you know the other two.

3
The Electromagnetic Spectrum
4
Atmospheric Windows

• Not all light from space makes it through the
  earths atmosphere. In fact, only visible light,
  radio waves, and some infrared light makes it to
  the ground. The rest of the electromagnetic
  spectrum can only be observed from space.

5
The Doppler Shift

• The wavelength emitted by an object is not always
  the wavelength you observe. If you are moving
  towards an object, you will see more waves per
  second (i.e., a higher frequency, like swimming
  upstream). The light will appear bluer and be
  blueshifted. Conversely, if you are moving away
  from an object, its light will be redshifted.

????????????v
?????????
? c
The faster the relative motion, the larger the
red or blue shift.
6
Scattering of Light

• Dust in the Earths atmosphere (or in space) can
  scatter light. In general, short wavelength
  (blue) light gets scattered more than red light.
  Thats why the sky is blue.

7
Scattering of Light
Dust in the Earths atmosphere (or in space) can
scatter light. In general, short wavelength
(blue) light gets scattered more than red light.
Thats why the Sun is red at sunset.
The long path through the atmosphere means all
the blue photons are scattered away.
8
Scattering of Light
In interstellar nebulae, stars behind large piles
of dust will be reddened. Other parts will
appear blue, due to the scattering by dust.
This is just like the daytime sky.
9
1st Type of Radiation Blackbody (or Thermal)

• Anything that is hot (i.e., above absolute zero)
  produces light at all wavelengths a continuous
  spectrum. But the amount of light given off at
  each wavelength is very sensitive to the objects
  temperature.

• For hotter objects
• Peak intensity shifts to shorter (bluer)
  wavelengths
• ?peak ? 1/T
• more light is created
• Total energy ? T4

10
1st Type of Radiation Blackbody (or Thermal)
11
Our bodies are a lot cooler than the Sun. So at
which wavelength range do our bodies produce most
of their light? A) Gamma ray B) Visible C)
Infrared D) Ultra-violet E) X-ray
12
We are all shining!
13
2nd Type of Radiation Emission Line

• In the Bohr model of the atom, the nucleus
  contains protons and neutrons. Circling around
  the nucleus (in orbitals) are electrons. Since
  electrons are attracted to the protons, they
  normally orbit in their lowest energy state
  (i.e., closest to the nucleus), called the ground
  state.

Important electrons can only orbit at very
specific distances from the nucleus, and these
distances are different for different elements.

14
2nd Type of Radiation Emission Line

• Suppose something collided into an electron
  orbiting in the lowest energy level. Some of the
  energy of the collision could kick the electron
  up to a higher level, or an excited state.

15
2nd Type of Radiation Emission Line

• Suppose something collided into an electron
  orbiting in the lowest energy level. Some of the
  energy of the collision could kick the electron
  up to a higher level, or an excited state.

Eventually, when the electron falls back down, it
has to give this energy back. It does so by
giving off a photon of light.
Ephoton E2-E1
Since each orbital has a very specific level,
electron transitions between the orbitals emit
very specific amounts of energy. The spectrum
from this process would not be continuous.

E1
E2
E3
16
2nd Type of Radiation Emission Line
17
2nd Type of Radiation Emission Line
18
2nd Type of Radiation Emission Line
19
Emission Line Spectra

• Since every element has a different set of atomic
  orbital energies, the emission line spectrum of
  every element is different. They are as unique
  as fingerprints!

20
Blackbody Spectra
21
Emission Line Spectra
22
Absorption Line Spectra

• An object (like a star) emits a hot blackbody
  spectrum. Somewhere between you and the star
  (like on the outside of the star) is some cooler
  gas. That gas can absorb the photons which
  correspond to the atoms energy levels. The
  result is an absorption spectrum.

23
Absorption Line Spectra

E1
E2
E3
E2-E1
24
Absorption Line Spectra
25
Absorption Line Spectra from the Sun
26
Why does the Sun produce absorption lines?
(1 Million K)
27
?
C
B
A
28
?
C
B
A
29
?
C
B
A
30
Continuous, Emission, and Absorption Spectra
31
Imagine yourself as an astronaut orbiting the
Earth. You look out the window of your vehicle
and see a flare on the surface of the Sun. You
know that solar flares emit light at all
wavelengths. How long until your ship is hit by
the dangerous x-rays and gamma-rays? a) 8
days b) 8 hours c) 8 minutes d) 8 seconds e)
No time at all. You are already getting hit by
the x-rays and gamma-rays.
32
X-rays
optical
33
An electron in an atom absorbs a photon and
jumps from level 1 to level 3. It then falls from
level 3 to level 2 and emits a photon, and then
falls from level 2 to level 1 and emits another
photon. Which of the statements below is
true? a) Each of the 2 emitted photons has a
higher frequency than the absorbed photon. b)
Each of the 2 emitted photons has a higher energy
than the absorbed photon c) Each of the 2
emitted photons has a longer wavelength than the
absorbed photon. d) The combined energy of the 2
emitted photons is less than that of the absorbed
photon. e) The speeds of the two emitted photons
are different from that of the absorbed photon.
34
2nd Type of Radiation Emission Line

• Suppose something collided into an electron
  orbiting in the lowest energy level. Some of the
  energy of the collision could kick the electron
  up to a higher level, or an excited state.

Eventually, when the electron falls back down, it
has to give this energy back. It does so by
giving off a photon of light.
Since each orbital has a very specific level,
electron transitions between the orbitals emit
very specific amounts of energy. The spectrum
from this process would not be continuous.
Ephoton E3-E1

E1
E2
E3
35
2nd Type of Radiation Emission Line

• Suppose something collided into an electron
  orbiting in the lowest energy level. Some of the
  energy of the collision could kick the electron
  up to a higher level, or an excited state.

Eventually, when the electron falls back down, it
has to give this energy back. It does so by
giving off a photon of light.
Since each orbital has a very specific level,
electron transitions between the orbitals emit
very specific amounts of energy. The spectrum
from this process would not be continuous.
Ephoton E3-E1

E1
E2
E3
36
2nd Type of Radiation Emission Line

• Suppose something collided into an electron
  orbiting in the lowest energy level. Some of the
  energy of the collision could kick the electron
  up to a higher level, or an excited state.

Eventually, when the electron falls back down, it
has to give this energy back. It does so by
giving off a photon of light.
Ephoton E3-E2
Since each orbital has a very specific level,
electron transitions between the orbitals emit
very specific amounts of energy. The spectrum
from this process would not be continuous.
Ephoton E3-E1

E1
E2
E3
37
2nd Type of Radiation Emission Line

• Suppose something collided into an electron
  orbiting in the lowest energy level. Some of the
  energy of the collision could kick the electron
  up to a higher level, or an excited state.

Eventually, when the electron falls back down, it
has to give this energy back. It does so by
giving off a photon of light.
Ephoton E3-E2
Emitted photons Lower energies Longer
wavelengths Lower frequencies
Ephoton E3-E1

Ephoton E2-E1
E1
E2
E3
38

• While moving toward the Earth, a spaceship
  emits a photon of infrared radiation toward the
  Earth. Which of the following could happen at the
  surface of the Earth?
• a) The photon is detected at radio wavelengths.
• b) The photon is detected at optical
  wavelengths.
• c) The photon is detected at X-ray wavelengths.
• d) The photon is detected at gamma ray
  wavelengths.

39

• While moving toward the Earth, a spaceship emits
  a photon of infrared radiation toward the Earth.
  Which of the following could happen at the
  surface of the Earth?
• a) The photon is detected at radio wavelengths.
• b) The photon is detected at optical
  wavelengths.
• c) The photon is detected at X-ray wavelengths.
• d) The photon is detected at gamma ray
  wavelengths.

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41
Summary

• Wave characteristics of light
• wavelength (?), frequency (?), speed (c), energy
  (E)
• The electromagnetic spectrum
• light at different wavelengths
• Atmospheric absorption
• Light at some wavelengths is absorbed by the
  atmosphere and cant reach the ground
• Doppler shift
• Light shifts wavelength if the light source is
  moving toward or away from you
• Why is the sky blue?
• The atmosphere scatters blue light more than red
  light

42
Summary

• 1st type of light blackbody (thermal)
• Produces a continuous spectrum
• As an object becomes hotter
• The peak of intensity shifts to shorter (bluer)
  wavelengths ?peak ? 1/T
• Total energy of its emitted light increases ? T4
• 2nd type of light line emission
• As an electron moves between discrete orbitals in
  an atom, light is emitted and absorbed at
  discrete wavelength
• The wavelengths of these lines are different for
  different elements, so they can be used as
  fingerprints in determining what stars are made
  of