Transcript Properties of Light

Properties of Light
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.
The Electromagnetic Spectrum
Atmospheric Windows
Not all light from space makes it through the earth’s 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.
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.
Scattering of Light
Dust in the Earth’s atmosphere (or in space) can scatter light. In
general, short wavelength (blue) light gets scattered more than red
light. That’s why the sky is blue.
Scattering of Light
Dust in the Earth’s atmosphere (or in space) can scatter light. In
general, short wavelength (blue) light gets scattered more than red
light. That’s why the Sun is red at sunset.
The long path through the atmosphere means all the blue photons
are scattered away.
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.
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 object’s temperature.
For hotter objects:
• Peak intensity shifts to
shorter (bluer)
wavelengths
peak  1/T
• more light is created
Total energy  T4
1st Type of Radiation: Blackbody (or Thermal)
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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
We are all shining!
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.
+
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.
+
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 = E2-E1
+
E1
E2
E3
2nd Type of Radiation: Emission Line
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2nd Type of Radiation: Emission Line
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2nd Type of Radiation: Emission Line
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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!
Blackbody Spectra
Emission Line Spectra
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
atom’s energy levels. The result is an absorption spectrum.
Absorption Line Spectra
+
E1
E2-E1
E2
E3
Absorption Line Spectra
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Absorption Line Spectra from the Sun
Why does the Sun produce absorption lines?
(1 Million K)
?
A
B
C
?
A
B
C
?
A
B
C
Continuous, Emission, and Absorption Spectra
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.
X-rays
optical
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.
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
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
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 Ephoton = E3-E2
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
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.
Emitted photons:
Lower energies
Longer wavelengths
Lower frequencies
Ephoton = E3-E2
Ephoton = E2-E1
Ephoton = E3-E1
+
E1
E2
E3
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.
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.
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
can’t 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
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