The Nature of Light
Light as a Wave
• Light and other forms of radiation carry
•
information to us from distance astronomical
objects
• Visible light is a subset of a huge spectrum of
electromagnetic radiation
• Maxwell pioneered the theory of electromagnetic
radiation (and light)
Q
Q
Q
Q
Wave
Q
Q
•
Diffraction
Interference
Can describe waves in terms of wavelength and
Wavelenth
frequency
Crest
Electric fields
Magnetic fields
Oscillating charges produce electric and magnetic
fields
Famous 4 equations (outside the scope of this course)
Trough
Moving at the speed of light
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
1
Electromagnetic Radiation
•
•
•
•
Lecture 6
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• The frequency/wavelength varies dramatically
• Most EM radiation cannot penetrate the Earth’s
radiation by its wavelength
Visible light has wavelengths between 400 and
700 nanometers
EM radiation with wavelengths just longer than
visible light is called infrared radiation (heat)
EM radiation with wavelength just shorter than
visible light is called ultraviolet radiation (UV)
Radio waves have long wavelengths (WKAR FM
is 3 meters)
Microwaves have about 3 cm wavelength
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
EM Radiation Spectrum
• Light differs from other forms of electromagnetic
•
ISP 205 - Astronomy Gary D. Westfall
atmosphere
3
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
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View of the Sky with X rays
Light as a Particle
• Light (and all EM radiation) exists in quantized
• If we could “see” with X rays instead of visible
units called photons
• A photon carries a specific amount of energy
• High frequency EM radiation has high energy
photons
light and we were above the Earth’s atmosphere
the sky would look like:
Q
Gamma rays
• Low frequency EM radiation has low energy
photons
Q
Long-wave radio
• Described by Quantum Mechanics
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
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ISP 205 - Astronomy Gary D. Westfall
Radiation and Temperature
6
Energy Emitted by Stars
• The temperature
•
Lecture 6
• The higher the temperature of an object, the more
of an object
determines what
wavelength of
EM radiation it
will emit
The wavelength
of the maximum
energy emission
is given by
Wien’s Law
energy is radiated at all wavelengths
• The higher the temperature, the “bluer” the star
looks
• The total energy radiated is given by the StefanBoltzmann law
Q
E = σT4 where E is the emitted energy, T is the
temperature, and σ is a constant
λmaxT = 2.9 x 10-3 mK
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
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ISP 205 - Astronomy Gary D. Westfall
Lecture 6
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Spectroscopy in Astronomy
Continuous Spectrum
• EM radiation carries information about the nature
• When white light (a superposition of light with all
of astronomical object
• Visible light is the most used
• Light can be
Q
Reflected
X
Q
From a mirror
Refracted
X
Q
•
•
wavelengths) is dispersed with a prism or a spectrometer,
all colors (wavelengths) are visible
Wavelengths shorter than 400 nm are invisible (UV)
Wavlengths longer than 700 nm are invisible (IR)
Through a lens
Dispersed
X
Separated by wavelength
+
+
Prism
Spectrometer
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
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Discrete Emission Spectra
10
• When white light passes through atoms light is
wavelengths characteristic of those atoms
Lecture 6
Lecture 6
Discrete Absorption Spectra
• When atoms are heated, they emit light at specific
ISP 205 - Astronomy Gary D. Westfall
ISP 205 - Astronomy Gary D. Westfall
absorbed at specific wavelengths
• Several elements were first observed in absorption
spectra from the sun
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ISP 205 - Astronomy Gary D. Westfall
Lecture 6
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Probing the Atom
Rutherford’s Model of the Atom
• The electron was discovered by J.J. Thomson in 1897
Q
• Rutherford’s results showed that most of the mass
Related to electricity, lightning
of the atom was concentrated in the nucleus
• Rutherford proposed a model similar to the solar
system with negative electrons orbiting a positive
nucleus
• In 1911, Ernest Rutherford bombarded a thin foil of gold with alpha particles
from naturally occurring radioactive radium
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
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ISP 205 - Astronomy Gary D. Westfall
The Hydrogen Atom
Q
Q
• The next most simple atom is
helium
• A helium atom has 2 neutrons
and 2 protons in its nucleus
with 2 electrons orbiting the
nucleus
Electron has charge -1
Proton has charge +1
Proton is 2000 times heavier
• The electron is bound to the proton in its ground state
• We know now that the electron does not orbit the proton
Q
the hydrogen atom because the 2 electrons interact
with each other
We cannot simultaneously know the position and energy of a particle to
arbitrary precision
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
Neutron not discovered until 1930 by Chadwick
• The helium atom is much more complicated than
Heisenberg Uncertainty Principle
X
The neutron and proton have almost the same mass but
the neutron has not charge
X
like the Earth orbits the Sun
Q
14
Other Atoms
• The simplest atom is the hydrogen atom
• Composed of 1 electron and 1 proton
Q
Lecture 6
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ISP 205 - Astronomy Gary D. Westfall
Lecture 6
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Isotopes
The Bohr Atom
• The chemical properties of atoms are determined by the number
•
•
• Rutherford’s model of the atom had some tragic
protons and the number of electrons
Light nuclei have roughly the same number of neutrons and
protons
Atomic nuclei can have different number of neutrons
Q
flaws
• Orbiting electrons are accelerating and should
radiate energy
Isotopes
Q
• Hydrogen has 3 naturally occurring isotopes
Q
Q
hydrogen atom could only exist in certain
quantized orbits
• Jumping between the orbits required the emission
or absorption of photons of a specific wavelength
Stable
Deuterium, 2H
X
Q
• Neils Bohr proposed that the electrons in the
Hydrogen, 1H
X
Stable
Tritium, 3H
X
Lifetime of the atom should be 10-10 seconds!
Radioactive
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
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ISP 205 - Astronomy Gary D. Westfall
Radiation and Absorption
• Visible light has wavelengths between 400 and
is emitted or absorbed. When that energy takes the form of electromagnetic
radiation then it has a frequency f (or as it is often called, ν) given by hν = |Ef E i|
700 nm
• Photons have energy E = hf = hc/λ
• Photons from visible light then have energies
between
Ef and Ei are the final and initial energies respectively.
• If Ei > Ef, then radiation occurs while if Ef > Ei, then absorption takes place.
• In the diagram the first six levels
•
•
•
ISP 205 - Astronomy Gary D. Westfall
are shown as well as the zero
energy level (n = ∞).
If the transition takes place from
any n to n =1, it is referred to as a
Lyman line.
Transitions to n = 2 are called
Balmer lines and so on.
Four of the Balmer lines are in the
visible range.
Lecture 6
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Photon Energies
• Whenever a hydrogen atom changes from one stationary state to another, energy
Q
Lecture 6
Q
700 nm
X
Q
400 nm
X
19
E = 6.62 x 10-34 * 3 x 108 / 700 x 10-9 = 2.8 x 10-19 J = 1.8
eV
E = 6.62 x 10-34 * 3 x 108 / 400 x 10-9 = 5.0 x 10-19 J = 3.1
eV
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
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Three Kinds of Spectra
Doppler Shift
• We will consider three kinds of spectra
Q
X
Q
Light bulb or other source
•
Emission
X
Q
• Relative motion affects
Continuous
Heated cloud of gas
Absorption
X
Continuous spectra passing through a cold cloud of gas
waves
If a source of waves is
moving toward you, the
frequency is higher and the
wavelength is shorter
• If a source of waves is moving away
•
•
•
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
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Red Shift
• Most of the objects in the universe seem to be
moving away from us
Q
Q
Q
Evidence for Big Bang
Red shift
v = c∆λ/λ
• We observe the red shift of specific emission lines
from known atoms
Q
Hydrogen or calcium have distinctive lines and are
almost always present
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
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from you the frequency is lower and
the wavelength is longer
Toward, shorter wavelength, blue
shift
Away, longer wavelength, red shift
A familiar example is sound
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
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