The Nature of Light
• 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)




Electric fields
Magnetic fields
Oscillating charges produce electric and magnetic
fields
Famous 4 equations (outside the scope of this course)
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
1
Light as a Wave
•
Wave


•
Diffraction
Interference
Can describe waves in terms of wavelength and
Wavelenth
Crest
frequency
Trough
ISP 205 - Astronomy Gary D. Westfall
Moving at the speed of light
Lecture 6
2
Electromagnetic Radiation
• Light differs from other forms of electromagnetic
•
•
•
•
•
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
3
EM Radiation Spectrum
• The frequency/wavelength varies dramatically
• Most EM radiation cannot penetrate the Earth’s
atmosphere
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
4
View of the Sky with X rays
• If we could “see” with X rays instead of visible
light and we were above the Earth’s atmosphere
the sky would look like:
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
5
Light as a Particle
• Light (and all EM radiation) exists in quantized
units called photons
• A photon carries a specific amount of energy
• High frequency EM radiation has high energy
photons

Gamma rays
• Low frequency EM radiation has low energy
photons

Long-wave radio
• Described by Quantum Mechanics
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
6
Radiation and Temperature
• The temperature
•
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
maxT = 2.9 x 10-3 mK
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
7
Energy Emitted by Stars
• The higher the temperature of an object, the more
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

E = T4 where E is the emitted energy, T is the
temperature, and  is a constant
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
8
Spectroscopy in Astronomy
• EM radiation carries information about the nature
of astronomical object
• Visible light is the most used
• Light can be

Reflected


Refracted


From a mirror
Through a lens
Dispersed

Separated by wavelength


Prism
Spectrometer
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
9
Continuous Spectrum
• When white light (a superposition of light with all
•
•
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)
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
10
Discrete Emission Spectra
• When atoms are heated, they emit light at specific
wavelengths characteristic of those atoms
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
11
Discrete Absorption Spectra
• When white light passes through atoms light is
absorbed at specific wavelengths
• Several elements were first observed in absorption
spectra from the sun
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
12
Probing the Atom
• The electron was discovered by J.J. Thomson in 1897

Related to electricity, lightning
• 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
13
Rutherford’s Model of the Atom
• Rutherford’s results showed that most of the mass
of the atom was concentrated in the nucleus
• Rutherford proposed a model similar to the solar
system with negative electrons orbiting a positive
nucleus
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
14
The Hydrogen Atom
• The simplest atom is the hydrogen atom
• Composed of 1 electron and 1 proton



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
like the Earth orbits the Sun

Heisenberg Uncertainty Principle

We cannot simultaneously know the position and energy of a particle to
arbitrary precision
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
15
Other Atoms
• 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

The neutron and proton have almost the same mass but
the neutron has not charge

Neutron not discovered until 1930 by Chadwick
• The helium atom is much more complicated than
the hydrogen atom because the 2 electrons interact
with each other
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
16
Isotopes
• The chemical properties of atoms are determined by the number
•
•
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

Isotopes
• Hydrogen has 3 naturally occurring isotopes

Hydrogen, 1H


Deuterium, 2H


Stable
Stable
Tritium, 3H

Radioactive
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
17
The Bohr Atom
• Rutherford’s model of the atom had some tragic
flaws
• Orbiting electrons are accelerating and should
radiate energy

Lifetime of the atom should be 10-10 seconds!
• Neils Bohr proposed that the electrons in the
hydrogen atom could only exist in certain
quantized orbits
• Jumping between the orbits required the emission
or absorption of photons of a specific wavelength
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
18
Radiation and Absorption
• Whenever a hydrogen atom changes from one stationary state to another, energy
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, ) given by h = |Ef Ei|

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
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.
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
19
Photon Energies
• Visible light has wavelengths between 400 and
700 nm
• Photons have energy E = hf = hc/
• Photons from visible light then have energies
between

700 nm


E = 6.62 x 10-34 * 3 x 108 / 700 x 10-9 = 2.8 x 10-19 J = 1.8
eV
400 nm

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
20
Three Kinds of Spectra
• We will consider three kinds of spectra

Continuous


Emission


Light bulb or other source
Heated cloud of gas
Absorption

Continuous spectra passing through a cold cloud of gas
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
21
Doppler Shift
• Relative motion affects
•
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
•
•
•
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
22
Red Shift
• Most of the objects in the universe seem to be
moving away from us



Evidence for Big Bang
Red shift
v = c/
• We observe the red shift of specific emission lines
from known atoms

Hydrogen or calcium have distinctive lines and are
almost always present
ISP 205 - Astronomy Gary D. Westfall
Lecture 6
23