Chapter 5:
Light
Light!
Measuring the speed of light
Early attempts to measure the speed of light
were done in 1638 by an apprentice of Galileo
Hilltop to hilltop around Padua Italy using
hand lanterns
“The speed of light is at least 10 times
faster than the speed of sound.”
Rømer estimates the speed of light
1676…Olaus Rømer
Eclipses of Io by Jupiter. Found speed to be finite but
very large (didn’t know diameter of Earth’s orbit)
By the 1800’s two Frenchmen
were able to measure the speed
of light with some accuracy
1850…Fizeau & Foucault measure speed to be about
300,000 km/s
The Speed of Light in vacuum
is a fundamental constant of
the universe
The speed of light in vacuum is the same for
all observers everywhere in the universe
regardless of their circumstances. We
define the speed of light to be
c = 299792458 m/s exactly.
For most calculations, though, we can use
3.00 x108 m/s
We can use the speed of light
to define distances
The meter is defined as
1 meter  distance light travels in 1
299792458
seconds
The lightyear is defined as
1ly  distance light travels in one year
The year used in the definition is the sidereal year:
365 days 6 hours 9 minutes and 9.5 seconds
When light travels through
anything other than vacuum it
moves slower
We define the index of
refraction of a material
to be the ratio of the
speed of light in
vacuum to the speed of
light in the material
c
n
v
What is light?
Just as a cork
bobbing in
water creates
waves in the
water, charges
“bobbing” in
space create
electric and
magnetic waves
“Light” is an
Electromagnetic Wave
Basic Properties of Waves
Wavelength = l in meters
Frequency = f in cycles per second or Hertz (Hz)
Speed = v in meters per second
c fl
Each “color” is characterized
by its wavelength
Using c = lf we can see that the frequency of visible
light is in the 1014 Hz range
Visible light is only a very
small part of the
Electromagnetic Spectrum
Watch ClassAction Light & Spectra module EM
Spectrum Module animation
Photon Torpedoes Away!
Light also behaves like a particle: a photon
E
hc
l
 h
h  6.625  1034 J  s c = speed of light
 = frequency
l = wavelength
But didn’t we say that light was a wave?
What’s going on? How can it be both?
To understand how light can be
both a wave and a particle, we
need new physics
Quantum
Mechanics deals
with the very
small like atoms,
molecules,
subatomic
particles and
light particles.
The Bohr Atom
An early model of the atom. Like a mini-solar system, the
electrons orbit around tiny nucleus
The Nucleus: Protons, Neutrons and 99.98% of the mass
The problem is, it
doesn’t work!
According to
electromagnetic
theory, the electron
will radiate away all
its energy and
collapse into the
nucleus in a fraction
of a second.
The Quantum Mechanical Atom
The Heisenberg Uncertainty Principle of quantum mechanics
says you can’t tell where the electron is, only the probability
of where it might be. The electron is “delocalized”.
The energy associated with
the electron is quantized
Light comes from Electron
Transitions
E2  E1 
hc
l
 Ephoton
The energy of the emitted photon is exactly equal to the
difference in energy between the two states
A hot gas will emit specific
wavelengths
An atom can also absorb a
photon if it has the right energy
Most atoms have many
emission lines
Absorption and emission lines
have the same wavelengths
The same states are involved in
emission and absorption. The
only difference between
emission and absorption is the
way the electron is changing.
For emission the electron loses
energy and thus goes from a
high energy state to a low
energy state. For absorption,
the electron gains energy and
so goes from a low energy
state to a high energy state.
Three Views Spectrum Demonstrator in
ClassAction Light & Spectra module
The Solar Spectrum has lots of
absorption lines
All the naturally occurring elements are
present in the Sun
The Doppler Effect
The light is
redshifted if the
source is moving
away from the
observer,
blueshifted if it is
moving towards
 vs 
l1  1  l0
c

+ If the source is moving away from the observer
- If the source is moving towards the observer
What do we mean when we
say something is hot?
On a microscopic scale, temperature is a
measure of how fast things are moving
In astronomy,
we use the
absolute
temperature
scale
Absolute zero is the
temperature at which all
motion stops. Quantum
mechanics says that isn’t
possible so you can never
reach absolute zero
All objects
emit light
according to
their
temperature:
Blackbody
Radiation
Hotter
objects
glow
brighter
and
become
bluer
The
Blackbody
Spectrum
The hotter the object
is, the shorter the
wavelength of
maximum intensity.
The hotter the object
is, the greater the total
intensity of the light it
emits is.
Objects
reach
equilibrium
between
energy
absorbed
and energy
emitted
There is more to the
temperature of a planet
than just sunlight absorbed
and blackbody emission
What can be learned from
the light of a planet or star?
•Surface Temperature…Blackbody Spectrum
•Elemental Composition…Emission/Absorption
•Radial Motion…Doppler Effect
•Rotation…Doppler Line Broadening
•Surface Pressure/Density…Pressure Broadening