Download Chapter 5

Chapter 5:
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 and the best timing instruments
available at the time. His conclusion:
“The speed of light is at least 10 times
faster than the speed of sound.”
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 motion. We
define the speed of light to be
c = 299792458 m/s exactly.
For most purposes, though, we use
3.00 x108 m/s or 300,000 km/s
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
Different wavelengths of
light are created by
things of similar size
Even though light is an
electromagnetic wave, it
sometimes behaves like a particle
E
hc
l
 h
h  6.625  1034 J  s c = speed of light
 = frequency
l = wavelength
When things get very small,
Quantum Mechanics rules
The Rutherford Atom
Early model of the atom. Like a mini-solar system, the
electrons orbit around a tiny but massive nucleus
The Nucleus: Protons, Neutrons and 99.98% of the mass
The Bohr Atom
Neils Bohr
Can’t tell where the electron is, only
the probability of where it might be
The energy associated with
the electron is quantizedStates above
the ground
state are
excited
states
Atoms emit light when an
electron goes from a high
energy state to a low one
The energy of the
emitted photon is
exactly equal to
the difference in
energy between
the two states
hc
E2  E1 
 Ephoton
l
A hot gas will emit specific
wavelengths
Atoms absorb photons when an
electron is “bumped up” to a
higher energy state
If we pass white light through
a “cool” gas we can see
absorption
Most atoms have many
emission lines due to many
different electron energy levels
The Doppler Effect
The light is redshifted (longer wavelength) if the source
is moving away from the observer, blueshifted (shorter
wavelength) if it is moving towards
The Doppler effect can change
a stars spectrum in two ways
If the star is moving
away or towards Earth
the entire spectrum is
shifted
If the star is rotating the
absorption lines are
broadened
The Solar Spectrum has lots of
absorption lines
We know absorption comes from electron
transitions but where does the continuous rainbow
of color come from?
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
As the temperature
increases, the peak
of the blackbody
curve shifts to
shorter (bluer)
wavelengths. The
total intensity also
increases
dramatically as the
temperature
increases.
The Sun is a 5800 Blackbody
It isn’t a perfect
match, but it’s
close
It also has lots of absorption lines due to the gasses
in its’ atmosphere
How bright something appears
depends on how far away it is
Brightness versus
intensity is another
inverse square law
intensity
brightness 
2
4π r
What can be learned from
the light of a star?
•Surface Temperature…Blackbody Spectrum
•Elemental Composition…Emission/Absorption
•Radial Motion…Doppler Effect
•Rotation…Doppler Line Broadening
•Surface Pressure/Density…Pressure Broadening