Chapter 5
Electromagnetic Radiation (Light)
Radiation is also known as...
• Light
*pure energy
• Electromagnetic Waves
*energy-carrying waves emitted by
vibrating electrons
• Photons
*particles of light
What is light?
• Light can act either like a wave or like a
particle
• Particles of light are called photons
Waves
• A wave is a
pattern of
motion that can
carry energy
without carrying
matter along
with it
Properties of Waves
• Wavelength is the distance between two wave peaks
• Frequency is the number of times per second that a
wave vibrates up and down
wave speed = wavelength x frequency
Wavelength (λ) and Frequency (f)
wavelength x frequency = speed of light ~ 300,000,000m/s
λf=c
Light: Electromagnetic Waves
• A light wave is a vibration of electric and magnetic
fields
• Light interacts with charged particles through these
electric and magnetic fields
The Electromagnetic Spectrum
• A range of light waves extending in
wavelength from radio waves to gamma
rays
The Electromagnetic Spectrum
•
•
•
•
•
•
•
Radio Waves
Microwaves
Infrared
Visible Light
Ultraviolet
X-rays
Gamma Rays
mnemonic
Raging
Martians
Invaded
Venus (Roy G. Biv )
Using
X-rays and
Gamma Rays
What is the electromagnetic spectrum?
Thought Question
The higher the photon energy…
a) the longer its wavelength.
b) the shorter its wavelength.
c) energy is independent of wavelength.
Why be a physicist in late 1800s ?
One scientist lamented Newton and Maxwell had
done it all.
Nothing was left to discover. All that remained was to
repeat the experiments with greater and greater
precision.
But there were some curious problems with “classical
physics”
Problems of Classical Physics
Spectra from atoms, cold and hot
Blackbody radiation
Photoelectric effect
What is the structure of matter ?
Problem:The structure of matter
Thompson model of matter had negative electrons
stuck in positive pudding.
Rutherford alpha scattering experiment proved
positive nucleus had most of the mass in small area.
Positive particles repel other positive particles.
NUCLEI SHOULDN’T EXIST a new more powerful force
is required.
Led to planetary model of atoms.
Positive nucleus (protons and neutrons) had negative
electrons in orbits around them.
More on Structure Problem
• Charged particles have an electric field around
them.
• Motion of a charged particle causes a
disturbance in the field, a wave, i.e. radiation
• Orbiting electrons should radiate energy,
spiral into the nucleus.
Spectra from atoms:The Problem
Positive charges repel
other positive charges
with inverse square law.
Electrons in classical
orbits would emit
continuous spectra as
they spiral into the
nucleus.
Atoms shouldn’t exist.
Kirchhoff
In 1859 long before
atomic structure was
understood, Gustav
Kirchhoff formulated
three rules that
describe the three
types of spectra.
p. 99
Kirchoff’s Laws for Spectra
• Law I: Solids, liquids or dense gases radiate at all
wavelengths, a continuous spectrum.
• Law II : A low density gas excited to emit light will do
so at specific wavelengths, a bright line or emission
spectra.
• Law III : If light from a continuous spectrum passes
through a cool low density gas a dark line spectra will
result.
Line spectra are unique for each element !
Kirchoff’s Laws for spectra
Kirchhoff’s 1st Law: continuous spectra
1st law: Solids, liquids or dense gases radiate
at all wavelengths… we now know the light is
radiated by thermal radiation which is
sometimes called blackbody radiation
Properties of Thermal/Blackbody
Radiation
1. Hotter objects emit more light at all frequencies per
unit area.
2. Hotter objects emit photons with a higher average
energy.
Blackbody Radiation
The Problem: Attempts to
explain BB radiation failed.
Classical physics leads to an
equation that fits for long
wavelengths but has a term that
results in increasing intensity as
the wavelengths get short. This
was called the ultraviolet (uv)
catastrophe.
Wien’s Law
for
blackbody
radiators
Wien’s Law
l max = 3,000 ,000
T
Star Colors
• Reddish
 coolest star
• Orange-ish
• Yellowish
• White
• Bluish
 hottest star
Star
Temperatures
3000 K
4000 K
5000 K
6000 K
7000 K
10,000 K
15,000 K
30,000 K
Total Energy from BB
The area under the curve is the total energy
emitted by the blackbody.
Stefan-Boltzmann Law:
E = sAT4,
• E is the energy emitted per second
• σ is a constant 5.67 X 10-8 J/m2sec/K4
•
T is the Kelvin (absolute) temperature of the
object
• A is the area the energy is passing through.
Emission Spectrum
Something first has to excite the atoms. The surface of a star or near by hot
stars mayexcite nebula out to great distances.
Horsehead Nebula
Absorption Spectra
A continuous
spectrum passing
through a cooler
gas will remove
(absorb) the same
lines the hot gas
emits.
These are called
absorption or dark
line spectra.
Lab Absorption
Stellar Atmospheric Absorption
Solar Spectra
Next Problem: Photoelectric effect
Photoelectric effect:Experimental
Photoelectric effect Results:
KE of the ejected electrons
Photoelectric effect: The Problem
Classical physics would predict the brighter the light the
more energy the emitted electrons would have. Increasing
the intensity gives more electrons, but they all have the same
kinetic energy.
Each metal has a different threshold wavelength. Below this
threshold no electrons are emitted regardless of the intensity
of the light.
The energy depends linearly on the frequency.
E= hf = hc/λ
What can we find out from the light
from a distant source? A.
Summary so far:
1. Temperature from BB peak: Wien’s Law
2. Total energy from the BB temperature: StefanBoltzmann Law
3. Composition: Hot gases, emission nebula
Cooler gases, stellar atmospheres, nebula
All are descriptive only, not understood until …
Problems resolved
Unexplained events are due to either poor experiments or standing
on the edge of a great discovery.
The Birth of Modern Physics
Max Planck (c 1900) explained BB radiation by applying discrete
energy levels to the oscillators in a BB cavity, Planck’s constant, h.
Einstein took Planck’s idea a “quantum leap” forward by saying the
energy itself was emitted in packets of specific energy, photons.
Energy given by E = hf. Light has particle properties too. [ This won
him the Nobel prize].
It also explains spectra as well.
Atoms are mostly empty
Bohr , Schrodinger and others
developed “quantum
mechanics” to describe the
structure of matter.
We still use the idea of
orbitals with associated
energy levels, but it’s better to
call these probability
distributions.
If hydrogen nucleus were the size if a grape
seed the first electron orbital would be out
225 yards away.
Take a chemistry class.
Fig. 6-2, p. 94
Spectra explained,1
Quantum mechanics,
uses particle and wave
properties to explain
structure of atoms.
Electrons in energy
levels do not emit
energy.
Only photons with the
exact energy to match
differences in energy
levels interact, going or
coming, with atoms.
The lowest energy level is called the
ground state. Higher levels are called
excited states.
Energy Level Transitions
I’m Free
Not Allowed
Allowed
• The only allowed
changes in
energy are those
corresponding to
a transition
between energy
levels
Photon absorption
The excited atom is unstable and within a fraction of
a second returns to a lower energy level, reradiating
the photon in a random direction.
Hydrogen Absorption
Fig. 6-4, p. 96
Spectra explained , 2
Hydrogen Energy levels
Transitions between
levels lead to different
series of spectra.
To n=1 level Lyman
series .. High energy we
can’t see these.
To n=2 Balmer series…
These are what we see
To n= 3 , Paschen
series, in infrared.
p. 100
Energy levels for different atoms
Increasing atomic
number, more
protons pulls the
energy levels
closer and we get
more electrons
too,
Spectra become
more complex.
Each element has its signature spectrum.
Levels have limits on number of electrons in
them (Pauli Exclusion Principle).
Fig. 6-3, p. 95
Chemical Fingerprints
• Each type of atom has a unique spectral fingerprint
Spectra,3
Bright line, emission spectra result from electron transitions
to lower energy levels. Conservation of energy requires the
energy to be carried away. Photons have energy E = DE =hf,
DE is the energy difference between the levels in the atom.
Dark line , absorption, spectra result when an electron is hit
by a photon with energy matching a transition to a higher
level.
The electron jumps to a higher state. The energy is later
emitted in either a single photon or a series of photons.
Emission is in a random direction. This lowers the light
intensity in the original direction.
Stellar Spectral
Fig. 6-CO, p. 92
Solar Spectra
Fraunhoffer in early 1800’s found 600 dark lines in solar spectra. We now
know the source.
Doppler Effect
• The apparent change in wavelength or
frequency of a wave when the source,
observer, or both is in motion.
Everyday Doppler Effect : Sound
Figure 6.11: The Doppler effect. (b) The clanging bell on a moving fire
truck produces sounds that move outward (black circles). An observer
ahead of the truck hears the clangs closer together, while an observer
behind the truck hears them farther apart.
Doppler Effect: Light
True Velocity
Radial Velocity
Tangential
Velocity
Radar
Optical Doppler Shift
Figure 6.11: The Doppler effect. (a) A blue shift appears in the spectrum of
a star approaching Earth (top spectrum). A red shift appears in the
spectrum of a star moving away from Earth (bottom spectrum). (The
Observatories of the Carnegie Institution of Washington
Fig. 6-11a, p. 106
Radial Velocity from Doppler Data !!!!
Vr
c
Vr
c
=
Dl
l
The ratio of the radial velocity to the speed of the wave is equal to the ratio of
the shift in the wavelength to the original wavelength. Or
Vr = Δλ c/ λ
What can we find out from the light
from a distant source? B.
Summary so far:
1. Temperature from BB peak: Wien’s Law
2. Total energy from the BB temperature: StefanBoltzmann Law
3. Composition
4. Radial velocity
Still more to come…..
How does light tell us the rotation rate
of an object?
• Different Doppler
shifts from different
sides of a rotating
object spread out its
spectral lines
Spectrum of a Rotating Object
• Spectral lines are wider when an object
rotates faster
Collisions cause
shifts
Higher pressure
higher
probability of a
collision
Higher Temperatures also cause more
broadening
Thin cool gas in nebula give sharp lines
Demonstrations for Lab
1.View light from a single slit or from a light bulb
filament.
Vary the voltage changing the temperature. Law I
2. Observe light from selected gas discharge tubes.
Law II
3. Observe a solar spectrum by sunlight reflected
off a cloud
Law III