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Properties of light:
1. Propagation within a
uniform medium is along
straight lines.
2. Reflection occurs at the
boundary of a medium.
3. Refraction may occur where
a change of speed is
experienced.
4. Interference is found where
two waves are superposed.
5. Diffraction takes place when
waves pass the edges of
obstructions.
Two theories can explain the
properties of light: the particle
theory (corpuscular theory) and
the wave theory .
Particle theory:
Isaac Newton (Laplace)
Wave theory:
Christian Huygens (Robert
Hooke)
CORPUSCULAR THEORY
1. Rectilinear propagation:
particles moving at great speed
would curve very little due to
gravity (or other forces).
How could waves travel in
straight lines?
2. Reflection:
Elastic particles
striking a surface
would bounce off
in a regular way.
3. Refraction:
The rolling ball model. Water
attracts light particles the same
way gravity attracts a rolling ball.
Requires speed of light in water to
be faster than in air.
(This was not measured until
123 years after Newton’s death).
Jean Foucault found in 1850
that the opposite was true.
WAVE THEORY
Huygen’s Principle: Each point
on a wave front may be regarded
as a new source of disturbance.
Wave supporters could
satisfactorily explain reflection
and refraction, but not rectilinear
propagation.
(the basis for Newton’s rejection).
The discovery of the
interference of light in the
early 1800’s and its
subsequent use to explain
diffraction imply a wave
character.
These phenomena can’t be
explained very well by a
particle theory.
The final blow to the
corpuscular theory
came in 1850 with
Foucault’s
measurement of the
speed of light in water
compared to air.
Michael Faraday
developed the principle
of the electric
generator; he
postulated tubes
of force between
charged bodies.
James Clerk Maxwell
developed a series of
mathematical equations
predicting that heat, light, and
electricity all move in free
space at the speed
of light as electro-magnetic
disturbances.
Electromagnetic theory states
that the energy of an
electromagnetic wave is equally
divided between an electric field
and a magnetic field, each
perpendicular to each other, and
both perpendicular to the
direction of the wave.
Electromagnetic wave
a periodic disturbance
involving electric and
magnetic forces.
Heinrich Rudolf
Hertzexperimental
confirmation of the
theory by 1885.
Many believed that all
significant laws of physics
were now discovered.
Hertz himself soon discovered
an important phenomena
which would create problems
for wave theory. This
discovery set the stage for
quantum physics.
Electromagnetic Spectrum
25
10 Hz to 10 Hz
constant speed of
8
3 x 10 m/s
v = fλ
7
λ range is 3 x 10 m to
-17
less than 3 x 10 m
Eight major regions:
Hard gamma rays,
Gamma rays,
X rays,
Ultraviolet radiation,
Optical spectrum,
Infrared radiation,
Radio waves,
Power frequencies.
The intensity of
light follows the
inverse square
law, as did sound
intensity.
About the only thing left
for scientists to explain
involved EM radiation and
thermodynamics.
Specifically, the glow of
objects at high
temperature.
Hot objects do not perform
the way classical mechanics
predicts. Classical theory
predicts that as the wavelength
of light approaches zero
(frequency becomes greater),
the amount of energy being
radiated should become infinite.
Experimental data shows that
the energy reaches a peak, and
then approaches zero along
with the wavelength. This
contradiction is called the
ultraviolet catastrophe, because
the disagreement occurs at the
UV end of the spectrum.
Negatively charged zinc
plates lose their charge
when illuminated by UV
radiation.
Positively charged plates
are not discharged by
similar illumination.
PHOTOELECTRIC EFFECT
the emission of electrons by
a substance when illuminated
by electromagnetic radiation.
These electrons are called
photoelectrons.
First law of
photoelectric emission:
The rate of emission of
photoelectrons is
directly proportional to
the intensity of the
incident light.
Work must be done
against the forces that
hold an electron in a piece
of metal to make the
electron escape the
surface of the metal.
This work is called the
work function.
If photoelectrons acquire
less energy than the work
function, they will not escape.
If photoelectrons acquire
more energy than the work
function, the excess is kinetic
energy and appears as
velocity.
Photoelectrons emitted
from various atom layers
below the surface will be
emitted at various
velocities ranging up
to a maximum value
(electrons at the surface).
If the collector plate
potential is made
increasingly negative
(repelling the electrons
emitted), the current
decreases until it
reaches zero.
At this point all
electrons emitted are
being repelled back
to the emitter. This
is the stopping, or
cutoff, potential VCO.
This measures the
photoelectrons with the
highest kinetic energy.
This cutoff potential is
the same for all
intensities of light.
Second law of
photoelectric emission:
The kinetic energy of
photoelectrons is
independent of the
intensity of the incident
light.
Robert A. Millikan (American)
found that the cutoff potential
had different values for
various frequencies.
The cutoff potential depends
only on the frequency of the
incident light.
Therefore, the
maximum kinetic
energy of
photoelectrons
increases with the
frequency of the light
illuminating the emitter.
Also, for each kind of surface
there is a characteristic
threshold or cutoff frequency
fCO below which the
photoelectric emission of
electrons ceases regardless
of the intensity.
Only a few elements
demonstrate the
photoelectric effect
with visible light.
(alkali metals)
Third law of
photoelectric emission:
Within the region of effective
frequencies, the maximum
kinetic energy of photoelectrons
varies directly with the
difference between the
frequency of the incident light
and the cutoff frequency.
First law of photoelectric emission
doesn’t conflict with the EM theory,
because the magnitude of the
photoelectric current is proportional
to the light intensity. However, the
velocity of the electrons emitted is
not raised with an increase of
intensity, as the wave theory
suggests.
Also, light of any frequency
should cause emission if it
is intense enough. But
there are cutoff frequencies
below which emission does
not occur, even
at high intensity.
The wave theory suggests
that given enough time a
weakly illuminated
electron could “soak up”
enough energy to be
emitted, but no such lag
time exists.
In case you haven’t noticed,
the wave theory looks pretty
sick right now. It can’t
explain this new evidence
(while the particle theory
does), but the particle
theory can’t explain the
old observations.
Max Planck suggested that the
energy emitted by a source is
equal to a constant multiplied by
the frequency of the light
emitted. He suggested that light
is emitted and absorbed in
indivisible energy packets, or
quanta.We now call these
packets photons.
The energy in a photon
is determined by the
frequency of the radiation.
The relationship is
expressed in this
equation:
E = hf
E = hf
f is the frequency in hertz,
h is Planck’s constant
-34
(h = 6.63 x 10 J•s)
E is the energy of the
photon expressed in
joules.
This led Einstein to
publish a simple
explanation to the
photoelectric effect.
Quantum theory - the transfer
of energy between light
radiations and matter occurs
in discrete units called
quanta, the magnitude of
which depends on the
frequency of the radiation.
When a photon is absorbed by
a emitter surface, its quantum of
energy hf is transferred to a
single electron. If hf is equal to
the work function, w, the
electron has just enough energy
to escape the surface (cutoff
frequency).
If hf is greater than w,
the electron leaves with
the excess energy
being expressed as
kinetic energy and
therefore as velocity.
The maximum
kinetic energy is
expressed as:
½
2
mv
=
hf
w
max
Einstein’s photon
hypothesis has no
problems explaining all
experimental evidence
about electromagnetic
energy.
2
mv max
If ½
= 0,
then 0 = hf - w,
and it follows that:
hfCO = w
this is the cutoff frequency
The modern view of the
nature of light recognizes
its dual character: Radiant
energy is transported in
photons that are guided
along their path by
electromagnetic waves.