Properties of light: 1. Propagation within a uniform medium is along straight lines. 2. Reflection occurs at the boundary of a medium.

1
Properties of light1. Propagation within a
uniform medium is along straight lines.2.
Reflection occurs at the boundary of a medium.
2
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.
3
Two theories can explain the properties of light
the particle theory (corpuscular theory) and the
wave theory .Particle theory Isaac Newton
(Laplace)Wave theoryChristian Huygens (Robert
Hooke)
4
(No Transcript)
5
(No Transcript)
6
CORPUSCULAR THEORY1. Rectilinear
propagationparticles moving at great speed
would curve very little due to gravity (or other
forces).How could waves travel in straight
lines?
7
2. ReflectionElastic particles striking a
surface would bounce off in a regular way.
8
(No Transcript)
9
3. RefractionThe 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 Newtons
death).Jean Foucault found in 1850 that the
opposite was true.
10
(No Transcript)
11
(No Transcript)
12
WAVE THEORYHuygens 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 Newtons rejection).
13
(No Transcript)
14
The discovery of the interference of light in the
early 1800s and its subsequent use to explain
diffraction imply a wave character. These
phenomena cant be explained very well by a
particle theory.
15
(No Transcript)
16
The final blow to the corpuscular theory came in
1850 with Foucaults measurement of the speed of
light in water compared to air.
17
Michael Faraday developed the principle of the
electric generator he postulated tubes of force
between charged bodies.
18
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.
19
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.
20
Electromagnetic wave a periodic disturbance
involving electric and magnetic forces.
21
Heinrich Rudolf Hertz-experimental confirmation
of the theory by 1885.
22
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.
23
Electromagnetic Spectrum10 Hz to 1025
Hzconstant speed of 3 x 108 m/s v f? ?
range is 3 x 107 m to less than 3 x 10-17 m
24
Eight major regionsHard gamma rays,Gamma
rays,X rays,Ultraviolet radiation,Optical
spectrum,Infrared radiation,Radio waves,Power
frequencies.
25
The intensity of light follows the inverse square
law, as did sound intensity.
26
About the only thing left for scientists to
explain involved EM radiation and thermodynamics.
Specifically, the glow of objects at high
temperature.
27
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.
28
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.
29
(No Transcript)
30
Negatively charged zinc plates lose their charge
when illuminated by UV radiation. Positively
charged plates are not discharged by similar
illumination.
31
(No Transcript)
32
PHOTOELECTRIC EFFECT the emission of electrons
by a substance when illuminated by
electromagnetic radiation. These electrons are
called photoelectrons.
33
(No Transcript)
34
First law of photoelectric emission The rate
of emission of photoelectrons is directly
proportional to the intensity of the incident
light.
35
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.
36
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.
37
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).
38
If the collector plate potential is made
increasingly negative (repelling the electrons
emitted), the current decreases until it reaches
zero.
39
At this point all electrons emitted are being
repelled back to the emitter. This is the
stopping, or cutoff, potential VCO.
40
This measures the photoelectrons with the highest
kinetic energy. This cutoff potential is the
same for all intensities of light.
41
Second law of photoelectric emissionThe kinetic
energy of photoelectrons is independent of the
intensity of the incident light.
42
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.
43
Therefore, the maximum kinetic energy of
photoelectrons increases with the frequency of
the light illuminating the emitter.
44
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.
45
Only a few elements demonstrate the photoelectric
effect with visible light. (alkali metals)
46
Third law of photoelectric emissionWithin 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.
47
First law of photoelectric emission doesnt
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.
48
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.
49
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.
50
In case you havent noticed, the wave theory
looks pretty sick right now. It cant explain
this new evidence (while the particle theory
does), but the particle theory cant explain the
old observations.
51
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.
52
(No Transcript)
53
The energy in a photon is determined by the
frequency of the radiation. The relationship is
expressed in this equation E hf
54
E hff is the frequency in hertz,h is
Plancks constant (h 6.63 x 10-34 Js)E is
the energy of the photon expressed in joules.
55
This led Einstein to publish a simple explanation
to the photoelectric effect.
56
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.
57
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).
58
If hf is greater than w, the electron leaves with
the excess energy being expressed as kinetic
energy and therefore as velocity.
59
The maximum kinetic energy is expressed as ½
mv2max hf - w
60
Einsteins photon hypothesis has no problems
explaining all experimental evidence about
electromagnetic energy.
61
If ½ mv2max 0, then 0 hf - w, and it
follows that hfCO w this is the cutoff
frequency
62
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.
63
(No Transcript)
64
(No Transcript)
65
(No Transcript)
66
(No Transcript)