Santa Rosa Junior College

Physics Department
Spring 2006, Physics 4D
Instructor: Younes Ataiiyan
By: Fermin Pureco, David Henderson,
and Ivan Sanchez
 Hertz's Spark Gaps
-Heinrich Hertz, in 1887, made
observations of the photoelectric effect
and of the production and reception of
electromagnetic (EM) waves.
-His receiver consisted of a coil
with a spark gap, whereupon a spark would
be seen upon detection of EM waves. He
placed the apparatus in a darkened box
in order to see the spark better; he
observed, however, that the maximum
spark length was reduced when in the box
-When removed, the spark length would
increase. Hertz concluded his months of
investigation and reported the results
obtained.
-He did not further pursue QuickTime™ and a
investigation of this effect, nor did he TIFF (Uncompressed) decompressor
are needed to see this picture.
make any attempt at explaining how the
observed phenomenon was brought about.

Source: www.epsic.ch/Branches/Audiovideo/audio/la
adio/radio/hertz.htm
 Joseph John Thomson’s CRT
-In 1899, Joseph John Thomson
investigated ultraviolet light in
Crookes tubes.
-In the research, Thomson enclosed
a metal plate (a cathode) in a
vacuum tube, and exposed it to
high frequency radiation. It was
thought that the oscillating
electromagnetic fields caused the QuickTime™ and a
TIFF (Uncompressed) decompressor
atoms' field to resonate and, are needed to see this picture.
after reaching a certain
amplitude, caused a subatomic
"corpuscle" to be emitted, and
current to be detected.
-The amount of this current
Source: www.luminet.net/~wenonah/history/rife.htm
varied with the intensity and
color of the radiation. Larger
radiation intensity or frequency
would produce more current.
 Tesla’s Radiant Energy
-On November 5, 1901,
Nikola Tesla received the
U.S. Patent 685957
(Apparatus for the Utilization of
Radiant Energy)
-This apparatus describes
radiation charging and
discharging conductors by
”radiant energy".
-Tesla used this effect
to charge a capacitor
with energy by means of a Source: www.frank.germano.com/radiantenergy.htm
conductive plate.
 Lenard’s Observations
-In
-In 1902,
1902, Philipp
Philipp von
von Lenard
Lenard observed
observed the
the variation
variation
in
in electron
electron energy
energy with
with light
light frequency.
frequency. He He used
used aa
powerful
powerful electric
electric arcarc lamp
lamp which
which enabled
enabled him
him to
to
investigate large changes in intensity,
investigate large changes in intensity, and had and had
sufficient
sufficient power
power toto enable
enable him
him to
to investigate
investigate the
the
variation of potential with light frequency.
variation of potential with light frequency.
-His
-His experiment
experiment directly
directly measured
measured potentials,
potentials, not
not
electron
electron kinetic
kinetic energy:
energy: he
he found
found the
the electron
electron energy
energy QuickTime™ and a
TIFF (Uncompressed) decompressor
by relating it to the maximum stopping potential
by relating it to the maximum stopping potential are needed to see this picture.
(voltage)
(voltage) in
in aa phototube.
phototube. HeHe found
found that
that the
the calculated
calculated
maximum
maximum electron
electron kinetic
kinetic energy
energy isis determined
determined byby the
the
frequency of the light.
frequency of the light.
-For
-For example,
example, anan increase
increase in
in frequency
frequency results
results inin an
an
increase in the maximum kinetic energy calculated
increase in the maximum kinetic energy calculated for for
an
an electron
electron upon
upon liberation
liberation -- ultraviolet
ultraviolet radiation
radiation
would
would require
require aa higher
higher applied
applied stopping
stopping potential
potential toto Source: http://nobelprize.org/physics/
stop current in a phototube than blue light. However
stop current in a phototube than blue light. However laureates/1905/lenard-bio.html
Lenard's
Lenard's results
results were
were qualitative
qualitative rather
rather than
than
quantitative
quantitative because of the difficulty in performing
because of the difficulty in performing
the
the experiments
experiments
-The
-The current
current emitted
emitted by
by the
the surface
surface was
was determined
determined byby
the light's intensity, or brightness: doubling
the light's intensity, or brightness: doubling the the
intensity
intensity of
of the
the light
light doubled
doubled the
the number
number ofof electrons
electrons
emitted from the surface.
emitted from the surface.
Photoelectric Effect

Theory
 Concerning an Heuristic Point of View
Toward the Emission and Transformation
of Light
 ”It seems to me that the observations associated with blackbody
radiation, fluorescence, the production of cathode rays by ultraviolet
light, and other related phenomena connected with the emission or
transformation of light are more readily understood if one assumes that
the energy of light is discontinuously distributed in space. In
accordance with the assumption to be considered here, the energy of a
light ray spreading out from a point source is not continuously
distributed over an increasing space but consists of a finite number of
energy quanta which are localized at points in space, which move
without dividing, and which can only be produced and absorbed as
complete units.”

 A.
A. Einstein,
Einstein, Ann.
Ann. Phys.
Phys. 17,
17, 132
132 1905
1905
 Einstein’s Theory
 The photoelectric effect
is interpreted with
photons and the
conservation of energy
with the equation:
 hf =  + ½ mv2
 hf equals the energy
of each photon

Source: http://www.westga.edu/~chem/courses/chem410/410_08/sld017.ht
 Kinetic energy of emitted
electron vs. Light


they
frequency
Higher-frequency
Higher-frequency photons
should make
photons have
the
have more
electrons
they should make the electrons come flying out
more energy,
come
energy, so
flying out
so

faster;
faster; thus,
thus, switching
switching to to light
light with
with the
the same
same
intensity
intensity but
but aa higher
higher frequency
frequency should should increase
increase
the
the maximum
maximum kinetic kinetic energy
energy of of the
the emitted
emitted
electrons.
electrons. IfIf you
you leave
leave thethe frequency
frequency the the same
same
but crank up the intensity, more
but crank up the intensity, more electrons electrons
should
should come
come out out (because
(because there there are are more
more
photons
photons to to hit
hit them),
them), butbut they
they won't
won't come
come outout
any
any faster,
faster, because
because each each individual
individual photon
photon still
still
has the same energy. And if the
has the same energy. And if the frequency is frequency is
low
low enough,
enough, then then none
none of of the
the photons
photons willwill have
have
enough
enough energy
energy to to knock
knock an an electron
electron outout of
of an
an
atom.
atom. SoSo ifif you
you use
use really
really low-frequency
low-frequency light, light,
you
you shouldn't
shouldn't get get any
any electrons,
electrons, no no matter
matter how
how
high
high the intensity is. Whereas if you use
the intensity is. Whereas if you use aa high
high
frequency,
frequency, you you should
should still
still knock
knock out out some
some
electrons
electrons even
even ifif the
the intensity
intensity is is very
very low.
low.

Source: http://online.cctt.org/physicslab/
content/PhyAPB/lessonnotes/dualnature/
photoelectric.asp
 Simple Photoelectric Experiment

Source: http://sol.sci.uop.edu/~jfalward/particlesandwaves/phototube.jpg
Photoelectric Effect

Applications
 Applications
 The Photoelectric effect has numerous applications, for example
night vision devices take advantage of the effect. Photons entering
the device strike a plate which causes electrons to be emitted, these
pass through a disk consisting of millions of channels, the current
through these are amplified and directed towards a fluorescent
screen which glows when electrons hit it. Image converters, image
intensifiers, television camera tubes, and image storage tubes also
take advantage of the point-by-point emission of the
photocathode. In these devices an optical image incident on a
semitransparent photocathode is used to transform the light image
into an “electron image.” The electrons released by each element
of the photoemitter are focused by an electron-optical device onto
a fluorescent screen, reconverting it in the process again into an
optical image
Applications: Night Vision
Device

http://www.lancs.ac.uk/ug/jacksom2/
 Photoelectric Effect Applications


 Photoelectric
Photoelectric DetectorsDetectors In In one
one type
type of of photoelectric
photoelectric
device,
device, smoke
smoke can can block
block aa light
light beam.
beam. In In this
this case,
case, the the
reduction
reduction in in light
light reaching
reaching aa photocell
photocell sets sets off
off the
the alarm.
alarm. InIn
the
the most
most common
common type type ofof photoelectric
photoelectric unit, unit, however,
however, light light
is
is scattered
scattered by by smoke
smoke particles
particles onto onto aa photocell,
photocell, initiating
initiating
an
an alarm.
alarm. In In this
this type
type ofof detector
detector there
there is is aa T-shaped
T-shaped
chamber
chamber with with aa light-emitting
light-emitting diodediode (LED)
(LED) that that shoots
shoots aa
beam
beam of of light
light across
across the the horizontal
horizontal bar bar ofof the
the T.
T. A A
photocell,
photocell, positioned
positioned at at the
the bottom
bottom of of the
the vertical
vertical basebase ofof
the
the T,
T, generates
generates aa current current when
when itit isis exposed
exposed to to light.
light.
Under
Under smoke-free
smoke-free conditions,
conditions, the the light
light beam
beam crosses
crosses the the
top
top of
of the
the TT inin an
an uninterrupted
uninterrupted straight
straight line,
line, not
not striking
striking
the
the photocell
photocell positioned
positioned at at aa right
right angle
angle below
below the the beam.
beam.
When
When smoke
smoke is is present,
present, the the light
light is
is scattered
scattered by by smoke
smoke
particles,
particles, and and some
some of of the
the light
light is
is directed
directed down down the the
vertical
vertical part
part of of the
the TT to
to strike
strike the
the photocell.
photocell. When When sufficient
sufficient
light
light hits
hits the
the cell,
cell, the
the current
current triggers
triggers thethe alarm.
alarm.

Source: http://chemistry.about.com/cs/howthingswork/a/aa071401a.htm
 Photoelectric Smoke Detector

Source: http://www.bassburglaralarms.com/images_products/d350rpl_addressable_duct_smoke_detector_b10685.jpg
 Applications
 Solar panels are nothing more
than a series of metallic plates
that face the Sun and exploit the
photoelectric effect. The light
from the Sun will liberate
electrons, which can be used to
heat your home, run your lights,
or, in sufficient enough
quantities, power everything in
your home.

Source: www.futureenergy.org/ picsolarpannelsmatt.jpg

 Work Cited
Amar, Francois G. The Photoelectric Effect. 25 Sep 2003. Section of Chemistry 121 for fall
03. 11 May 2006
<http://chemistry.umeche.maine.edu/~amar/fall2003/photoelectric.html>
Blawn, Jeramy R. and Colwell, Catharine H. Physics Lab: Photoelectric Effect. 10 Jun 2003.
Mainland High School: Online Physics Labs. 11 May 20006
<http://online.cctt.org/physicslab/content/PhyAPB/lessonnotes/dualnature/
photoelectric.asp>
Helmenstine, Anne Marie. Photoelectric & Ionization Smoke Detector. 25 Feb 2006.
About.com. 11 May 2006
<http://chemistry.about.com/cs/howthingswork/a/aa071401a.htm>
Einstein, Albert. “Concerning an Heuristic Point of View Toward the Emission and
Transformation of Light.” American Journal Of Physics 5 May 1965: 137.
Nave, Rod. HyperPhysics. 19 Aug. 2000. Georgia State University. 06 May 2006
<http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html> .
Thornton T., Stephen, and Rex, Andrew. Modern Physics for Scientists and Engineers.
Canada : Thomson Brooks/Core, 2006
Photoelectric Effect. 24 Apr. 2006. Wikipedia Free Encyclopedia. 05 May 2006.
<http://en.wikipedia.org/wiki/Photoelectric_effect>.