- Heinrich Hertz observed electromagnetic waves in 1887 using a coil and spark gap receiver. Removing the apparatus from a darkened box increased the maximum spark length observed. - In 1905, Einstein published his paper explaining the photoelectric effect using the theory that light is quantized into discrete photon packets. His equation related the energy of photons to the kinetic energy of emitted electrons. - The photoelectric effect has applications in devices like solar panels, photoelectric smoke detectors, and night vision goggles.

1. Rai Sahab Bhanwar singh College
Physics Department
Spring 2015, Physics 4D
Instructor: Gyan Rao Dhote
By: Rohit Gyanchandani

2. 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 investigation of this
effect, nor did he make any attempt at
explaining how the observed phenomenon was
brought about.
Source: www.epsic.ch/Branches/Audiovideo/audio/la
adio/radio/hertz.htm

3. -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 atoms' field to resonate and, after
reaching a certain amplitude, caused a
subatomic "corpuscle" to be emitted, and
current to be detected.
-The amount of this current varied with the
intensity and color of the radiation. Larger
radiation intensity or frequency would
produce more current.
Joseph John ThomsonJoseph John Thomson’’s CRTs CRT
Source: www.luminet.net/~wenonah/history/rife.htm

4. 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 conductive
plate.
Source: www.frank.germano.com/radiantenergy.htm

5. Lenard’s Observations
-In 1902, Philipp von Lenard observed the variation in
electron energy with light frequency. He used a powerful
electric arc lamp which enabled him to investigate large
changes in intensity, and had sufficient power to enable him
to investigate the variation of potential with light frequency.
-His experiment directly measured potentials, not electron
kinetic energy: he found the electron energy by relating it to
the maximum stopping potential (voltage) in a phototube. He
found that the calculated maximum electron kinetic energy is
determined by the frequency of the light.
-For example, an increase in frequency results in an
increase in the maximum kinetic energy calculated for an
electron upon liberation - ultraviolet radiation would require a
higher applied stopping potential to stop current in a
phototube than blue light. However Lenard's results were
qualitative rather than quantitative because of the difficulty in
performing the experiments
-The current emitted by the surface was determined by the
light's intensity, or brightness: doubling the intensity of the
light doubled the number of electrons emitted from the
surface.

6. Photoelectric Effect
Theory

7. 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. Einstein, Ann. Phys. 17, 132 1905

8. Einstein’s Theory
 The photoelectric effect is
interpreted with photons and
the conservation of energy
with the equation:
hf = φ + ½ mv2
hf equals the energy
of each photon
Source: http://www.westga.edu/~chem/courses/chem410/410_08/sld017.htm

9. Kinetic energy of emitted
electron vs. Light frequency
 Higher-frequency photons have more energy, so
they should make the electrons come flying out
faster; thus, switching to light with the same
intensity but a higher frequency should increase
the maximum kinetic energy of the emitted
electrons. If you leave the frequency the same
but crank up the intensity, more electrons
should come out (because there are more
photons to hit them), but they won't come out
any faster, because each individual photon still
has the same energy. And if the frequency is
low enough, then none of the photons will have
enough energy to knock an electron out of an
atom. So if you use really low-frequency light,
you shouldn't get any electrons, no matter how
high the intensity is. Whereas if you use a high
frequency, you should still knock out some
electrons even if the intensity is very low.
Source: http://online.cctt.org/physicslab/
content/PhyAPB/lessonnotes/dualnature/
photoelectric.asp

10. Source: http://sol.sci.uop.edu/~jfalward/particlesandwaves/phototube.jpg
Simple Photoelectric Experiment

11. Photoelectric Effect
Applications

12. 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

13. Applications: Night Vision
Device
http://www.lancs.ac.uk/ug/jacksom2/

14. Photoelectric Effect Applications
 Photoelectric Detectors In one type of photoelectric
device, smoke can block a light beam. In this case, the
reduction in light reaching a photocell sets off the alarm. In
the most common type of photoelectric unit, however, light
is scattered by smoke particles onto a photocell, initiating
an alarm. In this type of detector there is a T-shaped
chamber with a light-emitting diode (LED) that shoots a
beam of light across the horizontal bar of the T. A
photocell, positioned at the bottom of the vertical base of
the T, generates a current when it is exposed to light.
Under smoke-free conditions, the light beam crosses the
top of the T in an uninterrupted straight line, not striking
the photocell positioned at a right angle below the beam.
When smoke is present, the light is scattered by smoke
particles, and some of the light is directed down the
vertical part of the T to strike the photocell. When
sufficient light hits the cell, the current triggers the alarm.
Source: http://chemistry.about.com/cs/howthingswork/a/aa071401a.htm

15. Photoelectric Smoke Detector
Source: http://www.bassburglaralarms.com/images_products/d350rpl_addressable_duct_smoke_detector_b10685.jpg

16. 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

17. 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>.