1. LEEJA S S
PHYSICAL SCIENCE
ROLL NO:11

2. Photoelectric Effect
Theory

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

4. 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

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

6. Photoelectric Effect
Applications

7. 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

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

9. 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

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

11. THANK YOU