Download AP Physics Day 45

AP Physics Day 45 – Photoelectric Effect and Compton Scattering
Date 1/8/2004
Overview:
HW Check & Review Ch 27: 1,3,4,6-8,11
Photoelectric Effect Activity – Collect Data & Analyze Part A.
Notes - Photoelectric Effect & Compton Scattering
HW:
Ch 27: 15-19,27,33,35,36,37,40-43
Finish Photoelectric Effect Activity
Papers:
Photoelectric Effect Activity
Materials:
Demos:
*Blackbody Radiation
200 W bulb
diffraction grating glass
variable transformer
Photoelectric Effect
Photoelectric Effect Device (w/battery)
Oscilloscope
Websites
Labs:
Photoelectric Effect Applet: http://lectureonline.cl.msu.edu/~mmp/kap28/PhotoEffect/photo.htm
Notes:
Notes – Photoelectric Effect & Compton Scattering
Planck wins Nobel Prize for his work in 1918, but that is after the idea receives a lot of support:
First, in 1905 the idea of light quanta are
used to explain another experimental
result: the photoelectric effect.
Photoelectric Effect – Light striking the
surface of a sheet of a photosensitive
metal can knock an electron free from the
metal. Continuous light keeps knocking
electrons free and you get a current
This is simple – the energy from the photon
lifts the electron out of the electrical
potential energy well that it is in inside an
atom and the electron is free.
If you shine a brighter light (with more
energy) on the material, more electrons
are knocked free and you get a bigger
current.
You can control this by putting a voltage across the plates to work against the electrons. Electrons lose
kinetic energy as they travel and will stop if they run out of kinetic energy before they hit the
electrode. The higher the voltage the more get stopped until you reach some stopping voltage, V0.
Kmax = eV0
Here is data for one particular color of
light at two intensities. The more
intense light does produce more current
and we can turn up the voltage to stop
the electrons. Notice that for this color,
the stopping voltage is exactly the
same, regardless of intensity
As you change colors, the cutoff voltage
shifts as well. The higher the
frequency of the light, the higher the
cutoff frequency is! Higher frequency
light gives electrons more Kinetic energy!
As you decrease the frequency of the light, there comes a point, where the light does not produce a
current, no matter what. This frequency is the cutoff frequency (f0), which depends on the metal
being used. You get at least some current above cutoff frequency even with very dim light.
Einstein explains all of these effects by extending Planck’s quantum idea. He named each of the little
bits of light energy photons.
Ephoton = Kelectron +  Where  = PEelectron (work function)
hf = Kmax + 0
0 is minimum PE
0
hf0 = 0 + 0
f0 = h
Samples
http://lectureonline.cl.msu.edu/~mmp/kap28/PhotoEffect/photo.htm
Compton Scattering
In some materials, a photon hitting an electron is not totally absorbed. Instead the photon hits the
electron and they both fly off in different directions, just like two pool balls. The trick is that for the
electron to gain KE and fly off, the photon must lose energy – which means that its frequency and
wavelength change as well.
The amount by which the wavelength changes depends on the angle
of the collision, shown by the angle at which the photon exits
 = f – i
h
 =
(1-cos)
mec