MODERN PHYSICS
Photoelectric Effect Lab
 When light hits a metal, it can knock off some electrons
 These electrons will have kinetic energy
 Here is how you determine the kinetic energy in lab:
o Put a voltage hill in the way
o Increase the voltage to make the current just stop
o The PE from your volt hill now equals the KE of your electrons
o
o If q = 1.6 x 10-19C, then the units are called Joules
o If q = 1e, then the units are called electronvolts

The graph that you get is this:



The slope is Planck’s Constant
The x-intercept is the threshold frequency
The y-intercept is the work function


A brighter light (greater intensity) only gets more electrons to come off, not off
any faster (no increase in KE)
Planck’s Constant


Notice how you can use Joules or electronvolts for the units of energy
Threshold Frequency
 The minimum frequency needed to get an electron to come off the metal
 For example, infrared will make nothing happen in this setup, but UV will send
the electrons screaming off the metal
Work Function
 The minimum energy needed to get an electron to come off the metal.

When
is zero,
=
. This
is also the threshold frequency
Photons
 The very fact that this lab works is evidence that light does not act like a wave (at
least, not all the time)
 When light hits something, it behaves like a particle
 A particle of light is called a photon
 A photon:
o Has zero, absolutely zero, mass
o Travels at c, the speed of light
o Has Energy
o Has Momentum, like everything else that has collisions
Photon Energy




Sometimes you have the wavelength and have to v = f to get the frequency
Notice that when light goes from one medium to another, the Energy is conserved
(because frequency remains the same)
The energy can be in Joules or electronvolts
Photon Momentum
 Momentum is always conserved in collisions
 When light hits an electron, there is a conservation of momentum
 That is called the Compton Effect
 The momentum of a photon is:
o
o But m is zero…
o
o
o
o
o


o
your reference tables have
After a collision, a photon will have less momentum
That means it will have less energy
o A lower frequency
o A longer wavelength
o But always the same speed, c
Waves and Particles
 Light can behave like a wave or a particle
 Other particles can behave like waves
 A beam of electrons shot through a narrow double slit will make an interference
pattern
 Moving electrons (moving particles) behave like waves
 The wavelength of a moving particle is:

o
This is called the Debroglie wavelength

Electrons have mass,

Electrons have momentum,

Electrons have kinetic energy,
Practice Questions:
Photoelectric Effect
2010 #7
2010B #7
2004 #6
2000 #5
Photons, waves, particles, energies, momentums, etc.
*2009 #7
*Energy Levels is in a few classes
2008 #7
2008B #7
+
+
2007 #7
E=mc2 is needed
+
2007B#7
2006 #6
2006B #6
*2005 #7
2005B#7
2004B #6
2002 #7
+
E=mc2 is needed
*Energy Levels is in a few classes