# Photo Electric Effect

```A. Research the photo electric effect and provide a written explanation
of the effect and its importance to modern physics
B. Write down the equation for the photo electric effect given
by Einstein ( KEmax = hf – f ) and explain each term.
C. Classical physics predicted that any color of light (as long as it was
bright enough) would knock electrons out of the metal.
What actually happens when red light is used?
Photo Electric Effect Experiment
A. Background: If a light particle (photon) hits an electron in a metal, the electron may be knocked
out of the metal. The kinetic energy of the ejected electron is:
KE = hf – f where h = Plank’s Constant,
f = the frequency of the photon ( Hz), and f = the work function of the metal. What is new here,
is that a photon is given an energy E = hf. To find the KE of the electron, a negative plate is used
to make it stop right before it hits the plate. Then KE = eV, where e is the charge of the electron
and V is the stopping voltage used. So, the equation becomes:
eV = hf - f
If you know the stopping voltage and the frequency of the light, you can find Plank’s constant
and the work function of the metal by plotting eV vs. f.
B. Design a lab to determine Plank’s constant and the work function for two different metals using
the Phet lab “Photoelectric Effect”.
Select “ show only highest energy electrons” and use the maximum intensity of light.
Choose a frequency and adjust the stopping voltage until the electrons turn around right
before they hit the negative plate. Get them as close as possible without hitting it.
record the wave length and the stopping voltage and repeat for 9 more different settings.
Repeat for a new metal.
Plot your data ( eV on the y axis and f on the x-axis ). Recall that f = c/l.
Using your graphs, determine the experimental values of h and f. Compare these to the theoretical
values using a percent error.
1. What is the significance of the work function, f?
2. For some colors of light, no electrons are ejected at all.
Why is this?
3. A 100Watt bulb, emits 100Joules of energy in 1 second.
If the light frequency is 6 x 1014 Hz, how many photons come
out of the bulb in 1 second? (Use Etotal = nhf )
A.Two data tables
B.Two graphs
C.Two curve fits
D.Two values for Plank’s constant.
E.Two % Errors
F.Summary
```