Activity 27: Wave-Particle Duality and the Photoelectric Effect

In today's activity, you will use the photoelectric effect to explore the properties of light.

Equipment needed: A spectral lamp, a photoelectric head, lens and diffraction grating, filters, a multimeter, and banana plugs.

Do NOT turn off the Mercury lamps at any time during the class. Turning them off and on will damage them. You may (and should) turn off the photoelectric head and multimeter when not in use.

Before You Start:
Answer the following questions to the best of your ability before doing the experiment. This will be a significant part of today's activity.

Light shines on electrons in a metal. The electrons in the metal must absorb the binding energy, or work function f, of the metal to be released. Any energy above f goes into kinetic energy of the electrons. The energy of waves is proportional to its intensity, and is spread over the entire area of the wave. If light is a particle, it carries energy proportional to its frequency and gives it all up in a single interaction.

What will happen to the kinetic energy of the electrons if the intensity of light is doubled, assuming light is a wave? What would happen if light is a particle instead?
What will happen to the number of electrons if the intensity of light is doubled, assuming light is a wave? What would happen if light is a particle instead?
What will happen to the kinetic energy of the electrons if the frequency of light is doubled, assuming light is a wave? What would happen if light is a particle instead?
What will happen to the number of electrons if the frequency of light is doubled, assuming light is a wave? What would happen if light is a particle instead?
Will light passing through a diffraction grating produce a pattern that depends on the color of light if light is a wave? What if it is a particle?

After you have thought about your answers, compare notes with your group members. Does everyone have the same predictions, or are there differing opinions? STOP. The class will discuss these results before continuing.

Taking Measurements
Copy the table below on your page, and fill it in according to the directions. To take a measurement with the photoelectric head,

Pivot the stand until the desired color of light falls on the opening in the white mask.
Open the cylindrical shield, and pivot the head on the stand so that the desired color falls on the black openings in the white tube.
CAREFULLY close the shield and depress the Zero button, without disturbing the alignment.
When the voltage on the multimeter stops fluctuating, record that value as your stopping potential V.

You may choose to use the spreadsheet at Act27xl.xls to make your plots

Color	l (nm)	f (Hz)	V (V)	K (J)
yellow	578.1
green	546.1
blue	435.8
violet	404.7
u.v.	365.0

Do NOT turn off the Mercury lamps at any time during the class. Turning them off and on will damage them. You may (and should) turn off the photoelectric head and multimeter when not in use.

1.	Calculate the frequency associated with each color in the table, using c = fl.
2.	Use the photoelectric head and the variable filter to measure the stopping potential for four different intensities of light for one color. If you choose yellow (recommended) or green, be sure to place the appropriate color filter on the mask behind the variable filter.
3.	Convert the stopping potential to kinetic energy of the electrons, using K = eV.
4.	Make a plot of K vs. I. Does it support the wave theory or the particle theory? Be sure to notice the scale on the plot - if there are variations in K, how do they compare to the variations in K due to frequency as found below?
5.	Use the photoelectric head to measure the stopping potential for each color of light. Remember to use the filters when measuring green and yellow.
6.	Convert the stopping potential to kinetic energy of the electrons, using K = eV.
7.	Make a plot of K vs. f. Does it support the wave theory or the particle theory? Be sure to notice the scale on the plot - if there are variations in K, how do they compare to the variations in K due to intensity?
8.	Find the slope and intercept of your plot using Excel. How does your slope compare to the accepted value of Planck's constant, h=6.63x10^-34 Js?
9.	How soon after you release the Zero buttons do electrons start flowing? Is this consistent with waves or particles?
10.	What part(s) of the experiment demonstrated the wave nature of light? the particle nature? Justify your answer using your predictions.
11.	Based on your observations, does light behave like a particle or a wave?
12.	Do electrons in the photoelectric experiment behave like particles or waves?
13.	Look at the pattern produced when electrons from a cathode-ray tube are passed through a crystal. Do they behave like particles or waves?

You will be asked to complete an evaluation of today's activity and lecture before the end of class. This evaluation counts as a free 5% of each activity grade. It will generally be done on WebCT in the last 5-10 minutes of class, but time constraints may lead to the occasional evaluation done outside of class.