Investigating the Photoelectric Effect

Purpose:
To investigate how the photoelectric effect gives evidence for light as a wave or light as a particle.

Background:

The photoelectric effect occurs when matter (usually a metal) emits electrons upon exposure to electromagnetic radiation. These electrons absorb the energy of the incident light and are knocked out of the surface of the metal.

Electrons ejected from a metal surface can be measured as an electric current. Finding the opposing voltage it takes to stop all the electrons gives a measure of the maximum kinetic energy (energy of motion) of the electrons.

From Physics 2000:
“Historically, light has sometimes been viewed as a particle rather than a wave; Newton, for example, thought of light this way. The particle view was pretty much discredited with Young's double slit experiment, which made things look as though light had to be a wave. But in the early 20th century, some physicists--Einstein, for one--began to examine the particle view of light again. Einstein noted that careful experiments involving the photoelectric effect could show whether light consists of particles or waves.”

Different kinds of light such as red or ultraviolet have different frequencies. The frequency of a wave is given in cycles per second. If light is a wave, the energy of the wave should depend only on its amplitude, and not its frequency. Therefore, according to the wave model of light, in the photoelectric effect

i. the kinetic energy of an ejected electron and the number of electrons ejected would increase with the intensity (the amplitude of the wave)

ii. for lights of the same intensities, but different frequencies, the number of electrons ejected should be the same as should the maximum kinetic energy of those ejected electrons.

Hypotheses

Please use the simulation below to answer the following questions. Explain your answers by describing what you observe.

1.  Does the intensity, frequency or both of the incident light affect whether or not an electron is emitted at all?

2.  If electrons are emitted, which factors (intensity, frequency or both) affect
a) the number of electrons?
b) the kinetic energy of electrons (observe how fast they are going)?

Click to Run

http://www-ed.fnal.gov/projects/photoe_lab/student/students.html

2. Read the “BACKGROUND” and the “USER’S GUIDE”. Download the “Photoelectric Virtual Lab Software” from the link at the bottom of the “USER’S GUIDE”. Once you download the software (which opens in Excel), you will need to enable macros, save the changes to the software, quit and then reopen it.

3. Using sodium metal, conduct experiments (printing your graphs) to show how the number of electrons ejected is affected by intensity (vary from 10% to 100%).

4. Using sodium metal, conduct experiments (printing your graphs) to show how the number of electrons ejected is affected by frequency (vary from 2 X 10¹⁴ hz to 10 X 10¹⁴ hz).

5. Using sodium metal, conduct experiments (printing your graphs) to show how the maximum kinetic energy of electrons ejected is affected by intensity (vary from 10% to 100%).

6. Using sodium metal, conduct experiments (printing your graphs) to show how the maximum kinetic energy of electrons ejected is affected by frequency (vary from 2 X 10¹⁴ hz to 10 X 10¹⁴ hz).

CONCLUSIONS:

7. Can your results be explained by the wave model of light? Why or why not? Explain in detail, citing evidence from your graphs.

8. Is there a frequency below which no electrons are ejected? If so, what is it? Get more data if necessary.

9. Max Planck suggested that electromagnetic radiation came in discrete packets or quanta of energy. These light ‘particles’ are called photons. The energy of each photon is determined by the frequency of the radiation. If an electron can only absorb one photon, how can your results for the photoelectric effect be explained by a particle model of light? Explain in detail, citing evidence from your graphs.