The Photoelectric Effect

Goals

Visualize and understand the photoelectric effect
Understand that each element requires a specific photon energy to release an electron.
Describe why an understanding of this experiment requires the photon model of light.

Prerequisite

Energy can transform from one form to another but the total energy is conserved

Introduction

The photoelectric effect occurs when light hits a target material causing some electrons to be released from the target. In 1887 Heinrich Hertz discovered the photoelectric effect. Surprisingly the results obtained were unexpected to physicists of that time. In the time of the experiment, most physicists agreed that light behaved as a wave because that type of  behavior explained the diffraction phenomenon among other phenomena. But, the wave behavior of light did not explain the experimental results of the photoelectric effect very well.  The unexpected results were not explained until Albert Einstein used a new model of light in 1905.  But what was unexpected about this experiment? And what led Albert Einstein to come up with new explanation? Let’s go back in time to the 1800s and see why this experiment was shocking.

Optional Reading 

See this link for more details about the history of this phenomenon.

Begin the Activity

In this activity you will work with a simulation of the photoelectric experiment.The simulation is very similar to the real experiment except you can see the “electrons.” In the experiment you will shine light on a piece of metal target and analyze the outcomes. To begin follow this link.

Running it on the Web or downloading it is your choice.  Once you have the simulation ready to go you should have a window that looks like this figure. 

Notes on running the simulation

• As a preliminary step set the voltage on the battery to 0.00 V.  We will leave the voltage at zero throughout this experiment.
• We will not use the graphs that can appear on the right.  So, leave them unchecked.
• For now we wish to look only at the highest energy electrons.  Check that box in the upper right.
• You can change three parameters: light color (wavelength measured in nanometers), the intensity of the light, and the metal in the target.

Choose a metal target i.e. Sodium. While keeping the voltage at 0 V, shine light on it. Change the parameters and record your observations in the first observations box on the next page.