PHOTOELECTRIC EFFECT

This exercise will help you to more fully understand the importance of the photoelectric effect by experimenting with a simulation that graphically shows you the effect of one photon of a given energy hitting a photocathode. 

The applet is pretty simple to use.  First you set your parameters by clicking on the Set Parameters button.   Then a new screen will appear that allows you to set the stopping voltage and photon energy using sliders.  There is a drop down list that can be used to choose the metal for the photocathode.  Once the parameters are set you then press the Start Experiment button.  A photon will then move toward the cathode.   If the photon is able to release a photoelectron, you will see the electron emerge from the photocathode and move toward the collecting plate on the other side. 

Before proceeding, familiarize yourself with the operation of the applet at this time.

Now there are three different experiments to be performed.  The first is to determine all of the cathodes that have a work function of 5.0 volts or less.  This means that you want to check all of the possible photocathode materials and make a note of which ones will emit a photoelectron when the photon energy is 5.0 eV.  As you check the materials, enter the result in the box below.

Next, scroll back to the applet and pick a photocathode material other than sodium.  Set the stopping voltage to 2.0 volts.   Now experimentally determine the SMALLEST photon energy that will just cause current to flow.  Record that energy in the following box.     Using the results of this 'Experiment', what is the work function for your metal? 

For the last part of the exercise, you should choose another photocathode material.    Next set the photon energy at 10.0 eV (the maximum possible value).  Now determine the MINIMUM stopping voltage which will prevent current from flowing.  Calculate the work function for this case and record the results.   Calculate the cut off wavelength for the work function that you just calculated. 

The one aspect of the photoelectric effect that this simulation cannot show is how the photocurrent depends on the intensity of the incident light (when photoelectrons are ejected).  This should be fairly easy to understand if you think of the light as a stream of photons.  The more intense the light, the more frequently a photon strikes the surface and the more rapidly photoelectrons are emitted - causing a larger value of photocurrent.