# Virtual Photoelectric Lab

This material is replicated on a number of sites as part of the SERC Pedagogic Service Project

## Summary

In this activity, students measure the relationship between kinetic energy of photoelectrons and the frequency of the incident light. The kinetic energy is measured in the usual way by determining the stopping potential for each frequency. In this version, a Java applet that simulates the experimental set-up is used. The simulation shows the behavior of an electron when struck by a photon. The stopping potential is determined by observing the behavior of the electron in response to a counter voltage applied across the emitter and collector in the phototube. The stopping potential is the voltage that allows the electron to cross the gap and just touch the collector before falling back.

## Learning Goals

To understand the particle model of the interaction between light and electrons in the photoelectric effect and the procedures used to measure the electron's kinetic energy. Also, students will see that the energy of the photons is proportional to the frequency of the incident light

## Context for Use

Educational level: High school and lower-division undergraduate
Setting: Lab
Time required: approx 50 min
Special equipment: Computer with Internet access and browser that supports Java.
Pre-requisite knowledge: Students should be familiar with the problems inherent in the classical model of the photoelectric effect and the basic features of Einstein's quantum model.

## Teaching Notes and Tips

Although this activity could be used in a traditional lab as an alternative to (or a pre-lab exercise for) a conventional experiment, it is most likely to find application in a distance learning environment. In any case, it should not be thought of as a 'second best' option to working with actual equipment. The simulation provides students with a concrete model of the process; this may lead to a higher level of understanding than a conventional experiment
If possible, instructors should demonstrate the simulation prior to doing the lab. This should include the method used to find the stopping potential (see "Quick Method for Finding Stopping Potential" in the instructions.
Data analysis is done graphically. Graphs can be done by hand, however, computer generated graphs (with slope and intercept determined by linear regression) should be the preferred method, particularly in a distance learning situation.
Standard spreadsheet software (e.g. Excel) is probably best for this; however, a link to a Java applet for this purpose is provided in the instructions. For the latter, students will need to do a screen capture to save the graph

## Assessment

The activity is assessed by having students write a lab report.