The Photoelectric Effect

Any student who has bought a Twix or a dozen eggs understands the idea of an object being quantized.  Teaching a student that light, a seemingly endless saturation of electromagnetic radiation, is also quantized offers many opportunities to prepare a student for what they will encounter in college.  Explaining a photon by means of black body radiation will simply leave students bedazzled.  Using the experiment conducted by Philipp Lenard (1902), a student is offered several situations to grow accustomed to methods of reasoning they will use constantly during college.


Key Concepts

Some key concepts to address while teaching will be the following:

            1.  A quantum.

            2.  The Photon.

3.  Trajectories of electrons ejected from both a retarding potential and an accelerating potential related to projectile motion.

4.  Defining current as the flow of electrons and how adjusting the potential across two plates in a circuit affects the “flow of electrons”

5.   Illustrating how something as small as an electron has kinetic energy, and how the electrons within a plate of metal are affected by being bombarded by photons.


Student Background

Students who are attending a high school physics class are more than likely interested in continuing their education in the sciences.  Checking prerequisites for the physics class may offer insight in a student’s background, such as taking chemistry prior would suggest that they have already been introduced to the idea of a photon.  It is fair to assume that many students will not be apt at dealing with electric circuits due to limited school budgets, and lack of exposure at that level in their education.  However, Lenard’s experiment is simple enough that the main focus can be directed to electron motion between the two plates.  All physics classes deal with projectile motion early on and the force of gravity on a ball can easily be juxtaposed with a retarding potential on an electron.  The key to explaining a modern physics idea to a high school student will be allowing them to discover the result by means of studying concepts they already understand, and then relating these results to what is actually happening.  In summary, to teach the photoelectric effect, students must understand the idea of projectile motion, an object having mass, and an object having kinetic energy.  A simple quiz or check of what chapters have been completed in that class will determine the knowledge of the class and their preconceptions on the subject.


Key Resources

Almost all physics book address the subject of the photoelectric effect.  I personally have three textbooks that range from just mentioning it in passing to a rigorous exploration.  The beauty of a physics topic is that an experiment can easily be designed to show the desired result. Teaching physics requires a different approach compared to your conventional class and a lot of professionals have researched this fact.  Hence, thousands of books address specific topics and offer strategies for teaching complicated ideas.  One such book, I that I use constantly, is A Guide to Introductory Physics Teaching by Arnold Arons.  As there are thousands of books, there are even more websites that deal with teaching physics topics.  One such website is  These websites offer studies on student difficulties within a topic, and labs that are designed to facilitate learning.  Another resource identified in class is Eugene Wood who works with the PhysTec program.  I have not yet contacted Eugene, but I plan on contacting him soon.


Interactive Engagement

Since budgets at schools are often tight, it may be unrealistic to be able to find an evacuated chamber and fire up an arc of high intensity light.  However, a multimeter in simple circuit with a slide resistor would show the potential changes used to study the photoelectric effect.  Also an applied voltage to a plate could be used and considered a high intensity light.  I have not worked out the complete dynamics of a hands on experiment but I have thought about it.  If a hands on experiment is completely out of the question, an engaging discussion will suffice to teach the material.  Many students often ask how they will ever use physics.  This discussion is interesting, simple, and depicts exactly how key concepts are used to discover a phenomenon that is present through all types of electromagnetic radiation.



Since the class will be taught more like a lab then a normal class session, simply watching and asking questions will affirm how well the class has grasped the concepts.  As usual, a quiz at the end of class, that is more conceptual, will be the easiest, quickest, and most effective way to assess the entire classes understanding.  A final lab would also check the understanding of the students.  An example of a question that would test the understanding of the students is:  Draw three curves plotting current vs. frequency of light that would be accepted by Philipp Lenard as results to his experiment.



I believe teaching the photoelectric effect to a high school class would be entertaining.  When I was in a high school student, modern physics seemed like an entirely different realm of education.  I believe a lot of students would find the discussion very interesting and it may serve as a “sales pitch” to continue their education in physics.
























Optics Teaching Project

The Photoelectric Effect

Progress report











Justin Klein

Sept 29, 2003












Phys 352


Fall 2003