The Photoelectric Effect

Photoelectric Observations Summarized:

Compare your observations to these three concepts

1. In the photoelectric effect light which strikes a metal causes electrons to be emitted.

2. This process requires some threshold wavelength to cause an electron ot be emitted from the metal. For a wavelength longer than this threshold, no electrons are emitted. The threshold wavelength is dependent on the metal, and it is constant for each metal.

3. As the wavelength decreases for a specified metal, the speed (and thus the Kinetic Energy) of the emitted electrons increases.

Conclusions from the Photoelectric Effect Experiment

Observations such as these let to an important conclusion.  The energy in light comes in small packets.  Each of these packets is called a quantum of energy or a photon.   To see a representation of photons of light return to the simulation.  Under the Option menu click on Show Photons and change the intensity of the light.  then under Options click on Control the number of photons.  Now watch what happens as you change the number of photons and the wavelength of those photons.  From this representation it becomes clear that the low wavelength photons have high energy while the high wavelength photons have relatively low energy.

Notes about photons in the visualization

To create a visualization of this nature the creators must take some liberties with reality.  The dots flowing from the lamp are not how light actually looks but artists representations of photons.  In particular the speed fo these "photons" are much slower than the speed of light relative to the speed of the electrons. 

Energy considerations

We have not measured the energy in this simulated experiment.  However, we know that Energy is conserved, and thus we can conclude that the energy of the incoming photon will be equal to the kinetic energy of the electron plus whatever energy is needed to knock the electron out of the metal.  We will call this last energy the electron-metal binding energy.  As an equation we write write: Eincoming photon=KEelectron+Eelectron-metal binding. Equivalently: (Kinetic Energy)max for electron = Ephoton-Eelecton-metal binding. This equation tells us that increasing the incident photons energy will increase the Kinetic Energy of the emitted electrons. Recall Ebinding is constant for each metal.

If we were to do some careful measurements on the relation between energy of the electrons and the wavelength of the light, we would find that the kinetic energy fo the electrons is inversely proportional to the wavelength.  Connecting this conclusion back to the photon's energy, we conclude that Ephoton is proportional to (1/wavelength).  Using wavelength x frequency = speed of a wave we get the second important conclusion from this type of experiment:
Energy = h * frequency  (E=hf)
where h is a constant, called Planck's constant.

Notes about energy in the visualization

As described directly above the energy of photons is related to the frequency.  Thus. photons are different from other objects.  The speed of the photon is not r=used in calculating the energy.  The visualization does show an important consideration for speed.  Change the wavelength and watch what happens to the photons' speeds.  Once you have determined what happens to the speed as the wavelength, click here to see our result.

Optional Reading 

More about Planck's Constant
Information about Max Planck


With observations like the ones you made, Albert Einstein in 1905 gave a new explanation of photoelectric effect. Regarding the dependence of KE of ejected electrons on light energy and NOT intensity he proposed that light should be made of tiny particles or quanta which are now called photons. Whereas the minimum energy of light required to overcome the electron binding to the target surface led him to think of light energy has specific values and not continuous values. In other words each quanta i.e. photon has specific quantized energy. With this proposal of photon model by Albert Einstein in 1905 was part of the birth of Quantum Mechanics. And for this simple, yet revolutionary idea of quantized light, Albert Einstein received his Nobel Prize in 1921.

Optional experiment

Coming soon, a closer look at the energies in the photoelectric effect.

Some other Summaries

 As a summary you may want to look at this "conversation" about the photoelectric effect of this web page which includes the equations.

The first two minutes of this YouTube video (by the Lanton Star Centre) covers the material that we have investigated here. After that it goes into detail that we do not need yet.


Questions

For these questions enter your answers in the response frame below the questions.

1) This YouTube video has a nice summary of the basic effect. However, they make an error about iron. For the purpose of the photoelectric effect, iron and copper are very similar. Watch the video then run the simulation for copper. What is the error and what would be the correct description for the video?

2) Return to the simulation and uncheck the box "Show highest energy electrons". Just by looking at the speed of the emitted electrons judge their relative energies. (a)  What variations in energy do you see for a fixed wavelength?  (b) Can you speculate on why the electrons have different energies for a fixed wavelength?  (through out the course we will ask questions which begin with "Can you ..."  A logical answer is "no."  However, these questions are to constructed to motivate you to think about something we will study later.  So, think about and give your best answer.  No answer is wrong for this type of question.)


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