If you have not done so already, see “What’s This?” under the Help menu. You may also find it useful to have the Photoelectric applet open on the screen as you read the following descriptions (if necessary, click on the link photoelectricEffect.swf). Try manipulating some of the variables while reading the text below and explore the Options menu.
Incident Light
A representation of a portion of the electromagnetic spectrum at the top of the applet screen covers a range from near ultraviolet (200 nm) to the near infrared (800 nm). Using the slider you can select any particular wavelength for the light that will shine onto the cathode. For finer control, click in the wavelength box and enter your value.
According to Einstein’s hypothesis, light is composed of a stream of photons each with a specific energy given by
where f =frequency of light; l = wavelength; h = Planck’s constant
Each photon can transfer a specific quantum of energy to an atom at the cathode. The frequency (or wavelength) of the light is an important variable that is manipulated in photoelectric effect experiments.
As you know from your own experience, a light can vary in brightness but still be the same colour (same f and l). Therefore, the energy of a photon is the same whether the light is dim or bright, but there are more photons per second for a brighter light. This variable is called intensity and is controlled by slider at the bottom left of the applet screen. If you want a more precise control of intensity, you can click on the box (to the left of the slider scale) and enter any value between 0 and 1. Try changing the intensity and note what happens in the simulation of the incident light.
Cathode and Anode
The photoelectric effect occurs with many different metals. In the applet, you can choose from a variety of metals in the Option menu. Comparing the photoelectric effect of different metals can lead to many other questions.
An incoming photon, if it has enough energy, can eject an electron from an atom at the cathode. Einstein assumed that an electron ejected perpendicular to the cathode surface would have the greatest velocity (see Background). These are the electrons that are simulated in this applet. If the power supply is set at zero, a photoelectron reaching the anode completes the circuit and a very small current is registered on the ammeter (p = 10
). Note the connections on the power supply; it will apply a voltage that creates an electric field between the cathode and anode to repel the photoelectron. Try increasing the voltage with the slider and note that the photoelectrons slow down and can be stopped with a sufficient voltage applied. The lowest voltage that just stops the electrons (as measured on the ammeter) is called the stopping potential. When you get close to the stopping potential, click on the box to the left of the voltage slider to manually set a voltage for more precise control.
The stopping potential is used to determine the maximum kinetic energy of the photoelectrons.
Key Design Feature of the Millikan Photoelectric Experiment
The concept of the photoelectric tube is simple—shine a light onto a cathode and collect or stop the photoelectrons at the anode. A similar tube was used by Lenard in his experiments. What distinguished Millikan’s design was that he was able to construct a tube with what he called “a machine shop in vacuo”. By this, Millikan meant that he had a hinged knife inside the evacuated tube that he manipulated by an external magnet. In this way he was able to cut a clean surface on the cathode. This was particularly important for very reactive metals like sodium that are easily oxidized by any air present. Oxide layers on a metal greatly interfere with the photoelectric effect and produce unreliable results.