 | THE
PHOTOELECTRIC EFFECT VIRTUAL LAB |
STUDENTS' HOMEPAGE - BACKGROUND - USERS' GUIDE
Background
Since the late 1880s, physicists had observed that, under certain conditions, the simple act of shining light on metals could liberate a small, but measurable, number of electrons. In other words, light could produce electricity. What happens when you turn on and off the light below.
Brief History of the Photoelectric Effect
The behavior of light with metals was so unusual that the exact mechanism defied explanation for many years. Physicists had a model in mind when thinking about light. They believed that light behaved like a wave, with amplitudes and frequencies. This was a powerful model, supported by Thomas Youngís famous double-slit experiment at the beginning of the 19th century and by the magnificent pioneering work in electromagnetics performed by Michael Faraday and James Clerk Maxwell. Indeed, Maxwellís four equation of electromagnetics stand as one of the greatest acts of unification in the history of physics: Not only did he discover that electricity and magnetism were one and the same phenomenon, but also that light itself was a form of electromagnetic energy. Indeed, the "speed of light" should more completely be referred to as the "speed of electromagnetic waves."
Given this deep, fundamental connection between electricity and light, it was not wholly unexpected that there would be some interplay between the two. But describing the interplay proved difficult. First, though the photoelectric effect was observed by Heinrich Hertz in 1887, the electron itself was not discovered and isolated until 1897. Merely establishing the properties of the particle that carried the electric charge was not enough, however, to explain the photoelectric effect.
For good reasons, physicists could not reconcile the photoelectric behavior of light with the known properties of waves. The wave model, which had scored such great triumphs throughout the 19th century, was insufficient to describe the photoelectric effect.
Your job, in this virtual lab, is to gather enough information to see why the wave model is insufficient and to nominate another model. By the way, the person who first correctly explained the mechanism behind the photoelectric effect was Albert Einstein in 1905. Though he created Special Relativity and explained Brownian motion in the same year (!!), it was his work on the photoelectric effect that won him his Nobel Prize.
Applications of Photoelectric Effect
There are several well-known applications of the photoelectric effect. One is the production and use of solar panels. Solar panels are nothing more than a series of metallic plates that face the Sun and exploit the photoelectric effect. The light from the Sun will liberate electrons, which can be used to heat your home, run your lights, or, in sufficient enough quantities, power everything in your home. Solar panels are an interesting and attractive form of alternate energy. So why, particularly in the ecologically conscious 2000s, arenít they used more extensively? The answer is "efficiency." To supply a small town with its energy needs via solar panels would require an extensive area of land to be dedicated to solar panels. The tradeoff, presently, prevents the use of solar power on the large scale. But research continues in this promising field.
Another application of the photoelectric effect is in devices known as "photomultiplier tubes" or PMTs. A PMT is a cylindrical tube with a window on one end and wires, or metallic pins, on the other. Light enters through the window and strikes the "cathode," the first in a series of metallic plates that are embedded inside the tube. The photoelectric effect will allow for some electrons to be liberated from the cathode. These electrons are guided to the next plate through the action of a large electric field that is maintained between the two plates. Each electron arrives with significant energy; energy sufficient to liberate 3-4 more electrons. The process repeats itself at the next plate with a similar gain in electrons. With enough plates ? ten is not an unusual number ? a tremendous number of electrons can become available at the last plate, called the "anode." By counting the electrons at the anode, physicists can determine the energy of the light that entered through the window. PMTs are used in high-energy physics labs around the world; they are vital to the process of recreating collisions between sub-atomic particles. For example, at Fermilab, protons and anti-protons are accelerated to tremendous energies before they collide into one another, creating a spray of exotic particles and high-energy photons. Detectors like PMTs allow physicists to carefully reconstruct the collision and make new discoveries about the nature of matter and energy.