Phet photo electric effect lab 27

THE PHOTOELECTRIC EFFECT

Name ____________________________ Date __________________ Period _____

The photoelectric effect is one of the key experiments that supported early quantum theory. Light, prior to the early 20th century, was considered to be a wave phenomenon. In most ways, this idea reflects reality well—for instance, light is bendable when passed through a lens. The energy of a wave is given by amplitude of the wave squared, so a light wave of a certain frequency should be able to have any value for energy as long as there is a bright enough light source.

However, when red light was shone on a metal surface, no electrons were ejected even when the brightest red light sources were used. On the other hand, when blue light was shone on the same metal surface electrons were ejected even when the source of light was weak (and brighter blue lights ejected more electrons) How could this be? The energy didn’t seem to depend on the amount of light hitting the metal but instead the frequency of light that hit the metal.

Planck put us on the path leading out of this thicket of confusion when he theorized that light and other forms of energy comes in “packets” or discreet “bundles”. Light, in this theory, is considered to be a particle, which we now call a photon. The photoelectric effect was explained by Einstein when he conjectured that Planck’s bundles of energy (i.e. photons) were “knocking loose” the electrons—but only if the photons had enough energy to do the job (a two year old isn’t able to knock a football player off his feet, but a bull undoubtedly could). Einstein’s ideas gave further support to the theory that light energy really is not continuous with infinitely small increments of change (a wave), but is in fact “chunky”.

Today’s lab involves a simulation of the photoelectric effect. You will be checking various metals for the point at which they begin to shed electrons, based on a specific threshold frequency—the exact point when the photons have enough energy to knock the electrons loose. This energy is called the work function (W) for the metal. Different metals hold on to their electrons more strongly or weakly due to atomic structure, so the work function for various metals varies. The formula for calculating W is as follows:

hn = E_k + W

Where

• h is Planck’s constant
• n is the frequency of the light
• E_k is the kinetic energy of the ejected electron
• W is the work function

The kinetic energy of the electron refers to its actual movement once ejected. E_k can effectively be ignored if we just reach the amount of energy to loosen the electron but not get it moving (E_k in these circumstances will essentially have a value of zero). You will be trying to achieve the lowest possible speed for the electrons you eject from the virtual metal surface. W can be obtained by calculating frequency and using Planck’s constant. The work function will be in joules, so in order to compare to published lists of work function values—which are in electron volts—your final value will require conversion into this unit.

The “equipment” you will be working with looks like this:

1.         Bring up the internet and go to the following site:        http://phet.colorado.edu/index.php

2.         Look for “Run our Simulations”, click “On Line”, then “Light and Radiation” (in      the left hand column), then “Photoelectric Effect”, then “Run Now”.

3.         Keep battery voltage at 0. Turn light intensity up to 100%. You will be testing           sodium first (metals are changeable in the upper right hand box).

4.         Adjust wavelength to a value which just allows electrons to leave the surface at a      lowest possible speed.

5.         Calculate W in electron volts using the following values and formulas:

            c = ln, where c = 2.998E17 nm/sec, l is in nm and n is in s^-1

            h = 6.626E-34 J×s

            1 electron volt = 1.60217646 × 10^-19 joules

            hn = E_k + W

            ***remember—E_k is set at zero

6.         Calculate work function (in eV) for all other metals, including the mystery metal.

7.         Look up work function values on the internet. Identify the mystery metal, and          check the values obtained for the other metals.