In 1905, during his "miracle year," Albert Einstein published five papers. These included special relativity, which dealt with space and time, as well as general relativity, which related mass and energy through the equation E = mc2. However, Einstein won his only Nobel Prize for work he did that same year on the photoelectric effect.
At the time, it was known that light shining on certain materials could knock out electrons to produce a current. It stood to reason that the stronger the light, the greater the current. Researchers also found that how much of a kick the electrons got (or how much kinetic energy they had) depended on the color of the light. Many scientists expected a stronger light would also release an electron with greater energy. It took Einstein's brilliance to understand why the color (or frequency) of the light played such a key role in determining how much energy the electrons came away with. The consequences of this insight, along with the contributions of many other scientists, lead to the development of quantum mechanics, which is the basis for the modern electronic world.
This project introduces you to the idea of the photoelectric effect and guides you to recreate the type of data Einstein interpreted.
What You Need
- piece of zinc metal
- sandpaper or steel wool
- short jumper wire
- source of ultraviolet light (a carbon arc lamp or possibly a strong "black light")
- source of visible light (incandescent lamp)
- plate of glass
- electroscope, either purchased or built as a project
Photoelectric effect apparatus
- photoelectric effect apparatus, such as the Daedelon EP-05 (available from www.daedelon.com)
- variable DC voltage source
- voltmeter or multimeter configured as a voltmeter
- various light sources of known frequency: this includes laser pointers of known wavelength, incandescent, carbon arc, or ultraviolet lights
- color filters with known wavelength of transmitted light
This part introduces the basic idea of the photoelectric effect and brings you to the dilemma Einstein addressed.
- Rub the piece of zinc with a piece of sandpaper or steel wool. This removes oxides to expose the metal.
- Discharge the electroscope by touching your finger to the electrode.
- Using a very short jumper, attach the zinc to the electroscope.
- Darken the room.
- Shine the light from an ultraviolet source onto the zinc.
- Observe the effect on the electroscope leaves.
- Discharge the electroscope and compare the effect of the ultraviolet source and the visible source. Also compare the effect of shining the ultraviolet source through a pane of glass that transmits mostly visible range light, but hardly any ultraviolet light.
- Charge the electroscope positively and observe the effect of shining ultraviolet light on the zinc.
- Charge the electroscope negatively and observe the effect of shining the ultraviolet light on the zinc.
Photoelectric effect apparatus
This approach uses a metal target in a vacuum tube. Because the currents that need to be measured are so small, it is helpful to have the detector very close to the source of the current. This procedure goes through the generic steps to make this measurement with specific references to the EP-05 operation (more detailed instructions are available with that apparatus):
- Set up the fluorescent lamp to focus on the detector (photodiode).
- Attach a voltmeter to read the stopping voltage (stopping potential) across the photodiode. (The connections are the red and black banana jacks on the EP-05.)
- Place the blue filter over the opening going into the photodiode. The apparatus should be set up as shown in Figure 121-2.
- Darken the room. If necessary, construct a light shield from a cardboard box to protect the photodiode from stray light.
- Adjust the stopping potential, so all the electrons are turned back and there is no photocurrent. (This is accomplished by turning the "voltage knob" to the full clockwise position.)
- Now, adjust the stopping potential to its minimum value. (This can be done by turning the voltage knob as far in the counterclockwise position as possible.)
- Adjust the radiation intensity by changing the distance between the light source and the detector to read about 10 on the intensity scale. You are now calibrated and ready to make some measurements.
- Measure the current reading and write down the reading on the voltmeter.
- In several steps, increase the stopping potential and record the current reading at each step. Five readings should be sufficient to define a linear relationship.
- The data should produce a linear relationship similar to the one shown in Figure 121-2 between stopping potential and voltage.
- The voltage required to produce zero current is a key point that determines the value of the work function for the metal (in the photodiode).
- Repeat the previous steps using the green filter.
- Replace the fluorescent lamp with a tungsten incandescent lamp. Install the red filter and repeat the previous steps until current versus voltage curves for the red filter.
- A laser or LED of known wavelength can also be used as a source of illumination. A diverging lens (biconcave) may be helpful in spreading the laser beam to fill the opening area of the photo diode.
- For each color, plot the current versus voltage and extrapolate the curve to find the threshold stopping voltage that results in zero current.
- Plot the stopping voltage versus the frequency for each of the frequencies (colors) for which you took data.
Ultraviolet light shining on a piece of zinc results in a charge separation. This charge causes the leaves of a negatively charged electroscope to separate further and causes the leaves of a positively charged electroscope to come together. This indicates the charge is negative or, more specifically, consisting of electrons. Visible light does not result in this charge being developed in the zinc.
Using the photoelectric effect apparatus, we find that:
- The greater the frequency, the greater the stopping voltage required to limit the current flow. This relationship is linear. This means the kinetic energy of the freed electrons is proportional to the frequency of the light.
- Below a certain threshold frequency, no current is generated.
- Increasing the intensity of the light increases the current (for a given stopping potential and light frequency). However, increasing the light intensity does not have any effect on the kinetic energy of the freed electrons.
- The slope of the stopping voltage versus the frequency graph represents Planck's constant divided by the charge on one electron. The equation for this is:
- From the slope of the voltage versus frequency graph, Planck's constant can be determined from the slope multiplied by the electronic charge: q = 1.6 × 10–19 C. A slope of 4 = 10–5 gives the expected value of Planck's constant.
Because the wavelength of light is usually more readily available, the frequency can be determined from the equation: frequency = speed of light / wavelength
Why It Works
Light contains energy based on its frequency. The frequency of visible light is lower than that of ultraviolet light and it does not have enough energy to free electrons from a metal, such as zinc. As the frequency of the light increases, the energy each photon carries is raised above the threshold required to free electrons from the zinc.
The work function of a metal is a measure of how tightly electrons are held by the atoms of the metal. If the photon energy is greater than the work function of the metal, electrons are released. If the freed electrons encounter a stopping voltage (stopping potential), the amount of extra energy above the work function can be determined.
This can be summarized by the equation:
KE = Ephoton + W
where KE is the kinetic energy of the freed electron (measured by the amount of voltage required to stop the electrons).
Ephoton is the energy carried by the photon. W is the amount of energy just to free one electron from the metal with no extra energy to get it moving.
The energy in a photon was given by:
where h is Planck's constant and fis the frequency of the light.
Other Things to Try
A good software simulation of the results of this experiment can be found at http://phet-web. colorado.edu/wb-pages/simulations-base.html.
The key concept underlying this experiment is that light energy comes in specific amounts or packages called quanta or photons. These photons cannot be broken up into smaller units. The higher the frequency of the light, the greater the amount of energy contained in one photon. If a photon has enough energy to release an electron, an electric current can flow; otherwise, below that threshold, no energy will flow. The more energy the photon has, the more kinetic energy the electron processes when it is released.