The Photoelectric Effect

Introduction

When light of the appropriate wavelength and energy is shined onto a metal surface, electrons are given off. If the light is below a certain minimum or threshold energy, no electrons are released, regardless of the intensity of the light source. This phenomenon, called the photoelectric effect, was a paradox that could not be explained by the laws of classical physics. In 1905 Albert Einstein applied the new quantum theory of light to explain the photoelectric effect. In doing so, Einstein ushered in a new era in physics and chemistry, one that would transform not only science, but history as well. Demonstrate the photoelectric effect using a metal plate attached to a charged electroscope.

Concepts

• Photoelectric effect
• Energy and wavelength of light
• Quantization of energy
• Planck’s law

Materials

Safety Precautions

Ultraviolet light may damage the eyes and cause cataracts. Wear safety glasses when using an ultraviolet light source and do not look directly at the light. Wash hands thoroughly with soap and water before leaving the laboratory.

Disposal

Save all materials for future use.

Prelab Preparation

1. Practice charging the electroscope. Always ground yourself by touching the desk or bench before charging the electroscope in steps 2 and 3.
2. To charge the electroscope negatively by conduction, rub the rubber rod vigorously with wool, then gently slide the rod against the metal sphere on top of the electroscope. Repeat this process until the foil leaves of the electroscope are fully deflected. Note: The rubber rod and the electroscope must be in direct contact.
3. To charge the electroscope positively by induction, rub the rubber rod vigorously with wool. Touch the wire pole of the electroscope with your hand and bring the charged rubber rod near the metal sphere on top of the electroscope. Take your hand off the wire pole and move the charged rubber rod away from the electroscope. Note: The rubber rod should not touch the electroscope.
4. Polish the zinc foil and copper sheet with sandpaper. The oxide coating on the zinc surface will prevent the photoelectric effect.

Procedure

1. Balance the zinc plate against the top of the electroscope so that the plate is almost perpendicular to the ground. The plate should be touching the wire pole and metal sphere and should face in a direction which will make it easy to observe the deflection of the electroscope leaves. Use a piece of transparent tape to hold the zinc plate in place (see Figure 1).
   {13957_Procedure_Figure_1_Negatively charged electroscope with metal plate (rear view)}
2. Charge the electroscope negatively by conduction. Note: Recharge the electroscope as needed between the following steps.
3. Shine a flashlight or incandescent lamp on the zinc plate and record observations. (No effect.)
4. Take the electroscope outside in bright sunshine. Shield the zinc plate from the sun by keeping it shaded—place it in a cardboard box “shield” if possible. Charge the electroscope negatively by conduction. Step away from the electroscope and allow the sunlight to shine on the zinc plate. Record observations. (The electroscope leaves will collapse as the electroscope discharges.)
5. (Optional) Take the electroscope back inside the classroom and recharge the electroscope. Shine a black light or long-wavelength ultraviolet lamp on the zinc plate and record observations. (No effect.)
6. (Optional) Shine a short-wavelength ultraviolet lamp on the zinc plate and record observations. (The electroscope will discharge.)
7. (Optional) Recharge the electroscope. Place a piece of glass between the short-wavelength UV lamp and the zinc plate. Shine the UV light on the zinc plate through the glass plate and record observations. (No effect. Glass effectively blocks the short-wavelength UV light rays.)
8. Remove the glass plate and record observations. (The electroscope will discharge.)
9. Repeat steps 1–8 using the copper plate. (Copper has a higher threshold energy or work potential than zinc—the electroscope will not discharge using any of the light sources.)
10. (Optional) Charge the electroscope positively by induction and repeat the procedure. (This serves as a control procedure—the electroscope will not discharge using any of the light sources.)

Student Worksheet PDF

13957_Student1.pdf

Teacher Tips

• The electroscope and metal plates must be accurately charged as described in the Prelab Preparation section. If a negatively charged rod is brought near a negatively charged electroscope, the leaves will deflect further. If a negatively charged rod is brought near a positively charged electroscope, the leaves will collapse as the electroscope discharges.
• Rubbing a rubber rod with wool or animal fur gives the best results for charging the electroscope either negatively by conduction or positively by induction. Other materials may also be used—PVC and wool or animal fur may be used to charge the electroscope negatively by conduction, while glass and silk may be used to charge the electroscope positively by conduction.
• When charging the electroscope with the zinc or copper plate attached, it is best to charge the metal sphere on top of the electroscope rather than the zinc or copper plate. When the electroscope is charged, the metal plate will also be charged.
• Check with the biology or earth science teacher at your school for UV lamps. Ultraviolet light is typically used to demonstrate fluorescence in rocks and minerals.
• A combination long wave/short wave ultraviolet lamp with a sliding selector (Flinn Catalog No. AP5725) that allows one to select either long-wave or short-wave UV light is the best choice for this demonstration. A less expensive short-wave UV lamp is also available from Flinn Scientific (Catalog No. AP6658). The latter will discharge the electroscope, but it should be compared against a standard black light to confirm that only the short-wavelength ultraviolet light is effective.
• It is important to ground yourself both before charging the electroscope and again when the ultraviolet lamp is turned on and shined on the zinc or copper plate. Static charge build-up on the plastic casing of a black light or UV lamp will discharge the electroscope unless the demonstrator is grounded.
• If open windows are available in the science lab, mirrors may also be used to reflect sunlight onto the metal plate. However, it must be direct sunlight. Sunlight transmitted through glass will not possess short-wave UV radiation. The sunlight must be unfiltered.

Sample Data

Answers to Questions

1. The energy of a quantum of light is proportional to its frequency. Which UV light has higher energy—the long-wave of the short-wave? Explain.

   The short-wave UV light has higher energy than long-wave UV light because shorter waves result in higher frequency, which is proportional to that light’s energy.

2. How does the energy of light effect whether or not the light source can discharge a certain type of metal?

   Every metal has a distinct, minimum threshold frequency and energy required to dislodge its electrons. If a light source does not have a high enough frequency, it will not have enough energy to make the metal give up an electron.

3. Which type of metal has a higher threshold energy, zinc or copper?

   Copper has higher threshold energy. Sunlight and short-wave UV light could discharge the zinc, but none of the four light sources attempted in this demonstration could discharge the copper. Therefore, copper must require greater energy to dislodge its electrons.

Discussion

Albert Einstein’s explanation of the nature of the photoelectric effect represents a watershed event in the history of science—the dividing line between the laws of classical physics of the 19th century and the laws of quantum physics of the 20th century. The development of quantum physics transformed our understanding of atomic and electron structure and ushered in the atomic and nuclear age.

The fact that light will dislodge electrons from the surface of a metal does not in itself violate the laws of classical physics. Light is energy and, if the energy is high enough, it can cause the metal to lose electrons. According to classical physics, however, this effect should be observed with light of any frequency as long as the intensity of the light is great enough. This is not what is observed. The photoelectric effect is only observed when the frequency of the light is above a specific, minimum threshold frequency that depends on the nature of the metal.

The photoelectric effect was explained by assuming that the energy of a photon or quantum of light is proportional to its frequency (Planck’s law, E = hυ). If the energy of a photon is greater than the minimum energy needed to displace an electron from a metal, the metal will give up an electron. If more photons having this energy strike the metal, additional electrons will be lost, but their kinetic energy will not change. The kinetic energy of the electrons ejected from a metal depends on the frequency (energy) of the light source and the work function of the metal (the minimum energy discussed).

In this demonstration, only short-wavelength ultraviolet light will discharge a negatively charged electroscope with a zinc plate attached. The electroscope is charged with excess electrons. When the zinc metal loses electrons via the photoelectric effect, the resulting positive charge on the zinc surface “neutralizes” the negative charge of the electroscope and the leaves of the electroscope collapse as the electroscope discharges. Copper has a higher work function (threshold energy) than zinc. The negatively charged electroscope with a copper plate attached will not discharge using any of the light sources described in the Materials section.

References

This activity was adapted from Flinn ChemTopic Labs™, Volume 3, Atomic and Electron Structure; Cesa, I., Editor; Flinn Scientific, Batavia IL, 2003.