Evans SPH 4U1

Unit 9: Quantum Theory

Note 2: The Photoelectric Effect

Reference: Chapter 12.1

In 1905 Einstein persuaded the scientific community to accept the quantum theory of light to explain the photoelectric effect.

Physics became known as:

Classical Physics- physics before 1900

Modern Physics- physics after 1900

Photoelectric Effect:

First noticed by Heinrich Hertz when he was testing Maxwell's theory of electromagnetic waves.

When a light strikes a metal plate, electrons may be emitted from the plate and move away with a definite amount of Ek. The emission of electrons from certain materials by light of certain frequencies is called the PHOTOELECTRIC EFFECT.

Refer to class diagram or figure 7 on page 598 in text.

• Cathode is illuminated by light of a certain frequency and intensity causing photoelectrons to be emitted from the cathode.

• The photoelectrons travel to the anode, producing a photocurrent.

• If you vary the DC source to make the anode negative you can reduce the current and cause all but the faster photoelectrons to be returned back.

• A CUTOFF potential is reached where the photocurrent becomes zero. This CUTOFF potential = maximum Ek of the photoelectrons. (if the cutoff potential is 1.5 V, the maximum Ek of photoelectrons is 1.5 eV)

The following observations were found:

1. A photocurrent is observed when the light is above a certain frequency called the THRESHOLD frequency (fo). The threshold frequency is the minimum frequency at which electrons can be ejected from a photoelectric material. Above the threshold frequency, the more intense the light, the greater the current of photoelectrons.

2. If the frequency is less than the threshold frequency, NO electrons are emitted from the photoelectric surface regardless of the intensity (brightness) of the light.

3. The threshold frequency is different for different surfaces.

4. As the retarding potential applied to the anode is increased, the photocurrent decreases. For some cutoff potential Vo, the photocurrent becomes zero.  All electrons emitted are now turned back by the retarding potential before they reach the anode.

5. If different frequencies of light (above the threshold frequency) are directed at the same photoelectric surface, the cutoff potential will be different for each. The higher the light frequency, the higher the cutoff potential. The maximum kinetic energy of the emitted photoelectrons can be calculated from the cutoff potential.     where Vo is the cutoff potential

6. If different photoelectric surfaces are exposed to light of different frequencies, the graphs of the Ek for the ejected electrons versus the frequency is a set of parallel lines with the same slope (figure 11 page 600) (but they will have different threshold frequencies).

7. Once the threshold frequency is reached a photocurrent appears immediately when the light is turned on, even for extremely weak light.  No time is required to accumulate energy to free the electrons.

DO LAB EXERCISE 12.1.1 (formal report not required)