Photoelectric Effect

Question 1

Discovery of photoelectric effect

Answer 1

A metal contains conduction electrons, which move about freely inside the metal. These electrons collide with each other and with the positive ions of the metal.
Further investigations on the effect of electromagnetic radiation on metals showed that electrons are emitted from the surface of a metal when electromagnetic radiation above a certain frequency was directed at the metal. This effect is known as the photoelectric effect.

Question 2

Observations of photoelectric effect

Answer 2

Photoelectric emission of electrons from a metal surface does not take place if the frequency of the incident electromagnetic radiation is below a certain value known as the threshold frequency. This minimum frequency depends on the type of metal.
2
The number of electrons emitted per second is proportional to the intensity of the incident radiation, provided the frequency is greater than the threshold frequency.
3
Photoelectric emission occurs without delay as soon as the incident radiation is directed at the surface, provided the frequency of the radiation exceeds the threshold frequency,

Question 3

To explain the photoelectric Einstein stated that

Answer 3

When light is incident on a metal surface, an electron at the surface absorbs a single photon from the incident light and therefore gains energy equal to hf, where hf is the energy of a light photon.
• An electron can leave the metal surface if the energy gained from a single photon exceeds the work function of the metal. This is the minimum energy needed by an electron to escape from the metal surface. Excess energy gained by the photoelectron becomes its kinetic energy.

Question 4

Stopping potential

Answer 4

Electrons that escape from the metal plate can be attracted back to it by giving the plate a sufficient positive charge. The minimum potential needed to stop photoelectric emission is called the stopping potential V. At this potential, the maximum kinetic energy of the emitted electron is reduced to zero

Question 5

Conduction electrons

Answer 5

When a conduction electron absorbs a photon, its kinetic energy increases by an amount equal to the energy of the photon. If the energy of the photon exceeds the work function of the metal, the conduction electron can leave the metal. If the electron does not leave the metal, it collides repeatedly with other electrons and positive ions, and it quickly loses its extra kinetic energy.

Question 6

The vacuum photocell

Answer 6

A vacuum photocell is a glass tube that contains a metal plate, referred to as the photocathode, and a smaller metal electrode referred to as the anode. The microammeter in the circuit can be used to measure the photoelectric current. This is proportional to the number of electrons per second that transfer from the cathode to the anode

Question 7

More on the photoelectron emotion of and electron within a photocell

Answer 7

The intensity of the incident light does not effect maximum kinetic energy
• The maximum kinetic energy of the photoelectrons emitted for a given frequency of light can be measured using a photocell.

Question 8

The graph of kinetic energy against frequency

Answer 8

If the measurements for different frequencies are plotted as a graph of Ekmax against j, a straight line of the form y = mx + c is obtained. This is in accordance with the above equation as y = Ek and x = f. Note that the gradient of the line m = h and the y-intercept, c = -ф. The x-intercept is equal to the threshold frequency.