The photoelectric effect is best

explained by treating

light as a

Particle

• Albert Einstein explained the photo- electric effect in 1905.

• He won the Nobel Prize for his work in 1921.

• He also published his special theory of relativity in the same year.

I was only 26 in 1905!!

1879-1955

The Photoelectric effect occurs when light is shown on a metal. The light ejects electrons from the metal producing an electric current..

METAL

PHOTONS

ee

ee ELECTRONS

e

According to classical physics, high intensity light would eject electrons with greater kinetic energy. Experimental results contradicted this theory.

Experiments showed that the kinetic energy of the electrons

depended on the frequency of the light.

• Low frequency light, no matter how intense could not eject a single electron.

• High frequency light with a very small intensity was able to eject electrons with a large kinetic energy

Experiments were done to find the relationship between the kinetic energy of the ejected

electrons and the frequency of the incident light.

They resulted in the following graph:

frequency

y intercept

Slope hFo

Written Relationship::

The kinetic energy of the ejected electron is proportional to the frequency of the incident light.

frequency

y intercept

Slope hFo

Mathematical Model:

Kmax = hf -

Light is a particle, called a photon. Each particle has a energy that is directly proportional to the frequency of the light.

E = hfE = Energy

h = Planck’s Constant (6.63x10-34 Js)

f = frequency of the light

represents the work function of the metal.

It is the minimum energy needed to eject an electron

from the metal.

PRACTICAL APPLICATIONS

• The Photoelectric Cell

• video cameras

• electric eyes

• burglar alarms

• night lights

• solar power

• video games

Example:

The work function for a particular metal is 3.2x10-19J. If the metal is illuminated with light of wavelength 550nm. What is the maximum speed of the emitted electrons?

The maximum speed is:

3.0x105m/s

Sample Quiz

• The photoelectric effect is best explained by treating light as a ___________?

• Who won the Nobel Prize for the explaining the photoelectric effect?

• List two practical applications of the photo electric effect.