THE PHOTOELECTRIC EFFECT A simple demonstration to explain

THE PHOTOELECTRIC EFFECT A simple demonstration to explain the photoelectric effect. © Copyright Cheltenham Computer Training 1995 -2000 Albert Einstein. TM HUJ, www. albert-einstein. net

When red light is incident on a clean metal surface: Clean metal surface nno electrons are released, nhowever long light is shone onto it, nhowever intense the light source is. © Copyright Cheltenham Computer Training 1995 -2000

When UV light is incident UV lig on hat clean metal surface: Clean metal surface nelectrons are released instantaneously, nhowever weak the light source. © Copyright Cheltenham Computer Training 1995 -2000

Classically this cannot be explained because: If red light is shone onto the metal surface for long enough some electrons should gain sufficient energy to enable them to escape. © Copyright Cheltenham Computer Training 1995 -2000

Einstein put forward a theory: • Light energy is quantised. • Light consists of a stream of particles called photons. • The energy of each photon (E) depends on the frequency (f ) of the light. © Copyright Cheltenham Computer Training 1995 -2000

Frequency increasing © Copyright Cheltenham Computer Training 1995 -2000

Photon energy Red light photons therefore than violet light photons and even less than UV photons © Copyright Cheltenham Computer Training 1995 -2000

ONE PHOTON GIVES ALL ITS ENERGY e TO ONE ELECTRON © Copyright Cheltenham Computer Training 1995 -2000

A photon of red light gives an electron insufficient energy to enable it to escape from the surface of the metal. Red light photon Clean metal surface e e e surface electrons No electrons are released from the metal surface © Copyright Cheltenham Computer Training 1995 -2000

A photon of UV light gives an electron sufficient energy to enable it to escape from the surface of the metal. UV photon Clean metal surface e e e surface electrons Electrons are released instantaneously. Each photon releases an electron This is© called photoemission. Copyright Cheltenham Computer Training 1995 -2000

The demonstration © Copyright Cheltenham Computer Training 1995 -2000