THE PHOTOELECTRIC EFFECT

Albert Einstein 1879-1955 Nobel Prize in Physics 1921 © Copyright Cheltenham Computer Training 1995-2000

© Copyright Cheltenham Computer Training 1995-2000 Experimental set-up © Copyright Cheltenham Computer Training 1995-2000

When red light is incident on a clean metal surface: no electrons are released, however long light is shone onto it, however intense the light source is.

When UV light is incident on a clean metal surface: electrons are released instantaneously, however weak the light source.

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.

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. h x f E=

h is planck's constant red light has a smaller frequency Frequency increasing than violet light

Red light photons therefore E = h x f Photon energy Red light photons therefore have less energy than violet light photons and even less than UV photons

ONE PHOTON GIVES ALL ITS ENERGY e TO ONE ELECTRON

e e e e e e e surface electrons Clean metal surface 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 e e e e surface electrons No electrons are released from the metal surface

e e e e e e e surface electrons Clean metal surface 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 e e e e surface electrons Electrons are released instantaneously. Each photon releases an electron This is called photoemission.

© Copyright Cheltenham Computer Training 1995-2000

© Copyright Cheltenham Computer Training 1995-2000 Experimental set-up © Copyright Cheltenham Computer Training 1995-2000

© Copyright Cheltenham Computer Training 1995-2000 Intense Triple Intensity Double Intensity Initial Intensity The number of photoelectrons is proportional to the incident light intensity (frequency is constant) © Copyright Cheltenham Computer Training 1995-2000

© Copyright Cheltenham Computer Training 1995-2000 Freq The applied voltage or stopping potential depends on the frequency: – Higher frequencies generate higher energy electrons • Classical theory: cannot explain this. © Copyright Cheltenham Computer Training 1995-2000

© Copyright Cheltenham Computer Training 1995-2000 Different metals No electrons are produced below a certain frequency no matter the intensity of the incident light. • Classical theory: cannot explain this either ! Stopping potential © Copyright Cheltenham Computer Training 1995-2000

© Copyright Cheltenham Computer Training 1995-2000

© Copyright Cheltenham Computer Training 1995-2000 Takes Planck’s ideas a step further Suggests that the electromagnetic radiation is quantized. Quanta of light have energy E=hf (photons) – h: Planck’s constant – Travels at the speed of light: c = fλ © Copyright Cheltenham Computer Training 1995-2000

© Copyright Cheltenham Computer Training 1995-2000 Photoelectric effect: when a photon collides with an electron, it gives away all its energy (some of which is transferred to the electron as kinetic energy) hf = Φ + EK Energy of the incoming photon Kinetic Energy of the electron Work function (energy necessary for the electron to escape the material) © Copyright Cheltenham Computer Training 1995-2000

© Copyright Cheltenham Computer Training 1995-2000 ( h = 6.62 x 10-34 J.s ) ( 1eV = 1.6 x 10-19 J ) hf = Φ + EK What is the threshold frequency for the photoelectric effect on lithium (Φ =2.93eV)? 1. f= 5.41 x 1017 Hz 2. f= 7.08 x 1014Hz 3. f= 2.01 x 106 Hz What is the stopping potential if the wavelength of the incident light is 400nm? 1. V= 2.72 x 10-20 V 2. V= -1.34 V 3. V= 0.17 V © Copyright Cheltenham Computer Training 1995-2000