PHOTO ELECTRIC EFFECT

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

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

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.

Frequency increasing

Red light photons therefore than violet light photons and even less than UV photons Photon energy

GIVES ALL ITS ENERGY TO ONE ELECTRON e ONE PHOTON

eeeeeee 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 No electrons are released from the metal surface

eeeeeee 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 Electrons are released instantaneously. Each photon releases an electron This is called photoemission.

The demonstration demonstration

What is the Photoelectric Effect? A photon with energy hf strikes an electron and ejects it from the metal. hf = K.E. + w :- w = work to remove electron from metal K.E. = kinetic energy of ejected electron.

How does this explain the photoelectric effect? Think about hitting a ball into outer space. If you don't hit it hard enough, it will just come back down. No matter how many times you hit it. If superman hit it, he could get it into space. Similarly, no matter how many photons strike the metal, if none of them has sufficient energy to eject an electron from a metal atom, you won't get a current. If the energy the taken up by the electron is sufficient to allow it to be released from the metal atom, you will get a current. Here the photon is absorbed by the electron and ceases to be a particle.

Photoelectric effect (Albert Einstein) The photoelectric effect is a phenomenon where electrons are ejected from the surface of a metal (or some other material) when light shines on it shine light rays --> current increase intensity(total energy) --> current goes up That makes sense. The strange thing about this phenomenon, is that a certain minimum frequency of light is required before any current is detected.

Einstein proposed that this phenomenon could be explained by thinking of light as little packets of energy, known as photons. These photons, particles of electromagnetic energy, have an energy equal to E = hv, where h= 6.63 x J s. The higher the frequency of light - the higher the energy of the photon

© Copyright Cheltenham Computer Training

for various frequencies of light f. The value of h and W can then be found from the graph of stopping voltage v s light frequency.