photoelectric effect.pdf

7/28/2019 Photoelectric effect.pdf

1/16

PHOTOELECTRIC EFFECT


2/16

ESSENCE

Occurs for visible and uv radiation.


3/16


4/16

EXPERIMENTAL SETUP


5/16

EXPERIMENTAL RESULTS

Max kinetic energy of electrons independent of light intensity

For sufficiently big reversed bias,

only the fastest electrons lead tophotocurrent.

The stopping potential is

independent of light intensity.

Kmax = eV0


6/16

EXPERIMENTAL RESULTS

Threshold frequency below which no photocurrentIndependently of light intensity

Note linear dependence on frequency.


7/16

THE CLASSICAL TIME LAG

Potassium plate illuminated from 1m by 1W bulb

Pbulb = 1W = 1J/s

d = 1m

r = 1A

our approximation to theelectron size: radius of acircular area of the sizeof an atom

W0 = 2.1eV = 2.1 1.6 1019J = 3.4 1019J

energy required to removean electron from potassiumsurface

electronvolt: energy gained by anelectron moved across potentialdifference of 1 Volt


8/16

IN SHORT

Classical physics cannot explain results

of photoelectric experiments.


9/16

EINSTEIN'S POSTULATE OF A PHOTON

Radiation is composed of photonsIndivisible energy packets localized in space

More photons means higher intensity of radiation

One photon has a definite energy

Photoelectric effectA photon is absorbed by an electron

Ephoton = h


10/16

THEORY OF PHOTOELECTRIC EFFECT

Everything follows from energy conservationMoreover, one can ignore the energy of recoiling atoms

Of course momentum is also conserved, but to explain discussed

effects energy conservation is sufficient.

h = K+Wenergy of a photon

kinetic energy ofejected electron

work required toremove the electronfrom the metal


11/16

MAX KINETIC ENERGY

Max kinetic energy independent of light intensity

Intensity of light is given by the number of photons. More photonsmeans that more electrons will be ejected, but all of them with the

same max kinetic energy and therefore the same stopping potential.

Kmax = hW0

work function: minimal workrequired to remove the electron


12/16

THRESHOLD FREQUENCY

All photons with lower energy, equivalently lower frequency,

independently of how many of them hit the metal, will not beable to eject any electrons.

Threshold frequency

h0 =W0

the minimal energy required toremove the electron


13/16

STOPPING POTENTIAL

Linear dependence of stopping potential on frequency

Kmax = hW0

Kmax = eV0

gives

V0 =

h

e

W0

e


14/16

BREMSSTRAHLUNG

Inverse of photoelectric effectPhotons are created when electron brakes in nucleus' field

Artist impression.


15/16

HISTORY OF SCIENCE

Nobel prizes related to photoelectric effect

Albert Einstein (1921)for his services to Theoretical Physics, and especially for his discovery ofthe law of the photoelectric effect

Robert Millikan (1923)

for his work on the elementary charge of electricity and on thephotoelectric effect


16/16

SUMMARY

Radiation is composed of photonsMore photons more intense the radiation

One photon has energy

Photons are indivisible

Can only be absorbed or emitted as a whole

We still have to marry this particle-like picture with interference experiments.

Ephoton = h

photoelectric effect.pdf

Documents