©John Parkinson

1

MAX PLANCK

PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT

©John Parkinson

2

THE PHOTOELECTRIC EFFECT

THIS IS THE EMISSION OF ELECTRONS FROM MATTER WHEN MATTER IS ILLUMINATED BY CERTAIN TYPES OF ELECTROMAGNETIC RADIATION.

THE EFFECT OCCURS WHEN METALS ARE ILLUMINATED BY UV LIGHT AND CAN OCCUR WITH THE

ALKALI METALS FOR VISIBLE LIGHT.

IT WAS FIRST OBSERVED BY HEINRICH HERTZ IN 1887

©John Parkinson

3

Radiation

mA

Anode +ve Cathode -ve

electrons

The electromagnetic radiation releases electrons from the metal cathode. These electrons are attracted to the anode and complete a circuit

allowing a current to flow

vacuum

©John Parkinson

4

If the polarity is reversed, the pd across the tube can be increased until even the most energetic

electrons fail to cross the tube to A. The milliammeter then reads zero.

mA

A C

Radiation

electronselectrons

The p.d. across the tube measures the maximum kinetic energy of the ejected

electrons in electron volts.

V

©John Parkinson

5

At the end of the nineteenth century, Classical Electromagnetic Wave Theory thought of light

waves as being like water waves.

The wave’s Intensity or energy was directly proportional to the square of the Amplitude, A.

A

©John Parkinson

6

Potassium metal undergoes photoemission with blue and green light, but not with red light.

potassium metal

Emission!Emission!

Nothing!!

Blue light

Green light

Red light

©John Parkinson

7

THE CLASSICAL THEORY SUGGESTS TRYING MORE INTENSE LIGHT

potassium metal

Nothing!!Nothing!!

©John Parkinson

8

The Classical Theory must be wrong!!!!!

©John Parkinson

9

Quantum Theory of the Photoelectric Effect

In 1905 Einstein developed Planck’s idea, that energy was quantised in quanta or photons, in order to explain the photoelectric effect.

Electromagnetic radiation is emitted in bursts of energy – photons. The energy of a photon is given by E = hf, where f is the frequency of the radiation and h is Planck’s constant. [h = 6.6 x 10-34 Js]

But velocity of light = frequency times wavelength fc

Substituting c

f into E = hf

hc

EENERGYPHOTON

©John Parkinson

10

hc

EENERGYPHOTON

the visible spectrum

λ

frequency

violet light light 400 nm

red light light 700 nm

uv light < 400 nm

Blue photon Red photon

Which photon has the most energy ?????

BLUE !!!

©John Parkinson

11

Quantum Theory of the Photoelectric Effect

Because of the interaction of this electron with other atoms, it requires a certain minimum energy to escape from the

surface.

The photons are sufficiently localized, so that the whole quantum of energy [ hf ] can be absorbed by a single electron at one time.

The electron can then eithershare its excess energy with other electrons and the ion

lattice or it can use the excess energy to fly out of the metal.

The minimum energy required to escape depends on the metal and is called the work function, Φ.

©John Parkinson

12

For electron emission, the photon's energy has to be greater than the work function .

The maximum kinetic energy the released electron can have is given by:

EK = hf - Φ For every metal there is a threshold frequency, f0, where hf0 = Φ ,that gives the photon enough energy to produce photoemission.

It follows that the photo electric current is proportional to the intensity of the radiation provided the frequency of radiation is

above threshold frequency.

The number of photoelectrons emerging from the metal surface per unit time is proportional to the number of photons

striking the surface that in turn depends on the intensity of the incident radiation

EK = photon energy – the work function.

©John Parkinson

13

Maximum EK emitted electrons / J

Frequency f / Hz

metal A

Work function, Φ

Threshold frequency f0

metal B

EK = hf - Φ

Gradient of each graph = Planck’s constant, h.

©John Parkinson

14

f / Hz 1014

0 5 10 15

Max Ek / eV

1

2

Potassium Magnesium Aluminium

©John Parkinson

15

Summary

For any metal there is a minimum threshold frequency, f0, of the incident radiation, below which no emission of electrons takes place, no matter what the intensity of the incident radiation is or for how long it falls on the surface.

Electrons emerge with a range of velocities from zero up to a maximum. The maximum kinetic energy, Ek, is found to depend linearly on the frequency of the radiation and to be independent of its intensity.

For incident radiation of a given frequency, the number of electrons emitted per second is proportional to the intensity of the radiation.

Electron emission takes place immediately after the light shines on the metal with no detectable time delay .