THE PHOTOELECTRIC EFFECT

Objective:

Demonstrate the particle nature of light by discussing photoelectric effect.

Albert EinsteinTM HUJ, www.albert-einstein.net

How does a solar panel work?

Photoelectric effect

When red light is incident on a clean metal surface:

no electrons are released,no electrons are released,

however long light is however long light is shone onto it,shone onto it,

however intense the light however intense the light source is.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 surfacefor long enough some electrons shouldgain 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

ee e e eeesurface 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

ee e e eeesurface 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.

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Einstein’s Theory

• Light remains in the packets (photons) described by Planck– A beam of light is a stream of particles, each

having energy = hf

• When a photon collides with an electron, it must either be:– Reflected with no loss of energy– Absorbed, transferring all its energy to the

electron

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Conditions for the Photoelectric Effect

1. Emission only occurs if the frequency is above a minimum value – the threshold frequency f0

2. Emission starts as soon as the radiation falls on the surface

3. The number of electrons emitted is proportional to the brightness of the light

4. Electrons have varying K – up to a maximum which depends on the frequency of the radiation

5. The K of the electrons is independent of the brightness of the light

© Copyright Cheltenham Computer Training 1995-2000

1. Emission only occurs if the frequency is above a minimum value – the threshold frequency f0

– Classical Mechanics: No explanation can be made using the wave theory

– Quantum Mechanics: Electrons will only be emitted if the photons have enough energy to release them from the surface. The energy required is the work function (where = hf0). Below this frequency no photons have enough energy, so no electrons are emitted.

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2. Emission starts as soon as the radiation falls on the surface− Classical Mechanics: At low brightness the

energy would be spread out across the wavefront, and it would be some time before any electron receives enough energy to escape from the surface

− Quantum Mechanics: If the frequency is above f0 the first photon to arrive has enough energy so it may cause the emission of an electron

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3. The number of electrons emitted is proportional to the brightness of the light− Classical Mechanics: An increase in brightness

would be expected to increase the energy of the emitted electrons, not to increase the number

− Quantum Mechanics: The brighter the light, the more photons that arrive. Each photon is capable of emitting an electron

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4. Electrons have varying K – up to a maximum which depends on the frequency of the radiation

– Classical Mechanics: No explanation can be made using the wave theory

– Quantum Mechanics: The K of the electrons can be up to hf - the photon energy minus the work function. Most will have less than this as they lose energy in collisions as they exit the surface

© Copyright Cheltenham Computer Training 1995-2000

5. The K of the electrons is independent of the brightness of the light− Classical Mechanics: An increase in brightness

would be expected to increase the energy of the emitted electrons.

− Quantum Mechanics: Brighter lights mean more photons, but not more energetic photons, so the energy of the emitted electrons does not increase

© Copyright Cheltenham Computer Training 1995-2000

© Copyright Cheltenham Computer Training 1995-2000

K = qV0

K = maximum kinetic energy of the electron

= equal to the work done by the electric

field in stopping the electrons

q = charge of the electron (-1.602 x 10-19C)

note: do not include the negative sign in the computation

V0 = stopping potential in V or J/C

Cheltenham Computer Training 1995-2000

c = λf c = λ0f0

c = speed of the electromagnetic wave

= 3 x 108 m/s

λ = wavelength of the EM wave in m

f = frequency of the EM wave in 1/s or Hz

λ0 = cutoff wavelength

f0 = cutoff frequency

Electron-volt (eV)

• The amount of energy gained (or lost) by the charge of a single electron moved across an electric potential difference of one volt.

1 eV = 1.602 x 10-19 J

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Kinetic energy

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ReferencesCheltenham Computer Training 1995-2000

http://www.youtube.com/watch?v=MG4h0z8zvv4