THE PHOTOELECTRIC THE PHOTOELECTRIC EFFECTEFFECT

When red light is When red light is incident on a clean incident on a clean

metal surface:metal surface:

no electrons are released,no electrons are released,

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

however intense the light however intense the light source is.source is.

Clean metal surface

When UV light is When UV light is incident on a clean incident on a clean

metal surface:metal surface:

electrons are released electrons are released instantaneously,instantaneously,

however weak the light source.however weak the light source.

Clean metal surface

UV light

Classically this cannot be Classically this cannot be explained because:explained because:

If red light is shone onto the metal surfaceIf red light is shone onto the metal surfacefor long enough some electrons shouldfor long enough some electrons shouldgain sufficient energy to enable them to gain sufficient energy to enable them to

escape.escape.

Einstein put forward a Einstein put forward a theory:theory:

Light energy is quantized.Light energy is quantized.

Light consists of a stream of particles Light consists of a stream of particles called called photons.photons.

The energy of each photon (The energy of each photon (EE) depends ) depends

on the frequency on the frequency ((ff ) of the light. ) 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.

BLACKBODY RADIATIONBLACKBODY RADIATION

Significance of Black BodySignificance of Black Body• The black body concept has a The black body concept has a

significant role in thermal radiation significant role in thermal radiation theory and practice. theory and practice.

• The ideal black body notion is The ideal black body notion is important in studying important in studying thermal radiation and thermal radiation and electromagnetic radiation transfer electromagnetic radiation transfer in all wavelength bands. in all wavelength bands.

• The black body model is used as a The black body model is used as a standard with which the absorption standard with which the absorption of real bodies is compared.of real bodies is compared.

Definition of a black bodyDefinition of a black bodyA black body is an ideal body which allows the entire incident radiation to pass into itself (without reflecting the energy ) and absorbs within itself (without transmitting the energy). This propety is valid for radiation corresponding to all wavelengths and to all angles of incidence. This renders a black body an ideal absorber of incident radiation.

The UV CatastropheThe UV Catastrophe

Pre-1900 theory

Theory & experiment disagree wildly

Planck’s solution Planck’s solution

EM energy cannot be radiated or absorbedin any arbitrary amounts, but only in discrete“quantum” amounts.

The energy of a “quantum” depends on frequency as

Equantum = h fh = 6.6 x 10-34 Js

“Planck’s constant”

Quanta and the UV Quanta and the UV catastrophecatastrophe

Low frequency,small quantum,

Negligible effects

high frequency,large quantum,

huge effects

Withoutthe quantum

With the quantum

Comparison between Classical and Comparison between Classical and Quantum ViewpointsQuantum Viewpoints

ConclusionConclusionAs the temperature As the temperature

increases, the peak increases, the peak wavelength emitted wavelength emitted by the black body by the black body decreases.decreases.

As temperature As temperature increases, the total increases, the total energy emitted energy emitted increases, because increases, because the total area under the total area under the curve increases.the curve increases.

The curve gets The curve gets infinitely close to infinitely close to the x-axis but never the x-axis but never touches it.touches it.

Black-body RadiationBlack-body Radiation

peak = 2.9 x 10-3 m

T(Kelvin)

Lig

ht

inte

nsit

y

UV

IR

peakpeak vs Temperature vs Temperature

peak = 2.9 x 10-3 m

T(Kelvin)T

310 K(body temp)

2.9 x 10-3 m310 K

=9x10-6m

5800 K(Sun’s surface)

2.9 x 10-3 m5800 K =0.5x10-6m

infrared light

visible light