THE PHOTOELECTRIC EFFECT

When red light is incident on a clean metal surface: no electrons are released, however long light is shone onto it, 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 quantized. Light consists of a stream of particles called photons. The energy of each photon ( E ) depends on the frequency ( f ) of the light. 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.

BLACKBODY RADIATION

Significance of Black Body The black body concept has a significant role in thermal radiation theory and practice. The black body concept has a significant role in thermal radiation theory and practice. The ideal black body notion is important in studying thermal radiation and electromagnetic radiation transfer in all wavelength bands. The ideal black body notion is important in studying thermal radiation and electromagnetic radiation transfer in all wavelength bands. The black body model is used as a standard with which the absorption of real bodies is compared. The black body model is used as a standard with which the absorption of real bodies is compared.

Definition of a black body A 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 Catastrophe Pre-1900 theory Theory & experiment disagree wildly

Planck’s solution EM energy cannot be radiated or absorbed in any arbitrary amounts, but only in discrete “quantum” amounts. The energy of a “quantum” depends on frequency as E quantum = h f h = 6.6 x Js “Planck’s constant”

Quanta and the UV catastrophe Low frequency, small quantum, Negligible effects high frequency, large quantum, huge effects Without the quantum With the quantum

Comparison between Classical and Quantum Viewpoints

Conclusion As the temperature increases, the peak wavelength emitted by the black body decreases. As temperature increases, the total energy emitted increases, because the total area under the curve increases. The curve gets infinitely close to the x-axis but never touches it.

Black-body Radiation  peak = 2.9 x m T(Kelvin) Light intensity UVIR

peak vs Temperature peak vs Temperature  peak = 2.9 x m T(Kelvin) T 310 K (body temp) 2.9 x m 310 K =9x10 -6 m 5800 K (Sun’s surface) 2.9 x m 5800 K =0.5x10 -6 m infrared light visible light