THE PHOTOELECTRIC EFFECT

1. THE PHOTOELECTRIC EFFECT

2. When red light is incident on a clean metal surface: Clean metal surface e e e e e e no electrons are released, however long light is shone onto it, however intense the light source is.

3. When UV light is incident on a clean metal surface: UV light electrons are released instantaneously, however weak the light source. Clean metal surface e e e e e e e

4. 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.

5. 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. h x f E=

6. h is Planck's constant red light has a smaller frequency Frequency increasing than violet light

7. E = h x f Photon energy Red light photons therefore have less energy than violet light photons and even less than UV photons

8. ONE PHOTON GIVES ALL ITS ENERGY e TO ONE ELECTRON

9. A photon of red light gives an electron insufficient energy to enable it to escape from the surface of the metal. Red light photon Clean metal surface e e e e e e e surface electrons No electrons are released from the metal surface.

10. A photon of UV light gives an electron sufficient energy to enable it to escape from the surface of the metal. UV photon Clean metal surface e e e e e e e surface electrons Electrons are released instantaneously. Each photon releases an electron This is called photoemission.

11. BLACKBODY RADIATION

12. Significance of Black Body • 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 black body model is used as a standard with which the absorption of real bodies is compared.

13. 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.

14. The UV Catastrophe Theory & experiment disagree wildly Pre-1900 theory

15. 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 Equantum = h f h = 6.6 x 10-34 Js “Planck’s constant”

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

17. Comparison between Classical and Quantum Viewpoints

18. 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.

19. Black-body Radiation 2.9 x 10-3 m T(Kelvin) l peak = Light intensity UV IR

20. lpeak vs Temperature 2.9 x 10-3 m T(Kelvin) T l peak = 310 K (body temp) 2.9 x 10-3 m 310 K =9x10-6m infrared light 5800 K (Sun’s surface) visible light 2.9 x 10-3 m 5800 K =0.5x10-6m