Photons & The Photoelectric Effect

1. Photons & The Photoelectric Effect

2. Blackbody Radiation • All objects emit electromagnetic waves as a function of heat energy • A perfect blackbody emitter will absorb and reemit all of the electromagnetic radiation that fall on it

3. Blackbody Radiation • In 1900 Max Planck calculated black body radiation curves and assumed that the energy could only have discrete values • E = nhf n a positive integer f  frequency of vibration h Planck’s Constant 6.626 x 10 -34

4. Energy of a Photon • E = hf • Work Function minimum value of work needed to eject an electron • hf = KEmax + Wo • At f0 , KE max = 0 so hf0 = W0

5. Photoelectric Effect • 1873  James Maxwell affirmed light was a form of electromagnetic wave • Experiments performed by Hertz could not be explained by this wave model of light • One of these was the photoelectric effect

6. Photoelectric Effect • Photoelectric Effect  when light strikes a metal surface, the surface gives off electrons • Ejection of photoelectrons depends on the frequency of the light, not the intensity • KE of the photoelectrons depends on the frequency of the light, not the intensity • At low intensities ejection occurs almost instantaneously above a certain frequency

7. Photoelectric Effect • The key to the ejection of electrons is the frequency • This is known as threshold frequency (ft) • If light exceeds threshold frequency, the photoelectric effect will be observed

8. Threshold Frequency • Depends upon the work function of the surface • Work Function = hf • KEmax = hf – hft • Maximum kinetic energy = (Planck’s constant x freq of incoming photon) – work function

9. Momentum of a Photon • p = hf/c = h/l p  momentum h  Planck’s Constant f  frequency c  speed of light l  wavelength • p incident = p scattered + p recoil electron

10. Compton Effect • Compton showed that the difference between the wavelength of the scattered photon and the wavelength of the incident photon is related to the scattering angle • l’ – l = (h/mc)( 1 – cos q ) • h/mc = 2.43 x 10 -12 m – Compton Wavelength of the electron

11. De Broglie Wavelength • l = h/p l De Broglie Wavelength h  Planck’s Constant p  relativistic momentum of the particle