Modern Physics Wave Particle Duality of Energy and Matter Is light a particle or a wave? We have see that light acts like a wave from polarization, diffraction, and interference patterns. However, light does act like a particle! The quantum theory of the photoelectric effect shows us this.

Quantum theory Electromagnetic energy is emitted from and absorbed in discrete amounts or packets called quanta. The amount of energy is directly proportional to the frequency of light. E = hf = hc/ h is called Plank’s constant 6.63 x js The graph of Kinetic Energy of a photon vs. frequency the slope of the line is always Plank’s constant. Frequency is to energy at amplitude is to rate of emissions. Which color of light red or blue has greater enegy?

Blue! E = hf E = (6.63 x js) (6.4 x hz)blue E = 4.24 x j E = (6.63 x js) (4.1 x hz)red E = 2.72 x j You can convert these energies into electron volts by dividing by 1.6 x j/ev x j /1.6 x j/ev = 2.65 ev 2.72 x j /1.6 x j/ev = 1.7ev

Photoelectric Effect: electrons are emitted from a metal surface when light shines on it if the frequency is at least the threshold frequency. KE max = hf –w o Compton effect: Momentum of incident photon = momentum of scattered photon + momentum of recoil electron. hf = hf’ + KE DeBroglie Wavelength: Particles can exhibit properties of waves. = h/p