Modern Physics

1. 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.

2. 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 10 -34 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?

3. Blue! E = hf E = (6.63 x 10 -34 js) (6.4 x 10 14hz) blue E = 4.24 x 10 -19 j E = (6.63 x 10 -34 js) (4.1 x 10 14hz) red E = 2.72 x 10 -19 j You can convert these energies into electron volts by dividing by 1.6 x 10 -19 j/ev. 4.24 x 10 -19 j /1.6 x 10 -19 j/ev = 2.65 ev 2.72 x 10 -19 j /1.6 x 10 -19 j/ev = 1.7ev

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