Modern PhysicsWave 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 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?

Blue!

E = hfE = (6.63 x 10 -34 js) (6.4 x 10 14 hz) blueE = 4.24 x 10 -19 j

E = (6.63 x 10 -34 js) (4.1 x 10 14 hz) redE = 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

• 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 –wo

•

• 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