1. Chapter 31
Light Quanta

2. Light as a Particle
Called a photon
E = hf

3. Photoelectric Effect
Certain metals will emit electrons when exposed to light.
See the Effect

4. Photoelectric Effect
No time lag between turning on light and emission of electrons
Works for short wavelengths but not long wavelengths.
Number of electrons µ number of photons

5. Wave-Particle Duality
Light can act as a wave
Interference
Diffraction
Holography
Light can act as a particle
Photoelectric Effect
No experiment can show both natures at the same time.

6. Light as a Wave
E  1/l
E  p
l  1/p
l = h/p

7. De Broglie Wavelength Particles as waves Why don’t we see it? l = h/ p
A typical bullet (m = 0.02 kg, v = 330 m/s) has a de Broglie wavelength of
10-34 m

8. De Broglie Wavelength
But an electron traveling at 2% c has a de Broglie wavelength of…
10-10 m = 1 Å = about the size of an atom

9. Uncertainty Principle
There is a relationship between how well you can measure position and motion
If you want to measure where a particle is, you cannot know how fast it is moving
Also a joint uncertainty between energy and time