1. Speed of light (c) Celeritas = latin for swiftness

2. Wave Particle Duality
Earlier we studied light, a form of electromagnetic radiation, as a wave with frequency wavelength, amplitude and velocity. However, the wave model can not explain the interactions of light with matter. In these interactions light and other electromagnetic radiation acts as if it is composed of particles possessing kinetic energy and momentum.

3. When light strikes matter some of the light’s momentum is transferred to the matter. These interactions include the phenomenon known as the Compton effect and the photoelectric effect.
It is a mistake to think that light must be both a particle and a wave at the same time.
Light behaves like waves when it travels in empty space, and like particles when it interacts with solid matter.

4. Quantum Theory
We know that Einstein visualized particles of light as concentrated bundles of electromagnetic energy.
Max Plank called these bundles of energy - quanta (plural of quantum)
Einstein proposed that light itself is composed of quanta now called photons.

5. The energy of light is proportional to its frequency
Ephoton = hf Ephoton = energy of photon
h = Planck’s constant
f = frequency

6. Light as Particles
Each frame in the example figure below shows the image processing photon by photon.
Note that the photons seem to strike the film in an independent and random manner.
See image using overhead

7. Photon Particle Collisions The Photoelectric Effect
The photoelectric effect is the ejection of electrons from certain metals when light falls upon them. These metals are said to be photosensitive.
Investigators discovered that high-frequency light, even from a dim source, was capable of ejecting electrons from a photosensitive metal surface; yet low-frequency, even from a very bright source, could not dislodge electrons.

8. Dim vs. Bright Light – the more photons of light being emitted the
brighter or higher the intensity of the light.
(Remember the energy of light is proportional to its frequency.)
If the energy per photon is too small, then brightness or intensity of light does not matter. The critical factor is the color or frequency of the light.
Wave-Particle Duality Diagrams – PhotoElectric Effect

9. Photon Particle Collisions –The Compton Effect
The Heisenberg Uncertainty Principle

10. Matter Waves E = h ν = h c λ so, p = h c = h c λ λ
DeBroglie Hypothesis
E = h ν = h c λ
so, p = h c = h
c λ λ