Lecture 32

Readings

http://umdberg.pbworks.com/w/page/52122743/Three%20models%20of%20light

http://umdberg.pbworks.com/w/page/65464521/The%20photon%20model%20%282013%29

http://umdberg.pbworks.com/w/page/125510822/Workout%3A%20The%20photon%20model

http://umdberg.pbworks.com/w/page/53301740/Basic%20principles%20of%20the%20photon%20model

3 Models of Light

• Waves

• Rays

• Particles

• "Light" ( = Electromagnetic (EM) waves ) are really just 1 thing

  • Which "picture" or "model" we use depends on the application

• EM waves go by all/any of the following names:

  • Gamma Rays $\to$$\rightarrow$ short wavelength , high energy photons
  • X-rays
  • Ultraviolet
  • Visible light
  • Infrared
  • EHF , UHF , VHF
  • Microwaves
  • Radio Waves $\to$$\rightarrow$ long wavelength , low energy photons

• Does not include:

  • Sound , ocean waves
  • Ultrasound
  • Gravity waves

Photon Model

• Photo = "particle" ( but will have "wave" properties )

• A fixed amount of energy ( for a particular frequency / wavelength )

• For each wavelength $\lambda$$\lambda$ , a definite energy:

  • If $\lambda$$\lambda$ is in nano meters
  $$Energy(E)=\frac{hc}{\lambda }=\frac{1240}{\lambda }eV$$
  • Or if $\lambda$$\lambda$ is in Joules
  $$Energy(E)=h\ast f=6.6\ast {10}^{-34}\ast f$$
  • Remember $v=\lambda \ast f$$v = \lambda *f$
  • For EM waves in vacuum , $v=c$$v = c$ , the speed of light = $\frac{3\ast {10}^{8}meters}{1second}$$\frac{3*10^{8}\ meters}{1\ second}$

• Photon activity

  • 3 Possible Results

    • Vibration of Molecule (infrared) $\to$$\rightarrow$ medium energy
    • Rotation of Molecule (microwave) $\to$$\rightarrow$ low energy
    • Breaking of Molecule (ultraviolet) $\to$$\rightarrow$ high energy

• Photoelectric Effect

  • Hit matter with light (photon)

  • Electrons only emitted (ejected from molecule) when light has a high enough frequency

    • Remember, photons are light ( as in light waves)

      • They have no mass
      • They are not "matter"

    • Electrons do have mass

      • They are matter

    • Remember , each photon has a specific energy, determined by its freqency , $E=h\ast f$$E = h*f$

    • This phenomenon proves that light comes in discrete packets

  • But notice the contradiction:

    • Frequency is only makes sense when dealing with waves
    • Particles aren't waves

  • You must "suspend your logic" when thinking about light in these different ways