1. Origin of The Electromagnetic (EM) Waves
EM Waves are produced by:
The acceleration (oscillation/vibration) of charged particles (electrons, protons)
The transition of molecules, atoms, or atomic nuclei to lower energy states
The nuclear reaction in the Sun: visible and invisible EM waves
Burning fire emits thermal radiation, which is also a source of EM waves.
Electrons in atoms/molecules transiting from higher to lower energy states  emission of EM radiation .

2. The Nature of EM Waves
EM waves are the result of two oscillating fields at right angle, the electrical and the magnetic fields.
Electrical field
Electromagnetic waves were the results of the unification of the electrical and magnetic fields
Magnetic field
Electromagnetic Waves are radiant energy, travelling outward in all directions from its source

3. The Nature of EM Waves EM waves are transverse waves.
All EM waves travel in vacuum with the same speed: the speed of light.
Unlike sound waves, EM waves do not need a material medium o travel.
The speed of light in vacuum is constant and is:
c = 3 × 108 m/s
If EM waves travel that fast how can this speed be measured? (Michelson’s exp. 1905,)

4. The Electromagnetic Spectrum

5. Wavelengths and Frequencies of Visible Light
The wave speed c in terms of wavelength λ and frequency f is given by: c = λf
Light Colour
Wavelength range (nm)
Violet
390 – 455
Blue
455 – 492
Green
492 – 577
Yellow
577 – 597
Orange
597 – 622
Red

6. Properties of EM Waves: Dispersion
Dependence of the index of refraction, n, of a material on the wavelength (or frequency) of the EM wave.
Recall: Index of refraction, n = c(vac) /v (med)
The index of refraction (slightly) decreases as the wavelength increases.

7. Dispersion of Light
Dispersion of light uses a prism to spread or split white light into its colour components or spectrum
Which colour has the highest index of refraction?
Index of refraction depends not only the medium but also on the EM wavelength

8. Properties of EM Waves: Scattering
Scattering of light IS NOT the refection of light
Scattering is the result of the interaction of EM radiation with particles  absorption and reradiation of light by matter (i.e., gas, liquid, free particles)- no change in frequency.
The shorter the wavelength the higher the scattering 
α 1/λ4 Lord Rayleigh’s scattering.
What is the colour of the sky? Why?

9. Properties of EM Waves: Absorption
Photons with energy equal to the difference between energy levels are absorbed by matter, exciting its molecules/atoms to higher states.

10. Absorption IR and UV are commonly EM radiation absorbed by gases
Greenhouse gases such as CH4, CO2 and H2O have energy levels that differ by amounts comparable to infrared photon energies. These means IR gets absorbed by these gases leading to the greenhouse effect.
Ozone (O3) absorbs UV radiation (harmful radiation)

11. Absorption
Absorption of EM radiation by matter moves electrons to higher energy states:
temperature increase (green house effect for gases
fluorescence upon sudden emission of light (10-7 s)
phosphorescence upon emission of light in much longer time (much longer lifetime).

12. Properties of EM Waves: Transmission
Transmission is the transfer of EM radiation from one medium to another.

13. Lasers Light Amplification by Stimulation Emission of Radiation
Lasers are very orderly form of light produce very strong beams of light, which are:
Highly directional  all rays move in only one direction (e.g., laser pointers)
Monochromatic  rays have same colour  about same wavelength or frequency.
Coherent  all photons are in phase and stay in phase.
Is incandescent light coherent? Why?

14. Principle of Lasers It’s all about electrons in the atom
Electrons in the atom move from one energy level to another, depending on energy gain or loss

15. Stimulated Emission
Stimulated emission is the basis of laser operation A photon stimulates the electron to transit from higher level to a lower one The 2 photons are in phase

16. Population Inversion
To maintain the stimulated emission process in action more atoms in the excited states than in the ground states are needed  this is called population inversion this is achieved by a pumping process

17. Metastables States
Metastable states are required to maintain population inversion for ongoing stimulated emission.
Electrons’ lifetime increased form to 10-3 s.

18. Laser schematic

19. Principle of a He-Ne Laser
Ground States
Excited States
Metastable States
Absorption
Spontaneous emission Ne He
1.96 eV
20.66 eV
20.61 eV
Stimulated emission
18.70 eV

20. Lasers Applications Lasers are used everywhere:
Medical applications (treatment/application tools)
Communications (optical fibres transmission)
Military (laser guided weapons)
Industry (welding, cutting, controls, readers)
Computers, printers, bar code readers
Electronics (DVD players/recorders, game stations)
Measuring devices

21. Total internal reflection
Work problem 11, p. 604 in class

22. Homework
Q p
Summarize in your own words:
The EM spectrum and its segments
The laser operation