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 .

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

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,)

The Electromagnetic Spectrum

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 622 - 780

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.

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

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?

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.

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)

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).

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

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?

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

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

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

Metastables States Metastable states are required to maintain population inversion for ongoing stimulated emission. Electrons’ lifetime increased form 10-8 to 10-3 s.

Laser schematic

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

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

Total internal reflection http://www.youtube.com/watch?v=hBQ8fh_Fp04 Work problem 11, p. 604 in class

Homework Q. 1-11 p. 605-606 Summarize in your own words: The EM spectrum and its segments The laser operation