Electromagnetic waves

Transcription

1 Lecture 21 Electromagnetic waves Atomic Physics Atomic Spectra Lasers Applications

2 Electromagnetic Waves Electromagnetic Waves composed of electric and magnetic fields can be created by an oscillating charge

3 Electromagnetic Waves EM waves can be created by an oscillating charge A moving charge creates both an oscillating electric and magnetic field Q Stationary charge showing electric field lines Q Oscillating charge field lines follow charge EM waves created Q Charge stationary but EM waves continue to move away

4 Electromagnetic Waves Electromagnetic spectrum 10 1 Electromagnetic waves; unlike mechanical waves do not require material substance for propagation can travel in vacuum All EM waves (e.g. radio and X-rays) belong to the same class: only difference is frequency All EM waves travel at the same speed in vacuum c = 3.0x10 8 ms -1 Characteristic of all waves is that they can interfere constructively or destructively when superimposed

5 Electromagnetic Waves Example Lightening strikes 10 km away. (a) How long after the strike will you see the light? (b) How long after the strike will you hear the sound? (a) c = 3x10 8 m/s, s = 10 km, t =? s = vt t = s/v t = (10,000 m)/(3x10 8 m/s) = 3.3x10-5 s lightening flash almost instantaneous Speed of sound in air is 344 m/s (b) v = 344 m/s, s = 10 km, t =? s = vt t = s/v = (10,000 m)/(344 m/s) = 29 s

6 Electromagnetic Waves example If light has a frequency of 1.94x10 14 Hz what is its wavelength? c = f f = 1.94x10 14 Hz c = 3x10 8 m/s,, =? = (3x10 8 m/s)/(1.94x10 14 Hz) = 1.55x10-6 m What is significant about this wavelength?

7 Electromagnetic Waves Wave nature of light First proof---thomas Young 1801 Beams obtained by passing sunlight through two closely spaced narrow slits Superimposed two light beams and saw constructive and destructive interference Interference pattern (bright & dark regions) Slit widths x 1 x2 laser x 2 = x 1 + n constructive interference (bright) x 2 = x 1 + (n+½) destructive interference (dark) where n is an integer

8 Electromagnetic Waves Radio waves Wavelength: 1 m Marconi : Nobel Prize in 1909 for contributions to the development of wireless telegraphy Microwaves Wavelength: 1 cm Applications Radar systems Communications -Mobile phone networks Microwave ovens

9 Electromagnetic Waves Infrared radiation Uses Heat transfer by radiation Spectroscopy Night vision Wavelength: 1 mm - 1 m Cat Visible waves Wavelength: 400 nm nm

10 Electromagnetic Waves Ultra Violet Wavelength: 10 nm nm Characteristics Reacts with the skin to cause tanning, sunburn, and skin cancer Can be used to sterilize (kills microorganisms) Mostly absorbed by the ozone layer Disinfection penetrates cell walls and disrupts the cell s genetic material, impairs reproduction Optimum UV wavelength range to destroy bacteria is between 250 nm and 270 nm.

11 Electromagnetic Waves X rays Wavelength: Characteristics 0.1 nm - 1 nm Biologically hazardous Used in medical diagnostics and materials testing Wilhelm Roentgen Nobel Prize in 1901 for the discovery of x-rays Gamma Rays Wavelength: 0.01 nm Characteristics Produced in the nuclei of atoms (stars, nuclear reactors, nuclear bombs) Biologically hazardous Used in medical diagnostics and therapeutics

12 Atomic Physics Study of atoms and the physical principles underlying their characteristics Atomic Spectra Nature of a substance can be studied by measuring the intensity and wavelengths of radiation coming from it Hot neon gas emits wavelengths that give it a red appearance Gold illuminated with white light appears yellow due to wavelengths absorbed and reflected

13 Atomic Spectra Cool solids illuminated by white light Object s color is determined by absorbed wavelengths Hot Solids Emits infrared and visible light Spectrum is related to the object s temperature Hot Gases Atomic spectra EM radiation emitted by atoms and molecules Presence of spectral lines - a few strongly emitted frequencies Other frequencies are completely absent

14 Atomic Physics Structure of the atom 1911 Ernest Rutherford discovered that the nucleus is extremely small and dense 1913 Bohr proposed planetary model of the atom based on Rutherford s results Nobel Prize in 1922 for investigation of the structure of atoms and of the radiation emanating from them es/chemistry/laureates/1922/

15 Atomic Physics Planetary Model of the Atom Dense nucleus at center Nucleus made of neutrons and protons Has positive charge Electrons orbiting the nucleus Only certain electron orbits allowed + Energy of electron determined by orbit in which it resides

16 Atomic Physics Atomic Spectra Explained Generation of a photon: - Excited atom - Energy Levels + excited state absorption photon + - Emitted photon Ground state Generation of a photon: Electrons elevated to a higher orbit when atom absorbs energy Electron falls back to lower orbit due to attractive forces from positively charged nucleus Energy absorbed (difference in energy between two levels) is emitted Can be emitted as a photon of EM radiation

17 Atomic Physics Quantization of orbits: Only certain electrons orbits allowed Difference in energies Energy absorbed or emitted when electron changes orbit E E E i f Energy of emitted photon E E E hf i where h is Planck s constant = 6.6x10-34 J.sec and f is the frequency of the EM radiation f Units of energy Energy (Joules) = qv Energy (electron volt) = ev Charge on an electron = 1.6 x10-19 C 1eV = 1.6 x10-19 C x 1volt =1.6 x Joules 1eV =1.6 x Joules

18 Exercise From the energy level diagram below calculate the frequency and wavelength of the photons emitted and identify the type of radiation excited state photon Energy = 7.4 ev i Ground state f Energy = 0.0 ev E E E hf h = 6.63*10-34 Js E 7.4 ev 0.0 ev 7.4 ev Convert to Joules f ev J J J Js Hz c f 8 1 c 3 10 ms m 166 nm 15 f Hz uv light

19 Fluorescence Absorption Direct De-excitation Atom: Excited state The state of an atom that has absorbed energy Excited atoms eventually de-excite Absorbed energy is re-emitted Typically emitted as a photon Atom returns to the ground state Can return directly Can return in a series of smaller steps Fluorescence Different energy emitted than was absorbed

20 Applications Fluorescence Fluorescent light bulb Filled with gas Current passed through the gas Atoms of gas are excited Atoms de-excite by emission of UV radiation Fluorescent material coated on inside of tube absorbs UV radiation and emits visible light Substance identification Shine a UV light on minerals Certain minerals fluoresce Emit visible light Colour of light emitted indicates material

21 Laser Atomic transitions Electron energy levels, allowed states E 4 E 3 E 2 Excited states E 1 E 0 Ground state Atom: ground state Photon energy E=hf (energy absorbed) electron E 1 hf E E 0 Before excitation Atom: excited state E 1 E 0 After excitation

22 Laser Atomic transitions Spontaneous emission (10-8 sec) Atom: excited state Atom: ground state E 1 E E 1 hf = E E 0 Before de-excitation E 0 After de-excitation Stimulated emission Atom: excited state Atom: ground state hf = E E 1 E E 1 hf hf before E 0 after Excited atom returns to ground state and hence emits a 2 nd photon of the same energy E 0

23 Both photons are in phase and have the same energy (colour) (wavelength) Both photons can stimulate other atoms to emit photons that in turn stimulate the emission of more photons. Ordinarily more atoms in the ground state than excited state so there is a net absorption of energy Population inversion However if there are more atoms in the excited state than the ground state a net emission will take place Mirror Laser Laser Mirror 100% reflectivity Energy input 98% reflectivity Acronym: Light Amplification by Stimulated Emission of Radiation

24 Laser Typical Characteristics Collimated beam (uni-directional) Single wavelength in the uv, visible or infrared Intense beam Applications Check-out scanners CD $ DVD players Pointers Printers Eye surgery (reshaping cornea) Cuts tissue (burns tumours) Cuts metal Cuts patterns (many layers of cloth at once) Telecommunications (sent down optical fibres) Dentistry

25 Laser Dental Applications Laser Drill Replace turbine drill Preparation for fillings Eliminate local anesthetic injection Capable of killing bacteria located in a cavity No vibration Laser: (Er: YAG) Wavelength 2940nm light of this wavelength highly absorbed by water Laser beam preferentially absorbed by decayed tissue because of large water content compared with healthy enamel Result: selective ablation of decay, Conservation of healthy tooth no increase in pulp temperature Not suitable for removing amalgam fillings

26 Laser Dental Applications Early detection of caries Consequence of Water fluoridation Harder enamel Good resistance to decay Early detection of cavities more difficult Near-infrared laser-induced reflected fluorescence can detect early sub-surface decay Optical Coherence Tomography (OCT) use infrared laser light High resolution ( m) 3D images View inside of teeth and gums in real time

27 Laser Dental Applications Restorative materials rapidly cured (set) Reshape gum tissue (reduce prominence) Teeth whitening Oxidizing agents such as hydrogen peroxide or carbamide peroxide Laser aided teeth whitening Laser light used to activate and accelerate bleaching process