Electromagnetic waves

Transcription

1 Electromagnetic waves University of Pécs, Faculty of Medicines, Dept. Biophysics Scientists physicists, chemists, astronomers Sir Isaac Newton Sir William Herschel Johann Wilhelm Ritter Joseph von Fraunhofer Robert Wilhelm Bunsen Gustav Robert Kirchhoff Albert Einstein Louis-Victor de Broglie James Clerk Maxwell Heinrich Rudolf - Dispersion (664) - IR (800) - UV (80) - lines in the solar spectra (84) - interpretation of lines (86) - interpretation of lines (86) - light quantum (photon) (904) - matter-waves (924) - EM radiation theoretically (864) - EM radiation pragmatically (888) October 203 The light Electromagnetic spectrum Electromagnetic wave Transversal wave electric field strength - vector wavelength E B x x magnetic field strength- vector The vectors of the electric and the magnetic gradients are perpendicular to each other and to the direction of the propagation of the wave. James Clerk Maxwell (864) verified their existence theoretically. Heinrich Rudolf (888) confirmed their existence experimentally.

2 absorption The spectrum Spallation of one wave e.g. electromagnetic wave to its component frequencies. One intensity-like quantity represented as the function of an energy-like quantity. intensity, count rate (e.g. measurement of radioactivity), number of photons, transmittancy, absorbancy (extinction, OD) energy and energy-proportional quantities (e.g. frequency, wavelength, wavenumber) (nm) First law: a hot dense gas at high pressure produces a continuous emission spectrum of all colours. (Thermal radiation.) Second law: hot rarefied gas at low pressure produces an emission line spectrum (bright spectral lines in front of a dark background). Kirchhoff s Laws Third law: when light from a hot dense gas passes through a cooler gas, it produces an absorption line spectrum (bright spectrum with a number of dark, fine lines). The appearance of the spectra line-type (atoms) band (molecules) continuous (heated materials) I Line spectra (emission) of some elements He Hg Continuous emission Line-type emission Line-type absorption n Na Ne Ar Joseph von Fraunhofer ( ) Interaction of the light with matter Quanted energy uptaking (photon) Interaction of electromagnetic wave with atomic system (matter): reflection absorption transmission (scattering) 2

3 Electric energy levels of the atoms Bohr- and the quantummechanical atom model Postulates:. Electrons can only circle around the nucleus at definite levels (does not emit or absorb energy) stationary levels (unchanging). Energy level system of molecules 2. Atoms absorb or emit radiation only when the electrons abruptly jump between the different stationary levels, states. Important physical quantities and relations Frequency: n or f (/s) v = λ f Wavelength: (m) c v n = c / v Wavenumber: n (cm - ) Energy: E (J) h. f Einstein: energy of a photon (light-quantum) Extinct. coeff.: (M - cm - or (mg/ml) - cm - ) The dual nature of the light Region Wavelength range (mm) Wavenumber range (cm - ) Near Middle Far Wave (propagation) Diffraction Interference Polarization Particle (interaction) photoeffect Compton-effect The most useful I.R. region lies between cm -. Albert Einstein (905) : photoelectric effect photon (light quantum), its energy: E = h n (or E = h f) Louis-Victor de Broglie (924) : Matter-waves theory (All materials have wave nature as well.) λ = h/p, where p is the impulse => λ = h/m v 3

4 Huygens-Fresnel principle. All points on a wave front can be considered as point sources for the production of spherical secondary wavelets. 2. The interference of the secondary wavelets determines the further behaviour of the wave. a x s Interference s2 a sin To achieve max. gain: a sin n To achieve max. weakening: a sin ( n 2) Linearly polarized light Linearly polarized light Polarization The dual nature of the light Wave (propagation) Diffraction Interference Polarization Particle (interaction) photoeffect Compton-effect 4

5 absorption Photo- and Compton-effect, pair production Spectroscopy Spectra: The distribution of the intensity of the electromagnetic wave in terms of wavelength. (Greek: picture, colour) -scopo-, scop-, scept-, skept-, -scope-, -scopy, scopia, -scopic, -scopist Greek: see, view, sight, look at, examine (nm) Studies with EM radiations (e.g. light) Types and methods of spectroscopy. Spectroscopy of electric (atomic) energy levels Intensity - wavelength (frequency): VIS, IR, UV, Röntgen, Raman, Mössbauer, ESR, NMR, CT, MRI... Lifetimes of energy states: fluorescence/phosphorescence lifetime Polarisation (anisotropy): anisotropy decay, CD-spectroscopy 2. Spectroscopy of radioactivity (α-, β-, γ-particles, neutron, neutrino)... The purposes of the spectroscopy Qualitative and/or quantitative cognition of matter: Analysing the quality ( finger-print ) Analysing the quantity (intensity) Structural information (conformation) To follow the time scaled change of matter: (time-resolved spectroscopy) Changes of chemical constitution (e.g. under chemical reaction). Structural changes (acceptable for fast kinetic measurements) We can not see the molecule, but on the basis of the (change of the) spectrum and with the help of our physical knowledge we can implicate its structure. 5