Lec1_Spectroscopy_ML - Light is an integral part of life...

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Biophysical Methods Slide 1 Spectroscopy-1 Light is an integral part of life and of biophysics The Nature of Light: Waves or particles ? Socrates : particles emitted by the eyes and make contact with objects 17th century : Newton particles 19th century : Fresnel, Maxwell. .. waves 1900s : Planck, Einstein particles 1920s : Quantum mechanics no problem - kind of both Biophysical Methods Slide 2 Spectroscopy-1 Light as a particle Einstein 1905: Light is made of unbreakable “quanta” of energy. The particles (“photons”) possess a momentum p=h/ ! . Photons are particles with an energy proportional to the light frequency (and inversely proportional to the wavelength) E = h " = h c / ! ! = c / " " = c / ! E = energy ! = wavelength " = frequency h = Planck’s constant = 4 x 10 -15 eV seconds = 6.6 x10 -34 Joule seconds They move with velocity c = speed of light = 3 x 10 8 m/s
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Biophysical Methods Slide 3 Spectroscopy-1 Light as an electromagnetic wave x z y E and B fields are perpendicular B y is in phase with E x 2 2 2 2 x x o o E E z t ! μ " = E B k (plane wave solutions) time varying B-field induces E-field time varying E-field induces B-field 2 0 I E ! 2 * * * 0 x x y y z z E E E E E E = + + E Biophysical Methods Slide 4 Light as an electromagnetic wave Note - in most materials used in optics, ! o ~ ! so n is well approximated by But light travels slower in matter : The frequency of the light wave is unchanged, but the wavelength is reduced. The velocity of any electromagnetic wave in vacuum is: c = 1 0 0 μ 0 = 4 " x 10 -7 N s 2 C -2 , permeability of fee space , permittivity of free space 0 = 1 0 c 2 = 8.854. . # 10 $ 12 C 2 % N $ 1 % m $ 2 u = "# n , u n $ 1 % $ % c n = c u = 0 0 n " # 0
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Biophysical Methods Slide 5 Spectroscopy-1 Light as rays For reflection and refraction light is conveniently treated as rays Incident ray reflected ray Refracted ray # a # r # b # r = # a Biophysical Methods Slide 6 Spectroscopy-1 Light interactions with matter When light interacts with matter it can be: (1) Transmitted (no interaction) (2) Scattered (3) Absorbed - energy of molecule or atom is temporally increased 0 0 , I ! 0 , I 0 0 : , : , s s s s Rayleigh scattering I Raman scattering I " # Transmission: 0 I T I =
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Biophysical Methods Slide 7 Spectroscopy-1 Biological photodetectors in the eye Use light absorption Biophysical Methods Slide 8 Spectroscopy-1 Principles of light absorption Electromagnetic radiation (infrared, visible light, UV, x-rays) mainly interacts with matter via the electrons of atoms and molecules. The electrons can only occupy certain well-defined energy levels. Thus only light quanta (photons) with an energy equal to the energy difference between the initial state of the electron and some higher possible state, can be absorbed. The energy (E) of the light quantum is related to the light wavelength : , E = hc " = h # with c =speed of light and =c/ = frequency of light wave, h =Planck’s constant: =6.6252 10 -34 Js. E=h !
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Biophysical Methods Slide 9 Spectroscopy-1 Foundations of spectroscopy Assume a thin cuvette of thickness dx containing a solution of light absorbing molecules.
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Lec1_Spectroscopy_ML - Light is an integral part of life...

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