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Transcript
3. The nature of light 3.1 Light as a wave •Some Greek philosophers: light emitted from the eye detects surrounding •Newton: particles (corpuscles) emitted from bodies, collected in eyes •Huygens (1678): light propagates as a wave, motion of a wave front / light ray, ray optics or geometrical optics particular wave properties of light: physical optics. •1873, Maxwell: light as electromagnetic waves. • laws of reflection and refraction (Snell’s law) may be derived using Huygens principle “Every point of a wave front may be considered the source of secondary wavelets that spread out in all directions with a speed equal to the speed of propagation of the wave.” http://www.walter-fendt.de/ph14e/huygenspr.htm wave character of light: interference phenomena (e.g. colours of oil on water, or soap films) + diffraction “travelling around corners” • Theoretical Problem: medium for light waves? • Ether? properties? suppress longitudinal waves • detect (through interference) motion of earth through ether? NO (Michelson and Morley,1887) • Einstein’s theory of special relativity: no medium required BUT accept velocity of light as maximum velocity and relativity of time and simultaneity • Maxwell’s wave equations still hold, but new challenging experimental fact, addressed by Einstein (1905, annus mirabilis). 3.2 Light as a particle: the photo-electric effect 1886-1900: photoelectric effect experiments by Hertz, Halwachs, Lennard 1897 – e- discovered by J. J. Thompson (see section 4) Photo-effect: unexplainable by classical wave nature / description of light (YF 38.3) • minimum threshold frequency of light below which no electrons are emitted (dependent on material of cathode) • Speed of emitted electrons determined by stopping voltage V0: e V0 = 1/2 m v2max, (e, charge of electron) (YF, 38.4, 38.5) VAC: potential of anode relative to cathode problems for classical physics: • stopping potential V0 independent of light intensity, although intensity is energy per unit area per unit time • why threshold frequency fmin? (intensity of a wave is independent on frequency) • V0 is a linear function of frequency Einstein, 1905 (Nobel Prize 1921) • Energy quantised (Planck) • Postulate: light consists of small energy packages (photons) E = h f = h c/λ • Photon arriving at cathode absorbed by an electron. If energy of photon higher than material dependent work function Φ, then electron escapes from surface • Higher light intensity of light: more photons → more electrons • Φ, minimum energy needed, corresponding to minimum frequency fmin Visit the photoelectric effect applet at http://media.pearsoncmg.com/aw/aw_activphysics/aw_young_physics/part6.htm and also visit http://www.lon-capa.org/~mmp/kap28/PhotoEffect/photo.htm h f e V0 h V0 f e e work function Φ, Planck constant h, frequency f, charge of electron e, stopping potential V0 Graph of V0 as a function of f gives straight line of slope h/e and intersect Φ/e. Must know e to determine h or vice versa. charge of electron determined by Millikan (1909) (see section 4.3) 1e = 1.602×10-19 C → h = 6.626×10-34 J·s today: h = 6.62606876(52) ×10-34 J·s relative error = |h2006-h1909|/h2006 = 1.04×10-5 3.3 Matter waves and “duality” • Light: particle (but rest mass zero and moving at the speed of light) and wave properties. • de Broglie,1924: matter may behave wave-like, wave length λ of a particle of momentum p= mv: λ = h/p = h/(mv) (m: mass, v: speed) • experiments1927 (Davisson and Germer): beam of electrons through nickel crystal (lattice = diffraction grating) wave length very small for macroscopic objects (about 10-34m). Not of relevance in normal day life! Compton scattering effect (1923) further evidence of quantum nature of light. X-rays when interacting with matter are sometimes observed to scatter. When they scatter they appear to have longer wavelengths, observed to be dependent upon the angle of scattering as follows: h ' (1 cos ) mc (YF 38.28) (YF 38.26) Compton scattering can only be explained if the incoming photon is treated as a particle – this photon quantum of light as “particle” is then involved in a collision with an electron – scatters off the electron at an angle – losing energy and thus shifting to lower frequency (longer wavelength) - but photon must carry momentum! (YF 38.27) •Need to ascribe momentum to a photon as follows p h • Duality, i.e. having both wave and particle properties, is at the heart of physics at the atomic and sub-atomic level, described by quantum-mechanics. • Light and matter are neither particle nor wave. Depending on experimental situation one aspect might dominate over the other. • Hard to grasp intellectually, due to lack of everyday life experience.