Physics 2DL Lectures Vivek Sharma Lecture # 3 Discussion of Experiments Franck-Hertz Experiment : A prelude Bohr Atom : Discrete orbit Emission & Absorption line classical Bohrs quantization Franck Hertz Experiment: Playing Football ! Inelastic scattering of electrons Confirms Bohrs Energy quantization Electrons ejected from heated cathode At zero potential are drawn towards the positive grid G. Those passing thru Hole in grid can reach plate P and cause Current in circuit if they have sufficient Kinetic energy to overcome the retarding Potential between G and P Tube contains low pressure gas of stuff! If incoming electron does not have enough energy to transfer =E 2 -E 1 then Elastic scattering, if electron has atleast KE= then inelastic scattering and the electron does not make it to the plate P Loss of current (J) Franck & (G) Hertz Experiment Current decreases because many Electrons lose energy due to inelastic Scattering with the Hg atom in tube And therefore can not overcome the Small retarding potential between G P The regular spacing of the peaks Indicates that ONLY a certain quantity Of energy can be lost to the Hg atoms =4.9 eV. This interpretation can be confirmed by Observation of radiation of photon energy E=hf=4.9 eV emitted by Hg atom when V 0 > 4.9V Atomic Spectra Propagation of Plane Wave in Vacuum : Huygens Coherence of Light: if 2 light waves meet at a point are to interfere perceptibly, then the phase difference between them must remain constant with time; that is waves must be coherent. Degree of coherence of a source of light is the degree to which light consists of long, unbroken trains (packets) of Sinusoidal waves Coherent Sources : Laser, radiating atom (but they all have a spread of wavelengths) Incoherent Source : Light bulb Huygens theory of Wave propagation allows us to tell the future location of wave front All points on a wave front serve as a point source of spherical secondary wavelet. After a time t, the new position of wavefront will be that of a surface tangent to these secondary wavelets Wave packet Diffraction Phenomenon If a wave encounters a barrier that has an opening of dimensions similar to the, the part of the wave that passes thru opening will spread out (diffract) into a region beyond the barrier (spreading of Huygens wavelets) Narrower the slit, larger the diffraction Diffraction limits geometrical optics (ray tracing) Youngs Double Slit Interference Experiment Condition for constructive Interference : Overlapping waves must have same phase So the path lengths traversed by the two waves must satisfy L= d sin = m (m=0,1,2,3..) Destructive Inter. at the screen when 2 waves exactly out of phase d sin = (m+1/2) (m=0,1,2,..) With this simple idea, Young could measure the average wavelength of the sun (555nm) ! diffraction Michelsons Interferometer Interferometer: device to measure lengths or changes in lengths with great accuracy by means of interference fringes (big daddy of them all was designed by Michelson in 1881first American Nobel prize 1907) How it works: Light from source at P encounters beam splitter Beam splitter transmits and reflects of incident The 2 waves now head towards M1 and M2 mirrors Get reflected entirely and sent back along direction of incidence and then deflected towards telescope T Observer at T sees a pattern of zebra strip like fringes Path length when 2 waves combine at telescope=2d 2 2d 1 anything that changes this path diff will cause change in phase diff between two waves at the eye. E.g. If mirror M1 moves by then changes by and fringe pattern shifted by 1 (max min) Single Slit Diffraction Condition for First Minimum (a/2) sin = Diffraction Grating: Mechanism & Intensity Distribution Similar to double-slit except many more slits (ruling)~ 1000 Monochromatic light thru grating forms narrow interference Fringes(lines) that can be analyzed to determine of light If d = grating spacing, show that d Sin m ( condition for maxima) Width of line = / (N d Cos ) A Grating Spectrograph NaCl & X-ray Diffractio : Orientation Important! n=2dsin Bragg Scattering of X-Ray Light Electron Diffraction : Davisson Germer Expt Diffraction Pattern in Polycrystalline Al target Diffraction pattern produced by 600eV electrons incident on a Al foil target E.Coli Seen With Electron Microscope