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4. Investigations into the electrical properties of particular metals at different temperatures led to the identification of superconductivity and the.

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Presentation on theme: "4. Investigations into the electrical properties of particular metals at different temperatures led to the identification of superconductivity and the."— Presentation transcript:

1 4. Investigations into the electrical properties of particular metals at different temperatures led to the identification of superconductivity and the exploration of possible applications

2 Outline the methods used by the Braggs to determine crystal structure The Braggs used the diffraction of X-rays from a regular crystal to determine its structure When sources vibrate with same frequency and with a constant phase difference, you get interference effects. These are produced by constructive and destructive interference of the waves. Complete constructive interference occurs when waves are in phase at the same point. This happens when the difference (  D) in the length of paths travelled by each wave is a whole number of wavelengths  D=n For light (wavelength  that passes through two slits separated by a distance d, The constructive and destructive interference of the light from the slits produces maxima (bright) and minima (dark) areas in an interference pattern on a screen  D = n  d sin , where  is the angle to the 1st bright spot n  yd/L where y  is the distance to the 1st bright spot and L is the distance from the slits to the screen produced at the anode

3 Outline the methods used by the Braggs to determine crystal structure Diffraction is the spreading out of light (or any electromagnetic) waves around the edge of an object or when light passes through a small aperture. Interfering waves produce a DIFFRACTION PATTERN. The effects increase as the size of the aperture approaches the wavelengths. A diffraction grating contains a large number of slits, the more slits, the clearer the diffraction pattern. A standard optical diffraction grating does not have lines close enough to diffract X- rays because the wavelength is so small. A crystal lattice DOES have spacing which is small enough to produce a diffraction pattern from X-rays.

4 Outline the methods used by the Braggs to determine crystal structure X-ray tube CollimatorCrystal Photographic Screen They proposed that X-rays could penetrate the surface of matter and ‘reflect’ from the atomic lattice planes within the crystals The relationship between angle of reflection (  ), wavelength of radiation( ) and spacing of planes(d) was determined, providing a useful tool for studying crystal atomic structure & X-ray spectra. n  d sin  Maxima come from ‘in phase’ scattering when the distances to and from adjacent layers differ by whole multiples of wavelengths. The rays scattered from adjacent planes and interfered in a form of diffraction to produce maxima in specific directions.

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