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1Episode 323: DiffractionNote the spelling -double ff. The first recordedobservation of diffraction was by Grimaldi in 1665. The shadows cast by light sources were not quite the same size as the anticipated geometrical shadows. Furthermore, very close to the edges, the shadows were bordered by alternate bright and dark fringes or bands. What good observation –given the sources of light available in 1665!SummaryDemonstration: Observing the effect. (10 minutes)Demonstration: Ripple tank and/or laser. (20 minutes)Discussion: Deriving the formula. (20 minutes)Discussion: Resolution. (15 minutes)Student experiment: Resolution of spectral lines. (20 minutes)Demonstration:Observing the effectA distant light bulb viewed through the gap between the prongs of a tuning fork shows clear dark and bright bands. The effective gap can be altered by turning the tuning fork. Simply making a narrow gap between an index finger and thumb can produce the same effect.DemonstrationRipple tank and/or laserFor a larger scale demo from which to draw qualitative conclusions use either or both of:a ripple tanka single slit with laser light(The slit width can be changed by simply turning it about a vertical axis perpendicular to the beam.)TAP 323-1: Ripple tank diffractionSafetyProvided the laser is class 2 (less than 1 mW), the warning ‘Donot stare down the beam' is sufficient. Avoid specular reflections.
2Summarise findings:a pattern of intensity maxima and minima is formedthe pattern is symmetricalthe central maximum is wider than the other maxima (unlike with Young’s two slit fringes)the maxima and minima are evenly spacedthe intensity falls off with angle (distance from the central fringe)the spreading depends upon the wavelength and the gap width –less than gap width gives less spreading out; equal to gap width gives biggest effect; greater than gap width results in the wave being almost blockedenergy in the incident beam has been redistributedDiscussion:Deriving the formulaFor the minima in the diffraction pattern:sin = mdwhere mis an integer and dis the slit width.To derive this, we have to imagine that each point across the width of the slit is sending rays to every point on the screen.TAP 323-2: Diffraction at a single apertureDiscussion:ResolutionAt this point, you could discuss the idea of resolution. Resolution refers to the ability to distinguish two objects that are close together. The light from an object is diffracted by the aperture of the viewing instrument. Two neighbouring objects can be resolved provided that the peak from the central maximum of one is no closer than the first minimum of the other (and vice versa). This is called the Rayleigh Criterion.NBdAPIntensity-+