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lab39 - LAB 39:INTERFERENCE AND DIFFRACTION OF LIGHT...

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LAB# 39:INTERFERENCE AND DIFFRACTION OF LIGHT INTRODUCTION: The diffraction grating consists of a piece of glass or plastic with a large number of engraved evenly spaced parallel lines, usually 5,000 to 10,000 lines/cm. Diffraction refers to the "bending" of waves around sharp edges or corners. The slits of a grating give rise to diffraction and the diffracted light interferes so as to set up interference patterns. Interference is the effect produced when two or more wave trains are superimposed so as to either reinforce or cancel each other. The effect's magnitude depends upon the relative phases of the two waves. With light, interference occurs when two beams having identical frequencies are superimposed: darkness (destructive interference) being produced when two beams are united so that the positive vibration peaks of one match the negative vibration peaks of the other. These waves are said to be "out of phase". Conversely, a bright spot (constructive interference) is observed when two waves are matched peak to peak or "in phase". In most interference experiments the two beams are produced by splitting a single beam from a monochromatic light source to ensure that the frequencies will be the same. APPARATUS: Slit film, (large glass slide) grating, paper mm rule, Spectrum tubes, single filament lamp, 9X viewer. PROCEDURE: I. Qualitative Take the large glass slide and hold it with the emblem at the bottom left hand corner or, if it's labeled, with an "A" at the same corner. 1. Single Slits Look at the single filament lamp through the large slot at the bottom of column 'E' , from a distance of 3 to 6 meters. You should see a good quality image of the filament. Now look through the progressively narrower single slits of row 'E'. Describe what happens to the image as you go and comment on the width of the central stripe as you make the slit narrower, and on the width of any secondary stripes. Look at other spectrum tubes through the slit to assure yourself that the effects are not confined to single filament lamps! You may find one source or another more convenient in what follows.

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