# lab_04 - PH222 Laboratory O4 Single and Double Slit...

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PH222 Laboratory O4 Single and Double Slit Diffraction of Light Alyssa Woods ___________ Thursday _2:00-4:50___________ Group Members: Ryan Mejia

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Purpose: To determine the slit width for a single slit and the separation between the two slits for a double slit. Introduction : The wave nature of light was conclusively demonstrated by Thomas Young in the year 1801 when he was able to make white light diffract by passing a sunbeam through a pair of tiny slits. This phenomena, a consequence of the Superposition Principle which states the result of two wave forms superimposed on each other is the algebraic sum of the two is illustrated below: In this experiment, we will use a laser to pass through a double slit and single slit to produce a series of bright and dark spots where constructive and destructive interference take place. Double Slit Diffraction – Double slit diffraction occurs when a plane wave of light strikes a pair of slits separated by a distance “d.” The size of the double slits (openings) has to be on the order of or smaller than the separation between the two openings in order for us to observe double slit diffraction phenomena. Graphically: As the diffracted light waves spreads out, we will begin to see the two “wavelets” interfere with each other. When the difference in path length (we use the Greek symbol “ δ ” or “del” to signify path difference) that the two wavelets travel is equal to an integer number of
wavelengths, we get constructive interference. When δ is ½ integer number of wavelengths – we get destructive interference. If we put a screen on the wall a distance “L” away, we can see the diffraction pattern (series of light and dark spots) caused by the interference of the wavelets formed by the double slits. To find the location of the bright spots: δ = d sin bright =mt; where m=0, + 1, + 2,… Equation (1) where δ is the path difference, d is the distance between the slits, and bright is the angle from a point normal to the double slits to the bright spot. Note that for small angles o, we can approximate sin bright as sin bright = y bright /L Equation (2) where y bright is the height from the center bright spot to the next mode (bright spot) and L is the distance from the double slit to the screen, m is the mode (an integer), f is the wavelength of light illuminating the slits. A more convenient form may be to find the height “y bright ” above the center bright spot to the higher order bright spots – substituting Equation (3) into Equation (2), we get y bright = Lm/d where m=0,+ 1, + 2… Equation (4) Solving Equation (4) for d, the spacing between the slits, we get Equation (5) d= Lm/y bright Equation (5) By setting up a diffraction pattern from a double slit apparatus using a laser of known wavelength, and measuring the distances to the bright spots and the distance from the double slit to the screen, we can calculate the distance “d” between the slits. Single Slit Diffraction

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lab_04 - PH222 Laboratory O4 Single and Double Slit...

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