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Unformatted text preview: MASSACHUSETTS INSTITUTE OF TECHNOLOGY Department of Physics 8.02 Spring 2006 Experiment 9: Interference and Diffraction OBJECTIVES 1. To explore the diffraction of light through a variety of apertures 2. To learn how interference can be used to measure small distances very accurately. By example we will measure the wavelength of the laser, the spacing between tracks on a CD and the thickness of human hair WARNING! The beam of laser pointers is so concentrated that it can cause real damage to your retina if you look into the beam either directly or by reflection from a shiny object. Do NOT shine them at others or yourself. PRE-LAB READING INTRODUCTION Electromagnetic radiation propagates as a wave, and as such can exhibit interference and diffraction. This is most strikingly seen with laser light, where light shining on a piece of paper looks speckled (with light and dark spots) rather than evenly illuminated, and where light shining through a small hole makes a pattern of bright and dark spots rather than the single spot you might expect from your everyday experiences with light. In this lab we will use laser light to investigate the phenomena of interference and diffraction and will see how we can use these phenomena to make accurate measurements of very small objects like the spacing between tracks on a CD and the thickness of human hair. The Details: Interference The picture at left forms the basis of all the phenomena you will observe in the lab. Two different waves arrive at a single position in space (at the screen). If they are in phase then they add constructively and you see a bright spot. If they are out of phase then they add destructively and you see nothing (dark spot). The key to creating interference is creating phase shift between two waves that are then brought together at a single position. A common way to do that is to add extra path length to one of the waves relative to the other. In this lab the distance traveled from source to screen, and hence the relative phase of incoming waves, changes as a function of lateral position on the screen, creating a visual interference pattern. E9-1 Look here as function of time Consider two traveling waves, moving through space: Look here as function of time Constructive Interference Destructive Interference Two Slit Interference The first phenomenon we consider is two slit interference. Light from the laser hits two very narrow slits, which then act like in-phase point sources of light. In traveling from the slits to the screen, however, the light from the two slits travel different distances. In the picture at left light hitting point P from the bottom slit travels further than the light from the top slit. This extra path length introduces a phase shift between the two waves and leads to a position dependent interference pattern on the screen....
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