1.1.2 Interference by Division of Amplitude and Plhase Change on Reflection When light reflects from an interface between two media there may n Figure 3 from a lens of radius R resting on a flat...

1.1.2 Interference by Division of Amplitude and Plhase Change on Reflection When light reflects from an interface between two media there may n Figure 3 from a lens of radius R resting on a flat plate. The two reflected rays, r1 and r2, have different phases, f1 and f2. Rny r2 travels through air through an extra distance 2? and reflects from the air. glass interface at the flat glass plate. Since r2 travels in air when it reflects off the glass plate it undergoes a phase change f.-7. This happens because the speed of light in glass is slower than in air. The phase difference between r1 and r2 is f-0-f1. Including the extra path for r2 and d-p we see we see incident light reflecting Substituting for k in equation 7 we see for destructive interference So the condition for the minima is From Figure 3 we can determine ? = R-Ros(A). Since the angle A is very small under normal observing conditions we approx- imate cos(A)-1-A2/2. Then ? RA2/2. We also note that A = Sm/R so ? = . If we use two dark rings of orders m1, m2 then we find from equation 9 mi)A (10) A schematic setup for observing and measuring Newton' rings is in Figure 4. 1.1.3 Diffraction So far we treated the sources of light as point sources. In reality, the sources may often be distributed in space. An example of a distributed source is in Figure 5 which shows a single slit of finite width w. When electomagnetic radiation impinges on the slit we will observe at a far away point, P, a distribution of intensity which is characterized by the width of the slit, w, by the wave length of light, ?, and by the angle of observation, ?. The pattern we see is a diffraction pattern. Mathematically we can treat the diffraction as an interference of many(infinite) small sources acting coherently across width w. A detailed consideration shows that the intensity distribution is given by f/2 and the minima of the intensity is given by wsin(0) nA. (12) in equation 11 ? is the angle of observation and f is called the phase angle Using optical instruments there is always an entrance aperture which acts as a source of coherent light which will be manipulated by subsequent optics. The phenomenon of diffraction limits the angular separation we can investigate. 2 Objectives 1) Observe qualitatively and quantitatively the interference pattern from double slits. 2) Observe qualitatively and quantitatively the interference pattern from multi slits. 3) Using the multi slit arrangements determine the wave length of the laser's light. 4) Observe the effect of reflection on the phase of light 5) Observe qualitatively and quantitatively the diffraction patter o rving width.
Dec 17, 2021
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