4 - Experiment 4: Refraction and Interference with...

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Experiment 4: Refraction and Interference with Microwaves 1 Experiment 4: Refraction and Interference with Microwaves Introduction Many phenomena whose study comes under the heading of "physical optics" arise from certain integral relationships between the wavelength of the radiation and dimensions of the experimental apparatus. This experiment involve such ratios, these studies will be done with microwaves, i.e., very short radio waves having wavelengths of a few centimeters, rather than with visible light, where changes of system dimensions over a few wavelengths are very difficult to produce and control, and quite impossible to see directly. This experiment will take advantage of interference patterns (constructive interference to be precise) in order to determine fundamental values of both the microwave and the mediums through which it will travel. 1 Physics 1.1 Electromagnetic Radiation The term electromagnetic radiation encompasses a wide variety of radiative phenomena that seem at first to be quite different from one another. For example, radio waves, visible light, and the “gamma rays” which emanate from radioactive substances all fall into this category. In fact, they are manifestations of the same process, i.e., they all are propagating waves of electric and magnetic field energy. The only way in which they differ from each other is in their wavelengths (and frequencies). All electromagnetic radiation propagates at a velocity c = 3 x 10 8 m/sec in vacuum. Thus, while radio waves may have wavelengths of several thousand meters, light waves 5 x 10 -5 cm and γ -rays 10 -10 cm or shorter, they are the same kind of wave traveling at the same velocity (in vacuum). The phenomena of reflection and refraction of electromagnetic waves are particularly familiar in the instance of visible light, where our means of detection is so very good. Reflection, of course, is what makes most objects visible to us, and refraction is responsible for the functioning of eyeglass and camera lenses. Refraction is also responsible for the distortion of the image of an object when viewed through a glass of water. 1.2 Index of Refraction and Snell’s Law To discuss these processes in more detail, we must establish the notion of refractive index. This quantity, usually labeled n , is simply the factor by which an electromagnetic wave is slowed upon entering a material medium from a vacuum region. In other words, the velocity of the waves in this refractive medium is: v = c n (1) Now, it is evident that the number of waves per second incident from a vacuum region onto the surface of a refracting medium is the same as the number per second entering that medium, i.e., the
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Experiment 4: Refraction and Interference with Microwaves 2 frequency ( f ) of the waves must be the same in both regions. Otherwise, there would be a pile-up of waves formed at the interface. Therefore, because we know that f !
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4 - Experiment 4: Refraction and Interference with...

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