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lecture19 - 19. Lecture 19 19.1 Interference The principle...

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Unformatted text preview: 19. Lecture 19 19.1 Interference The principle of superposition implies than when two electromagnetic waves meet each other, the total electric and magnetic fields are the sum of the electric and magnetic field of each wave. This is quite important since, for example, the electric fields can add up or subtract depending on how they are oriented. When they are opposite and of the same strength they cancel each other and we have the very surprising property that two beams of light can cancel each other and give a dark spot. This is called destructive interference (whereas constructive is when they add up). However it does not seem to be something we see everyday!. In fact we know that two light bulbs always illuminate more than one. So what is going on?. The situation is that in a light bulb the atoms of the filament are emitting light independently, so from each light bulb we have an enormous number of pulses or wave packets that are not in phase with each other. So wave packets from different sources sometimes add up and sometimes cancel each other. In average the intensities just add up. Notice that this is not constructive interference. In constructive interference the electric fields add up. But the intensity is proportional to the square of the electric field so two beams of equal intensity give through constructive interference a beam of four times the intensity. On the other hand random interference gives just twice the intensity. In fig.107 we see the case of two wave packets interfering constructively, destructively or an intermediate case. The question is now if we can actually see interference and cancel light with light. The trick is to split each wave packet in two parts and then join them back so essentially each packet interferes with itself. How to do so we know from when we studied refraction, an incident beam is split into a reflected beam an a transmitted beam. This is the principle used in the interferometer. 19.1.1 Interferometer The interferometer (see fig.108) works by splitting a beam into two using a partially reflecting mirror (drawn in blue). The two beams are reflected from flat mirrors (drawn in green) and joined back together using the same partially reflective mirror. Part of the beam will be sent back to the source and lost. After the two beams are joined they are projected onto a screen. If the difference in the length of the two arms of the interferometer is an integer multiple of the wave-length then the interference in constructive. If it is a half-integer multiple is destructive. Otherwise we have an intermediate situation. The trick is that all wave packets emitted by the source interfere in the same way provided that the source is monochromatic, namely has a single wave – 123 – Figure 107: Two wave packets can interfere constructively or destructively. Also there are many intermediate situations as illustrated in the third plot. length. It is also important that the difference in length between the two arms of the interferometer is smaller than the typical length of the wave-packets (also known as coherence length). Otherwise a packet cannot interfere with itself. To summarize: ￿ mλ → Constructive ￿ ∆L = ￿ (19.1) 1 m + 2 λ → Destructive where m is an arbitrary integer. The wave length for visible light is of the order of 500nm so by moving the mirror so slightly one can go from constructive to destructive – 124 – interference. Not only is the interferometer a clear proof of the wave-like nature of light but it also provides a very sensitive instrument to measure length. Although this is not practical to use in everyday life, for certain physics experiments it is very useful. One such experiment which is under way is the detection of gravitational waves. Such waves would make the mirrors move and such motion can be detected. It requires great care since, for example, even small vibrations that change the position of the mirrors are enough to destroy the interference pattern. Figure 108: Interferometer. 19.1.2 Thin films Other situation where interference appears and is actually very easy to see is in thin films. For example soapy water on glass can form a thin film. It is known that when illuminated with white light such films appear colored as a rainbow (e.g. soap bubbles). The reason is that we have interference as shown in fig.109. The two reflected beams will interfere constructively or destructively. The difference in path length is 2d ∆L = (19.2) cos θ￿ – 125 – where d is the thickness of the film and θ￿ is the angle of refraction (given by Snell’s law). There is a subtlety which is that, as we are going to see later, when a beam is reflected from an interface there could be a phase shift of 180o equivalent to a halfwave length shift. The rule is that when coming from medium 1 and reflecting from an interface with medium 2: n1 < n2 → 180o phase shift n1 > n2 → no phase shift (19.3) (19.4) (19.5) where n1,2 are the indices of refraction of the two media. The other point to take into account is that the wave-length changes when going to a different medium. The wave length is related to the frequency through λ= c f (19.6) where c is the speed of light in the medium and f is the frequency which is the same in all regions (since the boundary conditions enforce that the electric and magnetic field oscillate in unison in all media). If we use λ1 the wave-length in medium 1 as a reference, the wave-length λ2 in medium two is given by λ2 = c2 n1 λ1 = λ c1 n2 (19.7) since the speed of light is inversely proportional to the refraction index. Of medium 1 is air we take n1 = 1 and obtain If n1 < n2 < n3 , or n1 > n2 > n3 then If n1 < n2 > n3 , or n1 > n2 < n3 then ￿ ￿ ∆L = ∆L = 2d cos θ ￿ 2d cos θ ￿ ∆L = ∆L = 2d cos θ ￿ 2d cos θ ￿ λ = m n2 ￿ ￿λ = m + 1 n2 2 λ = m n2 ￿ ￿λ = m + 1 n2 2 → Constructive → Destructive → Destructive → Constructive Since the angle at which there is constructive interference depends on the wavelength, different colors will be seen at different angles giving rise to the rainbow effect. The same happens for example for two flat glass surfaces separated by a thin layer of air. In fact the effect is more evident if the surfaces are not parallel but slightly at angle. Then dark and bright bands appear. An example is when a lens is positioned over a flat surface as seen in fig.111. – 126 – Figure 109: Thin-film interference. Care should be taken to include the extra 180o phase shift introduce when reflecting form a material of larger index of refraction. 19.1.3 Two slits Another experiment is the two slit experiment also knows an Young’s experiment. Light is shone toward an opaque surface with two very thin slits close to each other. A screen on the other side shows alternative bright and dark bands produces by interference. From fig.112 we see that the bright bands appear whenever2 : ∆L = d sin θM = mλ (19.8) where d is the distance between slits and θM is the angle at which we find a maximum. Also the integer m is known as the order of the maximum. The bright and dark bands are also called fringes. 2 Remember that in these formulas it is conventional to use m to denote an arbitrary integer. – 127 – Figure 110: Demo: Thin-film interference. The rainbow colors are produced by interference maxima located at different positions for different wave-lengths. – 128 – Figure 111: A thin film of air can be obtained between a lens and a flat specular surface. The interference pattern is known as Newton’s rings. – 129 – Figure 112: Young’s experiment, interference between light coming from two slits illuminated by the same source. – 130 – ...
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