Interference Diffraction And Polarization Of Light Pdf
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- Interference, Diffraction and Polarization of Light
- Optics Express
- Microscopy U - The source for microscopy education
The formation of an image in the microscope relies on the complex interplay between two critical optical phenomena: diffraction and interference. Light passing through the specimen is scattered and diffracted into divergent waves by tiny details and features present in the specimen. Some of the divergent light scattered by the specimen is captured by the objective and focused onto the intermediate image plane, where the superimposed light waves are recombined or summed through the process of interference to produce a magnified image of the specimen. The seemingly close relationship between diffraction and interference occurs because they are actually manifestations of the same physical process and produce ostensibly reciprocal effects. Most of us observe some type of optical interference almost every day, but usually do not realize the events in play behind the often-kaleidoscopic display of color produced when light waves interfere with each other.
Interference, Diffraction and Polarization of Light
In physics , interference is a phenomenon in which two waves superpose to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive interference result from the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency. Interference effects can be observed with all types of waves, for example, light , radio , acoustic , surface water waves , gravity waves , or matter waves.
The resulting images or graphs are called interferograms. The principle of superposition of waves states that when two or more propagating waves of the same type are incident on the same point, the resultant amplitude at that point is equal to the vector sum of the amplitudes of the individual waves.
If a crest of one wave meets a trough of another wave, then the amplitude is equal to the difference in the individual amplitudes—this is known as destructive interference. If the difference between the phases is intermediate between these two extremes, then the magnitude of the displacement of the summed waves lies between the minimum and maximum values. Consider, for example, what happens when two identical stones are dropped into a still pool of water at different locations.
Each stone generates a circular wave propagating outwards from the point where the stone was dropped. When the two waves overlap, the net displacement at a particular point is the sum of the displacements of the individual waves.
At some points, these will be in phase, and will produce a maximum displacement. In other places, the waves will be in anti-phase, and there will be no net displacement at these points. Thus, parts of the surface will be stationary—these are seen in the figure above and to the right as stationary blue-green lines radiating from the centre.
Interference of light is a common phenomenon that can be explained classically by the superposition of waves, however a deeper understanding of light interference requires knowledge of wave-particle duality of light which is due to quantum mechanics. Prime examples of light interference are the famous double-slit experiment , laser speckle , anti-reflective coatings and interferometers. Traditionally the classical wave model is taught as a basis for understanding optical interference, based on the Huygens—Fresnel principle.
The above can be demonstrated in one dimension by deriving the formula for the sum of two waves. The equation for the amplitude of a sinusoidal wave traveling to the right along the x-axis is. Suppose a second wave of the same frequency and amplitude but with a different phase is also traveling to the right. The two waves will superpose and add: the sum of the two waves is. A simple form of interference pattern is obtained if two plane waves of the same frequency intersect at an angle.
Interference is essentially an energy redistribution process. The energy which is lost at the destructive interference is regained at the constructive interference. Assuming that the two waves are in phase at the point B , then the relative phase changes along the x -axis. The phase difference at the point A is given by. Constructive interference occurs when the waves are in phase, and destructive interference when they are half a cycle out of phase. Thus, an interference fringe pattern is produced, where the separation of the maxima is.
The fringes are observed wherever the two waves overlap and the fringe spacing is uniform throughout. A point source produces a spherical wave. If the light from two point sources overlaps, the interference pattern maps out the way in which the phase difference between the two waves varies in space.
This depends on the wavelength and on the separation of the point sources. The figure to the right shows interference between two spherical waves. The wavelength increases from top to bottom, and the distance between the sources increases from left to right. When the plane of observation is far enough away, the fringe pattern will be a series of almost straight lines, since the waves will then be almost planar. Interference occurs when several waves are added together provided that the phase differences between them remain constant over the observation time.
It is sometimes desirable for several waves of the same frequency and amplitude to sum to zero that is, interfere destructively, cancel. This is the principle behind, for example, 3-phase power and the diffraction grating. In both of these cases, the result is achieved by uniform spacing of the phases. It is easy to see that a set of waves will cancel if they have the same amplitude and their phases are spaced equally in angle. A diffraction grating can be considered to be a multiple-beam interferometer; since the peaks which it produces are generated by interference between the light transmitted by each of the elements in the grating; see interference vs.
The intensity of the light at a given point is proportional to the square of the average amplitude of the wave. This can be expressed mathematically as follows. The displacement of the two waves at a point r is:. If the two beams are of equal intensity, the maxima are four times as bright as the individual beams, and the minima have zero intensity. The two waves must have the same polarization to give rise to interference fringes since it is not possible for waves of different polarizations to cancel one another out or add together.
Instead, when waves of different polarization are added together, they give rise to a wave of a different polarization state. The discussion above assumes that the waves which interfere with one another are monochromatic, i. This is not, however, either practical or necessary.
Two identical waves of finite duration whose frequency is fixed over that period will give rise to an interference pattern while they overlap. Two identical waves which consist of a narrow spectrum of frequency waves of finite duration but shorter than their coherence time , will give a series of fringe patterns of slightly differing spacings, and provided the spread of spacings is significantly less than the average fringe spacing, a fringe pattern will again be observed during the time when the two waves overlap.
Conventional light sources emit waves of differing frequencies and at different times from different points in the source. If the light is split into two waves and then re-combined, each individual light wave may generate an interference pattern with its other half, but the individual fringe patterns generated will have different phases and spacings, and normally no overall fringe pattern will be observable.
However, single-element light sources, such as sodium- or mercury-vapor lamps have emission lines with quite narrow frequency spectra. When these are spatially and colour filtered, and then split into two waves, they can be superimposed to generate interference fringes. A laser beam generally approximates much more closely to a monochromatic source, and thus it is much more straightforward to generate interference fringes using a laser. The ease with which interference fringes can be observed with a laser beam can sometimes cause problems in that stray reflections may give spurious interference fringes which can result in errors.
Normally, a single laser beam is used in interferometry, though interference has been observed using two independent lasers whose frequencies were sufficiently matched to satisfy the phase requirements. It is also possible to observe interference fringes using white light. A white light fringe pattern can be considered to be made up of a 'spectrum' of fringe patterns each of slightly different spacing.
If all the fringe patterns are in phase in the centre, then the fringes will increase in size as the wavelength decreases and the summed intensity will show three to four fringes of varying colour. Young describes this very elegantly in his discussion of two slit interference. Since white light fringes are obtained only when the two waves have travelled equal distances from the light source, they can be very useful in interferometry, as they allow the zero path difference fringe to be identified.
To generate interference fringes, light from the source has to be divided into two waves which have then to be re-combined. Traditionally, interferometers have been classified as either amplitude-division or wavefront-division systems.
In an amplitude-division system, a beam splitter is used to divide the light into two beams travelling in different directions, which are then superimposed to produce the interference pattern. The Michelson interferometer and the Mach—Zehnder interferometer are examples of amplitude-division systems.
In wavefront-division systems, the wave is divided in space—examples are Young's double slit interferometer and Lloyd's mirror. Interference can also be seen in everyday phenomena such as iridescence and structural coloration. For example, the colours seen in a soap bubble arise from interference of light reflecting off the front and back surfaces of the thin soap film.
Depending on the thickness of the film, different colours interfere constructively and destructively. Interferometry has played an important role in the advancement of physics, and also has a wide range of applications in physical and engineering measurement. Thomas Young 's double slit interferometer in demonstrated interference fringes when two small holes were illuminated by light from another small hole which was illuminated by sunlight. Young was able to estimate the wavelength of different colours in the spectrum from the spacing of the fringes.
The experiment played a major role in the general acceptance of the wave theory of light. Richard Feynman was fond of saying that all of quantum mechanics can be gleaned from carefully thinking through the implications of this single experiment. The results of the Michelson—Morley experiment are generally considered to be the first strong evidence against the theory of a luminiferous aether and in favor of special relativity.
Interferometry has been used in defining and calibrating length standards. Sixty years later, in , the metre in the new SI system was defined to be equal to 1,, This definition was replaced in by defining the metre as the distance travelled by light in vacuum during a specific time interval.
Interferometry is still fundamental in establishing the calibration chain in length measurement. Interferometry is used in the calibration of slip gauges called gauge blocks in the US and in coordinate-measuring machines. It is also used in the testing of optical components. In , a technique called astronomical interferometry was developed.
Astronomical radio interferometers usually consist either of arrays of parabolic dishes or two-dimensional arrays of omni-directional antennas. All of the telescopes in the array are widely separated and are usually connected together using coaxial cable , waveguide , optical fiber , or other type of transmission line.
Interferometry increases the total signal collected, but its primary purpose is to vastly increase the resolution through a process called Aperture synthesis. This technique works by superposing interfering the signal waves from the different telescopes on the principle that waves that coincide with the same phase will add to each other while two waves that have opposite phases will cancel each other out.
This creates a combined telescope that is equivalent in resolution though not in sensitivity to a single antenna whose diameter is equal to the spacing of the antennas furthest apart in the array. An acoustic interferometer is an instrument for measuring the physical characteristics of sound waves in a gas or liquid, such velocity , wavelength, absorption , or impedance. A vibrating crystal creates ultrasonic waves that are radiated into the medium. The waves strike a reflector placed parallel to the crystal, reflected back to the source and measured.
Quantum interference is quite different from the classical wave interference described above. Below, an enumeration of the important differences is provided. Quantum interference is, however, similar to optical interference. The best known example of quantum interference is the double-slit experiment.
In this experiment, electrons, atoms or other quantum mechanical objects approach a barrier with two slits in it. If the quantum object succeeds in passing through the slits, its position is measured with a detection screen a certain distance beyond and behind the barrier. When the object almost reaches the screen, the probability of where it is located is given by the above equation. In this context, the equation says that the probability of finding the object at some point just before it hits the screen is the probability that would be obtained if it went through the first slit plus the probability that would be obtained if it went through the second slit plus the quantum interference term, which has no counterpart in classical physics.
important phenomena such as interference, diffraction, and polarization. The study Describe how polarizers/analyzers are used with polarized light. • State the.
Diffraction refers to various phenomena that occur when a wave encounters an obstacle or opening. The diffracting object or aperture effectively becomes a secondary source of the propagating wave. Italian scientist Francesco Maria Grimaldi coined the word diffraction and was the first to record accurate observations of the phenomenon in In classical physics , the diffraction phenomenon is described by the Huygens—Fresnel principle that treats each point in a propagating wavefront as a collection of individual spherical wavelets. This is due to the addition, or interference , of different points on the wavefront or, equivalently, each wavelet that travel by paths of different lengths to the registering surface.
We head back to the recording studio to study interference and diffraction of sound waves. We investigate qualitatively how diffraction affects sound waves of various frequencies.
Microscopy U - The source for microscopy education
In , Albert Einstein was thinking about photons and excited atoms. He considered an atom excited by a certain amount of energy and what would happen if that atom were hit by a photon with the same amount of energy. He suggested that the atom would emit a photon with that amount of energy, and it would be accompanied by the original photon. The exciting part is that you would have two photons with the same energy and they would be in phase. Those photons could go on to hit other excited atoms, and soon you would have a stream of in-phase photons. Such a light stream is said to be coherent.
Previously in Lesson 3 , the behavior of waves traveling along a rope from a more dense medium to a less dense medium and vice versa was discussed. The wave doesn't just stop when it reaches the end of the medium. Rather, a wave will undergo certain behaviors when it encounters the end of the medium. Specifically, there will be some reflection off the boundary and some transmission into the new medium. But what if the wave is traveling in a two-dimensional medium such as a water wave traveling through ocean water? Or what if the wave is traveling in a three-dimensional medium such as a sound wave or a light wave traveling through air?
In physics , interference is a phenomenon in which two waves superpose to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive interference result from the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency. Interference effects can be observed with all types of waves, for example, light , radio , acoustic , surface water waves , gravity waves , or matter waves. The resulting images or graphs are called interferograms. The principle of superposition of waves states that when two or more propagating waves of the same type are incident on the same point, the resultant amplitude at that point is equal to the vector sum of the amplitudes of the individual waves. If a crest of one wave meets a trough of another wave, then the amplitude is equal to the difference in the individual amplitudes—this is known as destructive interference. If the difference between the phases is intermediate between these two extremes, then the magnitude of the displacement of the summed waves lies between the minimum and maximum values.
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