Light and Optics: We just learned that light is a wave (an 'electromagnetic wave', with very small wavelength). Haven't registered your existing Newport.com account. Fiber Optics Physics And Technology. 15-09-2016 2/2 Fiber Optics Physics And Technology. Other Files Available to Download . The OPTICAL PHYSICS research group has expertise in experimental and theoretical aspects of optical physics, atom optics, atomic & solid-state physics, and x-ray optics & physics. We are developing new tools for investigating. Experiment 4 - Physical Optics. APPARATUSShown in the picture below: Optics bench with laser alignment bench and component carriers. Laser. Linear translator with photometer apertures slide and fiber optic cable. Not shown in the picture above: Computer with Science. Workshop interface. High sensitivity light sensor with extension cable. Slit slides and polarizers. Incandescent light source. Tensor Lamp. INTRODUCTIONThe objective of this experiment is to familiarize the student with some of the amazing characteristics of a laser, such as its coherence and small beam divergence. The laser will be used to investigate single- and double- slit diffraction and interference, as well as polarization. Furthermore, several interesting diffraction phenomena that are hard to see with standard light sources can be observed easily with the laser. WARNING: Do not look directly into the laser beam! Permanent eye damage (a burned spot on the retina) may occur from exposure to the direct or reflected beam. The beam can be viewed without any concern when it is scattered from a diffuse surface such as a piece of paper. The laser beam is completely harmless to any piece of clothing or to any part of the body except the eye. It is a wise precaution to keep your head well above the laser- beam height at all times to avoid accidental exposure to your own or your fellow students' beams. Do not insert any reflective surface into the beam except as directed in the instructions or as authorized by your TA. The laser contains a high- voltage power supply. Caution must be used if an opening is found in the case to avoid contacting the high voltage. Report any problems to your TA. DOUBLE- SLIT INTERFERENCEIn the first part of the experiment, we will measure the positions of the double- slit interference minima. Schematically, a double- slit setup looks as follows: The incident laser light shining on the slits is coherent: at each slit, the light ray starts with the same phase. To reach the same point on the viewing screen, one ray needs to travel slightly farther than the other ray, and therefore becomes out of phase with the other ray. If one ray travels a distance equal to one- half wavelength farther than the other way, then the two rays will be 1. No light reaches this point on the screen. At the center of the screen, the two rays travel exactly the same distance and therefore interfere constructively, producing a bright fringe. At a certain distance from the center of the screen, the rays will be one- half wavelength (or 1. A bit farther along the screen, the rays will be one whole wavelength (or 3. Farther still, the ways will be one and one- half wavelengths (or 3. Thus, the interference pattern contains a series of bright and dark fringes on the screen. Let \(\theta\) be the viewing angle from the perpendicular, as shown in the figure below: Study the construction in Figure 3. The small extra distance \(x\) that the lower ray needs to travel is \(d\sin\theta\). If this distance is equal to an odd multiple of one- half wavelength, then the two rays will interfere destructively, and no light will reach this point on the screen: \begin. If \(y\) is the linear distance from the center of the pattern on the screen to the point of interference, and if the angle \(\theta\) is small, then \(\sin\theta \approx \tan\theta = y/L\). Thus, the positions of the minima are given by\begin. This type of interference — in which rays from many infinitesimally close points combine with one another — is called diffraction. We will measure the actual intensity curve of a diffraction pattern. The textbook or the appendix to this experiment gives the derivation of the intensity curve of the diffraction pattern for a single slit: \begin. Here is a plot of the intensity \(I\) from Excel: The image below demonstrates the intensity pattern; it shows the broad central maximum and much dimmer side fringes. Let us locate the minima of the single- slit diffraction pattern. When \(\alpha\) is zero, \(\sin\alpha = 0\), and the expression \(0/0\) is indeterminate. L'Hopital's rule resolves this ambiguity to show that \(\sin\alpha/\alpha \to 1\) as \(\alpha \to 0\). Thus, \(\alpha = 0\) corresponds to the center of the pattern and is called the central maximum. Elsewhere, the denominator is never zero, and the minima are located at the positions \(\sin\alpha = 0\) or \(\alpha = n\pi\), with \(n\) = any integer except 0. The centers of the side fringes are located approximately (but not exactly) halfway between the minima where \(\sin\alpha\) is either +1 or . To find the exact positions of the maxima, we need to take the derivative of \(I\) with respect to \(\alpha\) and set it equal to zero, then solve for \(\alpha\).)As mentioned above, the side fringes are much dimmer than the central maximum. We can estimate the brightness of the first side fringe by substituting its approximate position \(\alpha = 3\pi/2\) into Eq. We can see this most directly from the position of the first minimum in Eq. As we try to “squeeze down” the light, it spreads out instead. Consider the double- slit interference setup again. If the two slits were very narrow — say, much less than a wavelength of light (\(a \ll \lambda\)) — then the central maxima of their diffraction patterns would spread out in the entire forward direction. The interference fringes would be illuminated equally. But we cannot make the slits too narrow, as insufficient light would pass through them for us to see the fringes clearly. The slits must be of non- zero width. Their central diffraction maxima will nearly overlap and illuminate the central area of the interference fringes prominently, while the side fringes of the diffraction pattern will illuminate the interference fringes farther from the center. A typical example is shown below. POLARIZATIONConsider a general wave moving in the \(z\) direction. Whatever is vibrating could be oscillating in the \(x\), \(y\), or \(z\) directions, or in some combination of the three directions. If the vibration is along the direction of wave motion (i. Sound is a longitudinal wave of alternate compressions and rarefactions of air. If the vibration is perpendicular to the direction of wave motion (i. According to Maxwell's equations, light is electromagnetic radiation. The electric and magnetic field vectors oscillate at right angles to each other and to the direction of wave propagation. We assign the direction in which the electric field oscillates as the polarization direction of light. The light from typical sources such as the Sun and light bulbs is unpolarized; it is emitted from many different atoms vibrating in random directions. A simple way to obtain polarized light is to filter unpolarized light through a sheet of Polaroid. Such a sheet contains long, asymmetrical molecules which have been cleverly arranged so that the axes of all molecules are parallel and lie in the plane of the sheet. The long Polaroid molecules in the sheet are all oriented in the same direction. Only the component of the incident electric field perpendicular to the axes of the molecules is transmitted; the component of the incident electric field parallel to the axes of the molecules is absorbed. Consider an arrangement of two consecutive Polaroid sheets: The first sheet is called the polarizer, and the second one is called the analyzer. If the axes of the polarizer and analyzer are crossed (i. The diagram above shows that if the analyzer is oriented at an angle \(\theta\) with respect to the polarizer, then a component of the incident electric field \(E\cos\theta\) will be transmitted. Since the intensity of a wave is proportional to the square of its amplitude, the intensity of light transmitted through two polarizers at an angle \(\theta\) with respect to each other is proportional to \(\cos^2\theta\). This result is called Malus' Law, which we will test in this experiment. An interesting situation arises if a third polarizer is inserted between two crossed polarizers. No light passes through the crossed polarizers initially, but when the third polarizer is added, light is able to pass through when the third polarizer has certain orientations. How can the third polarizer, which can only absorb light, cause some light to pass through the crossed sheets? EQUIPMENTAt your lab station is an optics bench. A laser is located at one end of the bench, on a laser alignment bench, while a linear translator with a dial knob that moves the carriage crossways on the bench can be found at the other end. Between the laser and the linear translator are one or more movable component carriers. Fitted into a small hole in the linear translator is a fiber- optic probe connected to a high- sensitivity light sensor which, in turn, is connected by an extension cable to the Science. Workshop interface. Be careful with the probe. Do not bend the probe in a circle of less than 1. Also, do not bend the probe within 8 cm of either end. A slit of width 0. The linear translator (which is basically a carriage mounted on a threaded rod) moves the probe along the axis of the rod. An intensity plot of the pattern produced by a slit placed between the light source and the probe can be made by scanning the probe along the axis of the rod and taking readings from the high sensitivity light sensor. The probe can be attached to the high sensitivity light sensor by slipping the optic output connector (BNC plug) of the probe over the input jack on the high sensitivity light sensor. A quarter- twist clockwise locks the probe to the high sensitivity light sensor; push the connector towards the sensor box and a quarter- twist counterclockwise disengages it. The probe attenuates the light intensity reaching the selenium cell to approximately 6. This makes measurements of absolute intensity impossible. However, for these experiments, only the relative intensities are needed.
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