Experiment Reflection and Refraction, Study notes of Law

Download Experiment Reflection and Refraction and more Study notes Law in PDF only on Docsity! Experiment 15 Reflection and Refraction 1. Introduction A. Reflection When light strikes the surface of a material, some of the light is reflected. The reflection of light rays from a plane surface like a glass plate or a plane mirror is described by the law of reflection: "The angle of reflection is equal to the angle of incidence", or θi=θr These angles are measured from a line perpendicular or normal to the reflecting surface at the point of incidence, Fig. 1. Fig. 1. Reflection Fig. 2. Refraction B. Refraction When light passes from one medium into an optically different medium at an angle other than normal to the surface, it is "bent" or undergoes a change in direction, as show in Fig. 2. This is due to the different velocities of light in the different media. For θ1, the angle of incidence, and θ2, the angle of refraction, we have d tv1 1sin =Θ and d tv 2 2sin =Θ or 12 2 1 2 1 sin sin n tv tv == Θ Θ where the ratio of velocities is called the relative index of refraction. For light traveling initially in a vacuum, the relative index of refraction is called the absolute index of refraction or simply the index of refraction, and n=v/c where c is speed of light in a vacuum and v the speed of light in the medium. Snell's Law can then be written n1 sinθ1 = n2 sinθ2 where n1 and n2 are the indices of refraction of medium 1 and 2, respectively. 2. Procedure You will do three different experiments, listed below as A, B, and C. You may use a photometer and the fiber optic probe (recommended) or use your eye to detect maximum intensity of light. The laser or an incandescent light source can also be used in all three methods. When the laser is used no aperture mask is then needed. If you decide to use the laser, do not look into the beam. Observe reflected and transmitted light using a piece of paper. In the following sections we will assume that you will use the laser source, but if you or the lab assistant decide to use an incandescent lamp, follow the same steps but simply replace ”laser” with “lamp” when appropiate. A. Angles of Incidence and Reflection i) Position the laser (the incandescent light source) on the left end of the optical bench, and place the angular translator about 25 cm from the end of the laser (or light source housing). Make sure the 0o and 180o marks lie on a line parallel to the bench. Finally adjust the rotating table so that the scored lines run perpendicular and parallel to the bench. ii) Attach a viewing screen with a millimeter scale onto the magnetic holder. Switch on the light and adjust the laser so it hits the center of a millimeter scale marked on the screen. iii) Replace the viewing screen with the flat surface mirror. Make sure that the mirror surface is parallel to the scored line. Table 2. Index of refraction. Record only values of θ1 and d. You will calculate n and nave and variance later. Glass plate Acrylic plate Incident angle θ1 Displacement d [mm] n = sin θ1/sin θ2 ≈ (t/d) sin θ1 Inciden t angle θ1 Displacement d [mm] n = sin θ1/sin θ2 ≈ (t/d) sin θ1 C. Index of Refraction. This method is more precise than that in part B. i) Place the glass plate attached to the component carrier at the center of the rotating table. Place a square piece of paper about 5 centimeters on a side with a millimeter scale across the middle between the glass plate and the component carrier. Align the component carrier so the back surface of the glass plate is parallel to the scored line on the table. Place the viewing screen with the millimeter scale on the viewing arm. Turn the laser on and rotate the table to a convenient angle. You should see least two reflections on the screen as indicated in Fig. 5. Light is refracted toward the normal when passing from air to glass. Is the same true when light propagates from glass to air? By observing the positions of the image on the viewing screen, you can see that the refraction must be away from the normal at a glass-air interface. (See Fig. 5). ii) Observe on the screen two reflected laser beams from the plate. Measure the distance between them. You can measure the distance between the two images observed on the back of the plate using the paper with the millimeter scale or in the front of the plate on the screen placed on the viewing arm. Using the following symbols: D - the distance separating the laser spots on the back of the plate; t - the plate thickness; θ - the angle of incidence; and θ2 the angle of refraction, we have tan θ2 = D/(2t) Repeat measurement N times for different angles of incidence. Calculate θ2 for various θ. Then calculate n from n=sinθ/sinθ2 and average value of n and variance. Fig. 5. Secondary reflections iii) Repeat for the acrylic plate. Table 3. Index of refraction. In class record only values of θ1 and D. You will calculate values of n, nave, and variance σ later. Glass plate Acrylic plate Incident angle θ1 D [mm] θ2 n=sinθ1/sinθ2 Inciden t angle θ1 D [mm] θ2 n=sinθ1/sinθ2 3. Report 1. In the introduction discuss the total internal reflection and its applications. You may want to read a section in your textbook on total internal reflection. 2. In the result section present data in Tables 1, 2 and 3. 3. Calculate the average values of n and experimental errors, σ. 4. Index of refraction of glass varies between 1.50 and 1.80 depending on glass composition and the wavelength of light. Index of refraction of acrylic also varies with the wavelength, and for the red laser it is about 1.49. Compare your results with the expected values and discuss possible deviations. 5. In the conclusion section comment about relationship between reflection angle and incident angle for flat and spherical mirrors.