The Refraction Experiment - Lab Report Example

Introduction The speed of light varies with respect to the substance in which that light is travelling across. Light will travel relatively faster in a less dense medium as compared to a denser medium. Refraction is a property of light that occurs when it travels across mediums with varied densities at an angle. The refraction is viewed as a change in direction when light changes speed as it travels between materials of different densities. For instance, when light travels from air to water it will have a bending effect on the light as a result of the change in speed causing a subsequent change in direction of travel. Two angles are important in this perspective; the angle of refraction and the incident angle. Refractive index (RI) with regard to Snell’s Law provides a mathematical relationship between the incident angle and the angle of refraction. According to Snell’s Law RI = sin i/ sin r = n Where i is the incident angle, r is the angle of refraction and n is the refractive index. Figure 1 adapted from (The Physics Classroom, 2012) is an example of refraction showing both the angle of refraction and the incident angle. Figure 1 In other cases the light travels in a perpendicular line and RI cannot be measured by comparing the two angles. In such a case one can measure the RI by getting the ratio of the Real Depth (RD) to the Apparent Depth (AD). For instance, when determining the RI of water one can insert a measurement ruler until it touches the bottom of a beaker of known height (RD), and then comparing it with the depth as seen by the eye (AD). The following experiment consists of two parts; the first part used fresh tap water while the second experiment used salty water. Aims of the Experiment I. To measure the refractive index of water by obtaining the ratio between the real depth and the apparent depth. II. To establish whether the refractive index of water is affected by salinity. First Part Materials I. Three different sized glass beakers (without a lip). II. 2 metal pins. III. Measuring stick/ruler IV. Cork V. Retort stand VI. Mirror. Experimental Procedure The real depth (RD) of the container was measured. The container was then filled with ordinary tap water and a metal pin placed at the bottom of the container. A mirror was placed on top of the container so that the back of the mirror was in contact with water. The mirror covered only half of the water surface. A second pin was stuck in a cork and fixed with the retort stand and a clamp. The container was then looked directly while moving the second pin until there was no parallax between the image of the second pin in the mirror and the first pin in the container. After it was ascertained that there was no parallax, the height of the second pin above the mirror was measured and then recorded as the apparent depth (AD). Each of the above steps was repeated for each the different glass beakers and the refractive indices were measured and then their average computed. Second Part Materials I. 3 one liter bottles. II. Salt III. Teaspoon. Experimental Procedure Each of the one liter bottles were filled with ordinary tap water and three heaped teaspoonfuls of salt dissolved in first bottle, six in the second and nine in the third bottle respectively. The experimental procedure in the first part was then repeated and the refractive indices measured and their average computed. Experimental Results The calculations for the first part of the experiment were as follows: Beaker1 = RD/AD = 14.0/12.0 = 1.16 Beaker 2 = RD/AD = 12.5/8.4 = 1.49 Beaker 3 = RD/AD = 9.0/6.0 = 1.5 Table 1 shows the tabulated results of the first part of the experiment. Table 1 RD (cm) AD (cm) n Beaker 1 14.0 12.0 1.16 Beaker 2 12.5 8.4 1.49 Beaker 3 9.0 6.0 1.5 Average for n 1.38 The calculations for the second part of the experiment were as follows: Beaker 1 (3 tablespoons of salt) = RD/AD = 14.0/10.5 = 1.30 Beaker 2 (6 tablespoons of salt) = RD/AD = 14.0/8.8 = 1.59 Beaker 3 (9 tablespoons of salt) = RD/AD = 14.0/8.5 = 1.65 Table 2 provides the tabulated results of the second experiment RD (cm) AD (cm) n Beaker 1 (3 salt spoons) 14.0 10.5 1.30 Beaker 2 (6 salt spoons) 14.0 8.8 1.59 Beaker 3 (9 salt spoons) 14.0 8.5 1.65 Average for n 1.51 Table 2 A graph of apparent depth against relative depth was also drawn and the graph produced a straight line with the gradient of 1.2 [Insert the graph below and name it graph 1). Discussion The aims of the experiment were to determine the refractive index of water and to establish whether the salinity of water affected its refractive index. The refractive indices of the first experiment as demonstrated in Table 1 indicate that there was minimal difference. The average refractive index for the first experiment was found to be 1.38. According to literature, the refractive index of water is averagely 1.3 (Freeman, 2003). In essence, the experimental results were closer to the results found in literature. The fact that the refractive index of water has a value greater than 1 implies that light travels much faster in air than in water. In essence, air is less dense than water and as light travels from air to water the speed reduces. The average refractive index for saline water was found to be 1.51 indicating that saline water was denser than tap water. Another realization is that as the saline concentration of the water was being increased, the refractive index also increased. The results imply that an increase in salinity causes an increase in the density of water. When salt is dissolved in water the fact that it seems to disappear does not imply that it is not found in the water. A taste of the water will confirm that the salt is in the water but that it has dissolved. Table salt is made up primarily of Sodium Chloride. By dissolving the salt in water the NaCl is ionized into Na+ and Cl- ions which interact with H+ and OH- in water. The interaction causes the dissolution of the salt but it still proves that the salt is in the water. Refractive index is therefore a reliable way to compare the densities of two substances. The results have show that saline water is denser than tap water and that tap water is denser than air. The results of the experiment have also shown that a straight line graph is obtained when apparent depth is plotted against real depth in a graph. The graph is also positive indicating that as one value increases, the other value also increases. This indicates a direct proportionality relationship. The gradient of the graph indicates the constant that relates the two values. The constant in this case is the refractive index usually denoted as n. Refractive index is a reliable way of comparing the densities between two substances that are suspected of having different densities. Conclusion The experiment was relatively successful as demonstrated by the closeness of the experimental results to the results found in literature. However, this does not imply that the experiment was free of errors. One of the errors that were encountered was that not all the salt that was added to some of the beakers in the second part of the experiment dissolved. Furthermore, there were errors to do with making the measurements. Nevertheless, the experiment managed to arrive at the aim where it was able to establish the refractive index of water and also determine that salinity actually increased the refractive index of water. Reference List Freeman, M. H 2003, Optics, London: Butterworth Heinemann. The Physics Classroom 2012, The Mathematics of Refraction. Retrieved April 8, 2012, from The Physics Classroom: http://www.physicsclassroom.com/class/refrn/u14l2a.cfm Read More