INDEX OF REFRACTIONPineda, Ram Lewis N, PHY13L/A2 [email protected] Abstract This paper was written to further understand the concept of refraction – quantitatively. The study was conducted using a glass plate and air as the medium of light propagation. Both media were interchanged to establish a consistent relationship with the said topic. It was found out that that the value of the index of refraction is always greater than 1. The obtained data shows that the greater the value of the index of refraction of certain medium, the denser is the medium. This was verified by using the Snell’s Law, in which the experimental index of refraction was determined accordingly. Moreover, it is also found out that when a light ray travels in medium with considerable refractive index into much lesser refractive index a total internal reflection occurs between the interfaces of media. The results of this experiment were very promising in understanding the concept of refraction. Key Words: light, refraction, index of refraction, critical angle, Snell’s Law Introduction A. Light has a dual nature Sometimes it behaves like a particle (called a photon), which explains how light travels in straight lines Sometimes it behaves like a wave, which explains how light bends around an object. If light would be considered as a wave, it entails the principles of propagation and other properties of wave. These principles can be narrow down in the concept of reflection and refraction. Reflection is the change in direction of a wave front at an interface between two different media so that the wave front returns into the medium from which it originated. On the other hand, refraction is the change in direction of a wave passing from one medium to another caused by its change in speed. If a light ray travels and it is propagated in a certain medium it will travel in a constant density. However, if light travels in certain number of medium (media), at least of two medium, it is observed that light travels in different speed depending on the very structure of the material or the medium. The very reason of this refraction of light is that a change in speed of light takes place in the interface between the media. As a result of study, different media have different quantitative value of refraction. These quantitative values are often refereed as the index of refraction. 1 Then the angle must be determine using a protractor or can be calculated using the analysis of the right triangle formed by projecting the intersection of both extended line at the circle in the horizontal and vertical axis. Next. These two pins must be aligned and place the third pin anywhere in quadrant III in alignment with the pin 1 and pin 2. The materials used in the experiment The first part is about the determination of index of refraction of glass when light goes from glass to air. trace the exact size of the glass plate. push pins. So this part will be done on the data sheet containing two perpendicular axes and the circle. All data must be recorded. draw an extended line connecting points O and T to intersect the circle at H and another extended line connecting points O and K to intersect the circle at F. Figure 1. it will be divided into three parts. is to put a pin on the point O (origin) and pin 2 anywhere on the first quadrant and label it as point T.(A) Determination of index of refraction of glass when light goes from glass into air. To perform the experiment about the index of refraction the following materials are needed: glass plate. Next thing to do is to view the thickness of the lower edge of the glass plate and look for the images of pin 1 and pin 2. the glass plate’s lower edge must be placed along the line AA’. after doing so. Then. Remove the glass plate and identify the hole from the pin on the paper.Methodology In this fourth experiment. It is important that the two pins must touch the sides of the glass plate. On the datasheet with the drawing of two perpendicular axes AA’ and BB’ intersecting at the center of circle O. and either diode laser or a laser pointer. and (C) Determination of the critical angle for glass. 2 . (B) Determination of index of refraction of glass when light bends from air into glass. cork board. a laser must be placed to the lower side of the glass plate. thus the upper edge of the glass plate is actually touching the horizontal axis. All data must be recorded. one must view in the thickness of the glass plate in the third quadrant looking for the image of the second push pin. All places where the push pins are found out to be located must be traced from the center. A light ray will be observed emerging in the upper side of the glass plate. Then. Figure 3 Set-up for the second part of the experiment In the determination of the critical angle. Again. the first thing to do is to place and trace the glass plate in a clean sheet of bond paper. Still.Figure 2 Set-up for the first part of the experiment The second part of the experiment is most likely the same with the first part of the experiment. which is the last part of the experiment. It simply means that the procedure son in the first part is the same for the second part. The only difference is that the glass is place below the horizontal axis. the angle can be measured using a protractor or can be calculated as well. The laser must be somehow perpendicular to the glass plate this can be done by adjusting the laser until the incident ray of light appears to be almost parallel to the lower side of the glass plate. the position of both laser and the 3 . Solids are the best example of material which have a compact molecules. while gas are the best example of materials which a less compact or scattered molecules. which is also solid in nature. A. Theoretically. the angle with respect to vertical axis of the third push pin is expected to be more than the angle produce of the initially place push pin 2 due to the above explanation given. the position of the laser is set to be letter M and the point of the emerging light ray at the upper side of the glass plate was assigned to be letter E. It is important to note that all these angles are with respect to the vertical axis. See figure 4. All data must be recorded. it is noticeable that the angle in the first quadrant in much lesser than the angle in the third quadrant. We were able to determine the angle 4 . The very explanation of this findings is that light bends lesser in much denser material provided that its molecules is more compact than a less denser material that its molecules are less compact. the diode laser is pointed out at the lower end of the glass plate Results and Discussion The conducted experiment about index of refraction is relatively easy. served as the basis for where the push pin 3 to be placed. Again any letter will do to assign these points. again with respect to the vertical axis. In the case of our data. also it can be calculate by using the tangent function. In the case of our data. the theory was not violated since the angle of the third push pin is greater than the angle of the second push pin. The angle of this trace line must be measure with respect to the vertical axis. The following data and relationship established are provided below. the glass plate which is placed in the upper part of the horizontal axis. A line must be trace from point M up to point E.emerging light ray must be marked. Figure 4 Set-up for the last part of the experiment. Determination of index of refraction of glass when light goes from glass into air If you would take a look in figure 5a. In the experiment. which is the push pin 2 was initially place. the formula used in determining the experimental value of the index of refraction of the glass was derived from the equation of Snell’s law. but this angle was also calculated using the principle of right triangle. much denser material. See figure 5b. As explained above. is the index of refraction of glass. and we are tasked to determine the angle of refraction in the third quadrant. much lesser than the initial angle produced of push pin 2 in the first quadrant. higher angle. is the is the angle of refraction (All argument are in degrees) Table 1 Determination of index of refraction of glass PART A PART B Light bends from GLASS to AIR Light bends from AIR to GLASS Length of line GH 5 . less denser material. angle of incidence. Provided below is the Snell’s Law equation. (a) (b) Figure 5. Determination of index of refraction of glass when light bends from air into glass Basically. Also. Again push pin 2 was initially place in the first quadrant. making the angle of refraction of push pin 2. Since the glass plate was placed below the horizontal axis. where. specifically to locate where the image formed in the third quadrant by looking at the thickness of the glass. all findings and relationship established in the first and second part of the experiment is governed by the Snell’s law. less angle. (a) Actual data obtained in the frist part of the experiment. (b) Actual data obtained in the second part of the experiment Generally. B. the same principle is observed in the second part of the experiment. and is the index of refraction of air. in which the push pin 3 was placed. This theory is still observed in data obtained in the second part of the experiment.by using a protractor for accuracy reason. we are still able to gather reasonable data to emphasize the concept of critical angle. This means there is no refracted ray but the ray of light is totally refracted. The very reason why the critical angle exists is because a total internal reflection occurs. and second the angle of incidence must be greater than the critical angle. Determination of the critical angle for glass In the table provided it is observed that the percentage of error is most likely close to ten percent. Moreover. first is that the light is in the more dense medium and going to less dense medium. and (3) to determine the index of refraction of glass experimentally by applying the Snell’s Law. We were able to establish a relationship that is provided in the results and discussion. After the data was obtained we were able to compare the angle produce of the push pin. between the angle and the medium where the light bends. We found out that the medium truly affect the angle of refraction. However. but the percentage of error obtained in this part of the experiment is reasonable due to the structure of the glass plate.Length of line FL Index of Refraction of Air Index of Refraction of Glass (Experimental Value) Index of Refraction of Glass (Accepted Value) Percentage Error C. In the process of performing the experiment we were able to study the refraction of light on how it behaves in different media. (2) to compare the angle of incidence in the first medium to the angle of refraction in second medium. there are two requirements in order for a total internal reflection can occurred. With some extended research. like in the experiment the glass and air used as the medium. Table 2 Determination of critical angle Length of line MU Length of line EU Critical Angle (Experimental Value) Critical Angle (Accepted Value) Percentage Error Conclusion There are three objectives in this experiment: (1) to study the refraction of light. It observed the glass plate is not perfect rectangle as a result it highly affects the data obtained. The concept of the critical angle is still governed by Snell’s law. since the concept of critical angle is highly dependent on the angle how the laser is pointed into the glass plate. the third objective was accomplished by analysing our data obtained. The first and second objective of this experiment was accomplished both in the process of performing the experiment and after the data obtained. To 6 . html 7 ... since Snell’s law is very consistent. we also study the principle of the critical angle that is also can be explained mathematically by the Snell’s law. Wiley.. C. If innovations were introduced. J. Walker. Resnick. R. Vuille.. it is advisable to integrate it to the existing experiment in the purpose of further understanding..physicsclassroom. In addition to the stated objective. Cenage Learning. This paper can only support the existing theories ad principle about the said topic.gsu. Furthermore. Principles of Physics.phy-astr. it is advisable to some extended research to fully understand the concept of refraction. 2011 Manual [1] Physics Laboratory Part 4 URL [1] http://www. Essentials of College Physics. R. References Books [1] Serway.determine the experimental index of refraction of the glass we use the Snell’s law. 9th ed. A.edu/hbase/tables/indrf. There is no major problem encountered in calculating the experimental index of refraction. 2007 [2] Halliday. D.com/class/refrn/Lesson-2/Snell-s-Law [2] http://hyperphysics.