Refraction of Light


March 29, 2004


Lab Section 01



The experiment performed studied Snell’s Law of Refraction of racing light rays through water. A water filled plastic dish was placed on paper and the light was refracted as it entered the dish. Lines were drawn by looking through the water dish and lining up pins placed on certain areas on the paper. The angles between certain points on the paper and the perpendicular line were measured which were the angles of incidence of the rays. The average refraction of water for the six rays was 1.29; where as the known value for the refraction of water is 1.33.


By tracing rays of light through water in a plastic dish, Snell’s law was able to be used to determine the refraction of water. Snell’s law describes the change in direction and is found with the angle between the incident ray and the normal to the surface. In the laboratory, Snell’s law was verified by measuring the incident and refracted angles for several incident angles. The reason this is able to be calculated is because light rays that pass from one medium to another undergo a change in speed. As light passes from one medium to another having a different index of refraction, the direction of the light propagation changes. Refraction of light is used in Fiber Optics, cable television, and also swimming.


The equipment used in this experiment was fairly simple. The main piece of equipment used was a plastic dish with a semicircular shape. Water was placed in the dish to determine the refraction through the dish. The dish was placed on top of a white piece of paper that was on a small piece of cardboard which was used to hold metal pins in order to see the refraction. Those were the only pieces of equipment used in this experiment.


To start the experiment the piece of white paper was placed on the cardboard and x and y axes were drawn on the white paper. The plastic dish filled with water was placed on the lower part of the x axis. The direction that the light rays took as they entered and passed through the water were traced by viewing the pins through the dish. A pin was placed at the center of the straight face of the dish and a line was drawn perpendicular to the straight face and used as a reference to measure incident angles. A pin was placed at a location which was then labeled A through F. We then looked through the plastic dish from the curved face and observe the two pins through the water until the pins were aligned. A third pin was placed against the dish such that it was lined up as well. This line was traced and the angle between this line and the perpendicular line give the incident ray and the refracted ray.


The angles were obtained and the Snell’s law calculations were placed in the following table:


Table 1: Snell’s Law


Θi


±1°


Θr


±1°


sinΘi


±.005


sinΘr


±.005


n2


±.005


A








.122


.105


1.162


B


19°


15°


.326


.259


1.259


C


32°


24°


.530


.407


1.302


D


46°


33°


.719


.545


1.320


E


61°


41°


.875


.656


1.334


F


67°


45°


.921


.707


1.302


As shown in Table 1, the index of refraction of water was found for each angle. The average of the six values found was 1.29 ±.005 due to uncertainties. The index of refraction of air is 1.0. Graph 1 shows the index of refraction of water. Sinθr is on the y axis while Sinθi is on the x axis. The slope is ni/nr and is equal to .636 which shows that the refraction of water is 1.33 ±.005. Graph 2 shows the abscissa versus the ordinate and the horizontal asymptote was the critical angle for water. Somehow the graph showed a straight line with a linear relationship that looked almost identical to Graph 1. The cause of this is unknown because we plotted the points as we were told to in class and also as stated in the laboratory notebook. The relationship should not have been linear and should have been more exponential.


In conclusion, the average experimental refraction of water found was 1.29, which is very close to the known values for the refraction of water. This shows that the experiment had very little error in it and that the results are good. Somehow Graph 2 did not turn out as expected, but there was nothing we could do because we