PC1144 Physics IVSpeed of Light1Objective•Determine the speed of light by measuring the time taken for a light pulse to travelthrough a long optical fiber cable.2Equipment•Speed of Light apparatus•Long fiber optic cable (5m, 10m, 15m, 20m)•Short optical fiber cable (15cm)•Oscilloscope with a pair of probes3TheoryThe speed of lightc, with a value of 2.9979×108m/s in vacuum, is one of the most funda-mental physical constants. At sea level, the speed of light in air is only about 70 kilometers persecond less than it is in vacuum. At higher altitudes, where the air is less dense and where lightis impeded less by solid airborne particles, light speed increases. For most practical purposes,we can rate light speed in air versus light speed in a vacuum the same.When light passes through some medium other than a vacuum (such as solids or liquids),light speed is appreciably reduced. In water, the speed of light is about 25 percent less thanin a vacuum. In glass, light has even a tougher time; its speed drops by about 33 percent ascompared to its rate in a vacuum.To put these relationships into some form of mathematical perspective, we utilize the termindex of refractionnwhich refers to the ratio between the speed of light in vacuumcand itsspeed in some other mediumv:n=cv(definition of index of refraction)Substituting the numerical values of light speed in glass, we would obtain the index of refrac-tion for glass is about 1.50. In other words, the speed of light in glass is reduced by about onethird.Physics level 1 laboratoryPage 1 of 7Semester II, 2014/15
PC1144: Speed of LightPage 2 of 7In this experiment, you will use “time-of-flight” methods to measure the speed of light.The original attempt to do this was by the great Italian physicist Galileo. The method wassimple. Two people, call them A and B, take covered lanterns to the tops of hills that areseparated by a distance of about a mile. First, A uncovers her lantern. As soon as B sees A’slight, she uncover her own lantern. By measuring the time from when A uncovers her lanternuntil A sees B’s light, then dividing this time by twice the distance between the hill tops, thespeed of light can be determined. However, the speed of light being what it is, and humanreaction times being what they are, Galileo was able to determine only that the speed of lightwas far greater than could be measured using his procedure.Figure 1: Cross section of a typical optical cable.We will determine the speed of light by measuring the time taken for a light pulse to travelthrough a long optical fiber cable. Optical fiber cables consist of a translucent material, eitherplastic or glass, spun into thin tubes and wrapped orcladwith another translucent materialof lower index of refraction (see Figure 1). Due to the difference in index of refraction, a rayof light traveling in an optical fiber cable will undergototal internal reflectionin the centralcore.