Overview

In this lab we will try to determine how accurately the speed of light can be measured with a function generator, photo-detector, oscilloscope, laser, and a mirror. We will need stands for your laser and mirror capable of making small adjustments to the projected angles. In doing so we will also recommend improvements to the experiment so better data may be obtained.

Procedure

Connect your function generator by use of a t-splicer to both your oscilloscope and to your laser so that you have a zero point for measurements and reliable periodic pulses of light coming from your laser. In case the lag time for returned pulses from your laser is on to large a scale the location of your mirror at its first position will be used as a zero point for both measurements of change in time and distance; it is in both cases nice to have a reliable trigger. It will be easiest to set up all electronic equipment in one location of a room or long hallway then project the laser some distance to a mirror and back to the photo-detector that will also be attached to your oscilloscope. This is done so that only the mirror will need to be moved to make changes to the distance the light needs to travel to reach the detector. Here you can see that it is necessary to have both the laser and mirror on stands where fine adjustments can be made so that final incidence with the photo-detector will result in clear pulses on the oscilloscope. It may be useful as well to have your photo-detector on an adjustable stand as small differences in the angle at which your photo-detector receives pulses makes a large difference in your ability to resolve pulses in the nano second range we hope to be in the scale of. The closer you can keep the laser and the detector the easier it will be to maintain proper alignment as you move the mirror back, this will also allow you to make a small angle approximation for the distance the light travels from the laser to the detector. Your set-up should be similar to the diagram shown below. Your function generator should be set to produce a square wave of peak voltage that is consistent with proper voltage allowed for your laser.