Physics307L:People/Muehlmeyer/Formal
Measuring the Speed of Light
Author: Justin Muehlmeyer
Experimentalists: Justin Muehlmeyer and Alexander Barron
University of New Mexico
Department of Physics and Astronomy Junior Lab November 2008
jmuehlme@unm.edu
Abstract
To approximate the speed of light we measured the "flight time" of pulses of light emitted by a light emitting diode by measuring the time difference between LED emission and PMT reception of the light signal down a tube via a time amplitude convertor (TAC). By varying the distance the light signal travels we plot distance vs. flight time and use the linear-least squares method to approximate the slope of our data, which gives us the speed of light. We found that our best approximation to the accepted value of 2.99 X 108 m/s was 2.94 X 108 m/s, which came from large variations in distance and from using the "time walk" correction that accounts for the changing intensity of the light as the LED distance approaches the PMT.
Introduction
Materials and methods
Instrumentation
- PMT: Perfection Mica Company N-134
- Digital Oscilloscope: Tektronics TDS 1002
- LED: Cycles on and off at around 10KHz depending on the voltage applied. Recommended voltage is around 200 volts DC.
- LED Power Supply:
- PMT Power Supply:
- Cardboard tube wide enough to fit the PMT on one end and LED on the other.
- 2 Polarizing filters: one for the PMT, one for the LED.
Set Up
To measure the time difference between LED emission and PMT reception we have a time amplitude converter which converts the time difference of its inputs into a voltage that is proportional to that time difference. The two inputs are labeled the "start" time from LED emission, and the "stop" time from PMT reception. The output ratio of the time difference between the two inputs is 10 V = 50 nS, which we read on the digital oscilloscope.
Data
Results and Discussion
