# Physics307L:People/Gonzalez/Speed of Light

## Measuring the Speed of Light

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### Purpose

The purpose of this experiment is to measure the speed of light, we will be doing this by using and LED that will be sending short pulse signals through a range of 30cm to 120cm, a high speed detector will be used to measure when the light arrives from the LED.

### Background

The speed of light is one of the most fundamental constants in physics, it is also the fastest anything we know to exist can travel. Light's constant speed regardless of what reference frame its in gave rise to several theories, including that of the a luminous ether. In 1887 the Michelson and Morley experiment failed to find this ether, their failure in finding it was one of the strongest proofs against the ether theory. It wouldn't be for another 18 years that an answer would be found when Einstein concluded that the speed of light was independent of its reference frame, thus the speed of light is constant in all reference frames. This postulate along with the laws of physics are the same in all reference frames had tremendous consequences that would forever change the world of physics that was known in the early 20th century.

Several methods can be used to detect the speed of light, in this experiment we used a high speed detector and a time-amplitude converter (TAC)along with a photomultiplier tube(PMT) to measure the speed of light.

### Procedure

My lab partner and I both followed the procedure found in Dr. Gold's lab manual in order to detect the speed of light, the full details of the procedure and setup are found in my my lab notes. Once the lab was set up we monitored the amplitude of the voltage coming from the LED, and recorded the results in relation to the distance the light pulse had to travel to reach the PMT. The schematic in figure 1 shows a graphical representation of the experiment setup.

This is a schematic of the experiment, this photo of the schematic found in Dr. Gold's lab manual, it was taken from Alexandra's lab notebook.

### Data

After finding the data we plotted distance vs. voltage, I used a least squares fit line to find our slope, using linest on both Microsoft excel and Google docs I was then able to take the slope and convert it to meters/second using the TAC conversion found in the manual at the lab. Our conversion for the TAC for this particular setup was 5ns/volt.

To the right in figure 2 is the plot of the distance vs. volts.
Distance vs Voltage plot

my final results were:

$148(8)\frac{centimeters}{Volt}X\frac{Volt}{5*10^{-9}seconds}=2.96*10^{8}m/s$ being my best guess, while

$140\frac{centimeters}{Volt}X\frac{Volt}{5*10^{-9}seconds}=2.80*10^{8}m/s$ was my minimum value and

$156\frac{centimeters}{Volt}X\frac{Volt}{5*10^{-9}seconds}=3.12*10^{8}m/s$ was my maximum value.

the accepted value of the speed of light(c) = 299,792,458 meters/second.

### Error

Most of the error may have been due to the fact that we had to eyeball our readings from the oscilloscope, with the fluctuations the way they were it was our best guess in the end that determined which number to use for the data.

SJK 15:14, 19 December 2009 (EST)
15:14, 19 December 2009 (EST)
You don't point out that your measurements were actually consistent with the accepted value for the speed of light!

## Acknowledgements

My Lab partner John Callow
Dr. Koch