THE SPEED OF LIGHT             Physics 480L




I.  Introduction

	In this experiment you will use a light emitting diode (LED), a
photomultiplier tube (PMT) and fast pulse circuitry to make a direct
measurement of the speed of light by measuring the travel time of a
light pulse over a path length of about 25 meters in the laboratory.
The LED and photomultiplier are located at one end of an enclosed tube
mounted on the wall.  A plane mirror located at the other end of the
tube reflects the light pulse from the LED back to the photomultiplier.


NOTE:  The PMT will burn out if high voltage is applied with room light
falling on the tube.  ALWAYS be sure the PMT is protected before
applying voltage.


II.  Initial Setup

	The light emitted by the LED is made into an approximately parallel
beam by a Fresnel lens placed so that the LED is at its focal point.
The direction of the resulting beam is determined by the transverse
location of the lens with respect to the LED.  It may be necessary to
align the lens and mirror if no PMT signals are observed when the LED is
excited by a positive pulse from the pulse generator.

	Using the pulser, adjust a fast oscilloscope to display 20 ns pulses
of about -100 mV amplitude.  Connect the PMT to a high voltage power
supply and to the oscilloscope.  With the LED turned off, start at -500 V 
and increase the HV until you see pulses on the oscilloscope.

              DO NOT EXCEED -1100 VOLTS ON THE PMT

Check for light leaks by turning the room lights on and off and watching
the intensity of the oscilloscope display.

	Now apply 20 ns, well separated, positive pulses to the LED.
Triggering on the applied pulse, gradually increase the amplitude until
a signal is observed on the oscilloscope at a delay of about 100 ns
from the applied pulse.  This signal is due to photons reflected from
the surface of the Fresnel lens.  Calculate the expected delay for a
pulse reflected from the mirror and look for a signal at that position
on the trace.  If the signal is weak, adjust the angle of the mirror to 
maximize it.

	If no pulses are detected, it will be necessary to align the 
Fresnel lens. 	TURN OFF the high voltage to the PMT and disconnect the 
cable from the HV supply. Remove the mirror from the far end of the tube.
Using the pulser to supply positive pulses to the LED, adjust the lens so 
that it appears uniformly illuminated. Re-install the mirror, being sure 
to make a light-tight seal.

	Make a pulse height spectrum of the PMT pulses.  Compare with the
spectrum of pulses from the PMT when the LED is turned off.  Explain
the two spectra.


III.  Calibration of electronics

	The pulses from the PMT will be passed through a discriminator and
used to stop a time-to-amplitude converter (TAC)  The output pulses
from the TAC have an amplitude proportional to the time between the start
and stop pulses.  Split a pulse from the pulse generator into two with a 
tee and send both signals through delays. Delay one of the signals
and measure the time difference  using the universal counter. Sending the 
same two signals into the TAC and the MCA program, determine the channel 
number corresponding to the measured delay. Repeat for several different 
delays and make a calibration curve.


IV.  Measurements

	Now pass the signal from the PMT into the discriminator.  Adjust
the high voltage and/or the threshold to count photon-induced pulses.
Adjust the width of the discriminator output pulse to be as short as
possible but not allow multiple pulsing from the same PMT pulse.  Check
this by examining the discriminator output on the oscilloscope and
looking for pulses at a fixed time after the triggering pulse.

	Using the TAC examine the distribution of times between the pulse
applied to the LED and the observed photons.  There should be two
signals, separated by the delay time for the round trip.  You may need
to delay the PMT signal to place it closer to the LED pulse so you can
use a sensitive scale on the TAC. Study the rise time of either of
these signals.  Try to make it as short as possible by adjusting the
pulse applied to the LED.

	When you have determined the optimum pulse settings for the LED
make a careful time distribution. Perform chi square fits to the leading
edges of the two distributions and determine the delay due to the
roundtrip to the mirror.  Measure the distance between the lens and the
mirror and calculate the speed of light.  Calculate the error in your
result.  Compare with the known value.


V. Self Test

	1. How far does light travel in 1 ns?

	2. How long will it take for light to make the round trip from
	the LED to the mirror and back to the PMT in the lab apparatus?

	3. Explain how a PMT produces pulses even when there is no light
	hitting the photocathode.

	4. Approximately what error do you expect in measuring the distance
	between the Fresnel lens and the mirror? What error will this
	produce in the speed of light? How accurately would you have to
	measure the time to give the same error?