Product Instructions: Speed of Light Kit
OP94625
This apparatus is intended to allow direct measurement of the Speed of Light using the “time
of flight” method, that is, measure how long it takes the light to travel a known distance, and
calculated the speed using
speed = distance
time
Light is one of the fastest things in the universe, travelling great distances in very small amounts
of time. Most widely accepted theories state that nothing can travel faster than light.
This makes measurement of the time of flight over short distances (on the order of 10s of metres)
very difficult.
Only with very fast electronics such as a high-bandwidth oscilloscope is it possible to measure
the very short times required to obtain a reasonably accurate value for c in the lab, using
relatively inexpensive equipment.

ever look directly at the LED.
N
The light source is very high intensity, with a risk of permanent damage
to vision, especially in a darkened room. Carry out a risk assessment.
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Product Instructions: Speed of Light Kit
The Benchmark Speed of Light Kit consists of:
•
Transmitter (Tx)
•Mirror
•
Receiver (Rx)
•
Fresnel lenses to focus transmitted and received light
•
Connecting leads
•
Bases and rods for mounting transmitter and receiver
•
Power supply
Also required is a double beam oscilloscope with at least 20MHz bandwidth and 20ns/div
resolution (for example, EL81502).
The apparatus is supplied in a Gratnells tray with insert for easy transport and storage.
The transmitter features a high intensity LED, which is modulated by a 1MHz square wave.
This pulse signal is picked up by the receiver after the light has travelled a distance, and
the “sent” and “received” signals can be compared. The delay in the signal is caused by
a combination in the delay in the electronics, , and the time-of-flight of the light .
The delay in the electronics can be measured first by pointing the transmitter directly at the
receiver, thus reducing the distance the light has to travel to zero.
By subtracting the electronic delay from the overall delay, the time-of-flight of the light signal
can be calculated.
Mirror
Tx
Rx
Tx
Measure delay caused by electronics to
Measure total delay t
t c =t- t 0
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Rx
Product Instructions: Speed of Light Kit
Electrical Connection
The kit is supplied with a single plug-top power supply, and a 3.5mm-3.5mm connecting lead.
The transmitter has two power ports on the back. Plug the power supply into one of these power
ports – it does not matter which one.
Then, use the 3.5mm-3.5mm connecting lead to connect the power ports of the transmitter
and receiver.
Use the BNC leads supplied to connect:
• Transmitted pulse output of the transmitter to channel 1 on the oscilloscope
• Received pulse output of the receiver to channel 2 on the oscilloscope
• The trigger output of the transmitter to the trigger input of the oscilloscope
Power Supply
Tx
Rx
CH1
CH2
Dual Trace Oscilloscope
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Product Instructions: Speed of Light Kit
Calibration
Even if the light had no distance to travel, there will still be a time difference between the
transmitted and received signal caused by delays in the wiring, the response times of the
components and measuring devices etc.
It is important therefore to compensate for this, which can be done by measuring the time
difference between the pulses when the light has to travel a negligible distance.
Switch on the oscilloscope, and make sure there is power to the transmitter. Ensure also that the
transmitted pulse output of the transmitter is connected to channel 1 of the oscilloscope.
Set the oscilloscope timebase to 1µs/div. Adjust the sensitivity on channel 1 of the
oscilloscope until you can see the pulses. The channel should be a.c. coupled.
You should be able to see 10 pulses across the screen. The pulse frequency is 1MHz, meaning
the pulses are 1µs apart.
Use the x10 control on the oscilloscope to set the timebase to 0.1µs/div. You should now see
one pulse in greater detail.
1µs/div
0.1µs/div
Now, ensure there is power to the receiver, and that the received pulse output of the receiver
is connected to channel 2 of the oscilloscope. Set the sensitivity of channel 2 to 50ms/div.
The channel should be a.c. coupled.
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Product Instructions: Speed of Light Kit
Point the transmitter at the receiver, and separate them by about 50cm, or until the received
pulse is no long clipped (flat at the top). Ensure the fringe of the COPY MISSING?
Transmitted Pulse
Transmitted Pulse
Received Pulse
Received Pulse
Clipped
Correct
The transmitted pulse comes directly from the oscillator in the transmitter, so is a relatively clean,
square pulse.
The received pulse has been subject to conversion from electrical signal, to light, and back to
electrical signal, so is rounded and distorted.
t0
Use the divisions on the oscilloscope screen
to measure the distance between the end of
the transmitted pulse and the peak of the
received pulse. It is necessary to measure to
a precision of at least 0.01μs, which is 1/10
th of a division if the timebase is 0.1μs/div.
Multiply the number of divisions by the
timebase to find t 0 .
For example, if the timebase is 0.1μs/div, and
you measure a distance of 1.6 divisions, then:
t 0 = 0.16μs
Make a note of this time as it will be required later.
Take note of the distance between the transmitter and receiver D0 .
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Product Instructions: Speed of Light Kit
Speed of Light in Air
Setup
The transmitter, receiver and oscilloscope can be placed on the same table. The mirror should be
placed at the same height as the transmitter and receiver, on the other side of the lab. Nothing
should block the light travelling from the transmitter to the mirror, and back to the receiver.
Mount the transmitter and receiver on retort rods at the same height.
Place the Fresnel lenses in their holders as
illustrated. The centre of the Fresnel lens should
be at approximately the same height as the
diodes on the transmitter/receiver.
Place the large Fresnel lens in front of the
receiver. The small Fresnel lens can go in front
of the transmitter, but is not usually necessary.
Ensure there is power to the transmitter and
receiver. The transmitter will project a large
red dot.
Point the transmitter and receiver at the mirror
on the other side of the lab, ensuring the large
Fresnel lens is in front of the receiver.
The next page illustrates the ideal setup. The transmitter and receiver should be at a slight angle
to the mirror, so that the light from the transmitter reflects off the mirror into the receiver.
One way to achieve this is to move your head into the position of the receiver, at the same height,
and adjust the position of the transmitter until you can see the light from the transmitter reflected
from the mirror. Then, place the receiver where your head was when you could see the
reflected light.
Finally measure the distance from the transmitter to the mirror – this is half of the distance that
the light will travel.
If the receiver is in approximately the same position as the transmitter, then the flight path D will
be double the distance. Otherwise, you will also have to measure the distance from the mirror to
the receiver to get the total path.
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Product Instructions: Speed of Light Kit
Speed of Light Experiment Setup (connections not shown)
Large Fresnel Lens
Receiver
Light Path
Mirror
Transmitter
Small Fresnel Lens
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Product Instructions: Speed of Light Kit
Measurement
Keep the oscilloscope settings the same as the calibration procedure.
You should see a similar pattern on the oscilloscope – a clean square wave for the transmitted
pulse, and a rounder, less well defined pulse as the received wave.
Notice that the received pulse will be slightly more delayed than in was during the calibration
procedure. It may also be slightly more distorted, and of a lower amplitude, due to light
dispersion and losses in reflection.
Use the divisions on the oscilloscope screen
to measure the distance between the end of
the transmitted pulse and the peak of the
received pulse. It is necessary to measure to
a precision of at least 0.01μs, which is 1/10th
of a division if the timebase is 0.1μs/div.
t
Multiply the number of divisions by the
timebase to find t.
For example, if the timebase is 0.1μs/div, and
you measure a distance of 2.2 divisions, then:
t=0.22μs
Calculation
The time of flight tc of the light is the difference between the total time and the calibration time:
t
t
: delay on received signal
c= 0 - 0: calibration delay
t
t
t
And the speed of light is simply the distance over time:
c = D - D0 D: distance travelled by light
D0: calibration distance
c
t
Say you measured a total distance of 18m, a total time of 0.22 μs and a calibration time of 0.16
μs, this would give you:
18m
c=
= 3 x 108ms-1
-6
(0.22 - 0.16) x 10 S
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Product Instructions: Speed of Light Kit
Speed of Light in Optical Fibre
Setup
First time: drill a 2mm hole in two of the lens
holders to hold the fibre optic cable, as illustrated.
The holes should be drilled at a height where the
cable can be threaded through and point at the
LED of the transmitter or receiver.
From the 10m reel of fibre optic, pull out
approximately a metre at each end, but leave the
rest in the bundle for convenience.
Mount the clips on circular bases, and thread each
end through the hole you’ve drilled.
Position the stands so that the ends of the fibre
optic cable are facing the LEDs of the transmitter
and receiver.
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Product Instructions: Speed of Light Kit
Measurement
The measurement procedure is exactly the same as that of measuring the speed in air.
t
Use the divisions on the oscilloscope screen
to measure the distance between the end of
the transmitted pulse and the peak of the
received pulse. It is necessary to measure to
a precision of at least 0.01μs, which is 1/10th
of a division if the timebase is 0.1μs/div.
Multiply the number of divisions by the
timebase to find t.
For example, if the timebase is 0.1μs/div, and
you measure a distance of 2.1 divisions, then:
t=0.21μs
Calculation
t
The propagation time f of the light through the fibre is the difference between the total time
and the calibration time:
: delay on received signal
f= 0 - 0: calibration delay
t
t
t
t
t
And the speed of light through the fibre optic is simply the distance over time:
Df - D0 Df: length of fibre optic cable
f=
D0: calibration distance
f
c
t
The length of the fibre optic cable is 10m. Say you measured a total time of 0.21 μs and a
calibration.16 μs, this would give you:
cf =
10m
(0.21 - 0.16) x 10 S
-6
= 2 x 108ms-1
Refractive Index
The refractive index of a material is the ratio of the speed of light of the medium to that of a
vacuum (which is approximately 3 x 108ms-1).
3 x 108ms-1
2 x 108ms-1
= 1.5
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