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2. Description
Equipment set U8476460 is used for determining
the speed of light by electronically measuring the
time it takes to travel a certain path.
Extremely short light pulses are transmitted from
an LED light source via a beam splitter before fi-
nally reaching two photo-voltaic converters con-
nected to two amplifiers that send voltage pulses to
outputs "A" and "B" for evaluation on an oscillo-
scope. Output "B" provides the reference signal
while output "A" outputs a signal that is delayed by
the time it takes for light to travel from the source
to the mirror and back to the detector. The oscillo-
scope can be triggered via a pulse from output "C".
The equipment set U8476460-115 is for operation
with a mains voltage of 115 V (±10%), and the unit
U8476460-230 is for operation with a mains voltage
of 230 V (±10%).
3. Scope of delivery
1
Basic unit including light emitter, receiver and
integrated power supply
1
Fresnel lens on stem
1
Triple prism reflector on stem
3
BNC cables
3. Technical data
Basic unit
Light emitter:
Pulse rate:
Power input:
Voltage:
Dimensions:
Stem:
Weight:
Lens
Fresnel lens:
Lens surface:
Dimensions:
Stem:
Weight:
Mirror
Design:
Mirror diameter:
Dimensions:
Stem:
LED
30 kHz approx.
3 W approx.
115/230 V, 50/60 Hz
3
103 x 56 x 175 mm
150 mm x 10 mm diam.
1 kg approx.
f = 375 mm
245 mm x 245 mm
285 mm x 285 mm
54 mm x 10 mm diam.
200 g approx.
Micro-prism mirror
100 mm approx.
3
170 x 170 x 40 mm
54 mm x 10 mm diam.
4.1 Experiment set-up
The basic unit and the Fresnel lens are mounted on
riders along an optical bench and should be
aligned together with the microprism along the
optical axis.
The minimum distance between the basic unit and
the lens (distance a) should be approximately f, the
focal length of the lens. If a = f, then distance b
would be infinity.
The minimum distance between the light emitter
and the mirror should be approximately 150 cm if
the lens is positioned in the middle. If the distance
to the reflector is increased, then the ideal distance
for a comes down towards approximately 37 cm.
Distance b then tends towards infinity.
To achieve optimum results, take special care to
maintain sufficient accuracy in the horizontal and
vertical alignment of the basic unit and the lens.
The red spot projected onto the reflector is clearly
visible in the position illustrated and it continues
to appear in full on the reflector segment even at a
distance of up to approximately 8 m.
Outputs "A" and "B" of the basic unit are connected
to the Y inputs of the oscilloscope via high-
frequency leads of equal length and equal imped-
ance (see Fig. 2).
The synchronising trigger for the oscilloscope
should be set to "ext".
pulse is provided by output "C".
In order to receive a high proportion of reflected
light, the Fresnel lens and, if necessary, the micro-
prism mirror should be realigned so that they are
precisely in line. It helps when making the adjust-
ment to look along the line to the mirror from the
basic unit. The setting is optimal when the light
beam from the source is clearly visible, focussed on
the surface of the mirror.
In an experimental set-up involving greater dis-
tances (a + b) or unfavourable lighting conditions,
it may be easier to pick out the beam from the
position of the mirror. This means putting your
head just in front of the mirror and moving it till
you can see the light source through the lens. This
should enable you to adjust the lens and the mir-
ror more easily.
4.2 External synchronisation
Socket "C" outputs a synchronising trigger pulse
which is coupled to the light beam and precedes
the two beam pulses (reference pulse and test
pulse) from output "A" and output "B" by 60 ns.
This method allows older oscilloscope models to be
2
4. Operation
⋅
a
f
=
b
−
a
f
A synchronising trigger
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