3B Scientific Speed of Light Meter (230 V, 50__60 Hz) - Manual (Page 7)

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used for this experiment as long as their band-
widths are over 20 MHz.
4.3 Oscilloscope settings
Mode: Dual channel operation
Sensitivity: Y1/Y2 at 100 mV/DIV
Time base: Minimum value, e.g.
50 ns/DIV
External triggering: “EXT” (trigger from C)
(recommended for oscilloscopes with bandwidths
of less than 100 MHz)
The test pulse from output “A” should first be op-
timised to a maximum value by carefully position-
ing the components required for the experiment.
If the mirror and the lens are optimally positioned,
the amplitude of the test signal can be greater than
that of the reference signal for a distance of up to
8 m. This being the case, it is then possible to make
the amplitudes from outputs “A” and “B” roughly
equal by turning the lens very slightly from its
“ideal position”.
In order to measure or evaluate the delay between
“A” and “B” (time taken for light to travel the path),
both of the pulse maxima should be adjusted to
about the same height. This can be set by adjusting
one of the two “Y position” (Y-POS) knobs on the
oscilloscope.
4.4 Determining the speed of light
The speed of light c is simply given by the length of
the path travelled by the light divided by the time
the light takes to cover it.
The length of the beam path is double the distance
from the light emitter to the mirror since the light
travels this distance twice, there and back.
The time taken for the light to travel this distance
is read off from the oscilloscope by measuring the
distance between the maxima of the two pulses.
Fig. 1: Experiment set-up: A Microprism mirror, B Fresnel lens, C Optical bench, D Basic unit
Fig. 2: Connections between the basic unit and the oscilloscope