7.
Lines from lasers
In
order to follow a track in a bubble chamber photograph,
Sweepnik needed to know the direction in which the track
was going. This could be achieved by projecting a short
rotating line of light onto the film.
To
produce the line, light from a helium-neon laser was
passed through an astigmatic lens. The astigmatic lens
focused the light onto a line.
The
light was then passed through a rotating Dove prism.
A Dove prism is made of glass with its two ends at 45
degrees to the top and bottom surface, and all other
surfaces at right angles to each other. Light entering
the prism through one of the 45 degree surfaces is reflected
inside the prism before leaving through the other 45
degree surface. If the Dove prism is rotated, the light
exiting it will be rotated twice as much.
The
light leaving the Dove prism was a line image, rotating
around like the hand of a clock, but at 48 revolutions
a second. The angle of rotation could be accurately
measured from a coded disk attached to the Dove prism.
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