scientific
5
012-02675B
Interferometer
Figure 5 ADJUSTING M
1
LASER
REFLECTED
LASER BEAM
OPTICS BENCH
ALIGNMENT RAIL
VIEWING SCREEN
LASER BEAM
10 - 20 cm
Aligning the Interferometer
➦
NOTE: This alignment procedure is for
those using a PASCO scientific Optics Bench. If
you are not using an Optics Bench, tape a
straightedge to a flat level surface. The straight-
edge will provide a substitute for the alignment
rail of the optics bench.
➀
Place the laser and the interferometer on the Optics
Bench, approximately 10 - 20 cm apart (Figure 5).
Be sure that the edges of both units are flush
against the alignment rail of the bench. Place a
viewing screen as shown. (A blank sheet of white
paper taped to the cover of a book provides a con-
venient screen.) Turn on the laser.
➁
Loosen the thumbscrew that holds the beam-splitter
and rotate the beam-splitter so it is out of the beam
path of the laser as shown in Figure 5. Then loosen
the thumbscrew that holds M
2
, the movable mirror.
Adjust the rotation of M
2
so the laser beam is re-
flected directly back toward the aperture of the la-
ser. (The reflected beam need not be at the same
height as the incident beam, but it should strike the
front panel of the laser along a vertical line through
the aperture.) Hold M
2
in position and tighten the
thumbscrew.
➂
Rotate the beam-splitter so its surface is at an angle
approximately 45
o
with the incident beam from the
laser (see Figure 6). You will see two sets of laser
spots on the viewing screen, corresponding to the
two paths that the beam takes in reaching the
screen. (Each path results in more than one laser
spot because of multiple reflections within the
beam-splitter.) Adjust the beam-splitter so the two
sets of laser spots are as close as possible, then
tighten the thumbscrew to secure the beam-splitter.
Using the alignment screws, adjust the angle of M
1
until the two sets of laser spots are superimposed
on the viewing screen (the two brightest spots must
be superimposed).
➄
Place the lens holder on the optical bench as shown
in Figure 7. Be sure its edge is flush against the
alignment rail. Then place the 18 mm focal length
lens on the lens holder (it attaches magnetically).
Adjust the position of the lens on the holder so the
light from the laser, now spread out by the lens,
strikes the center of the beam-splitter. If you have
performed the alignment correctly, you will see an
interference pattern of concentric rings on the view-
ing screen. If the alignment is not just right, the
center of the fringe pattern may not be visible on
the screen. Adjust the alignment screws on M
1
very slowly as needed to center the pattern.
➃
