scientific
Interferometer
012-02675B
4
Operation
M1 (FIXED MIRROR)
M1 ALIGNMENT
SCREWS
MICROMETER
KNOB
M2 (MOVABLE MIRROR)
BEAM SPLITTER
where r is the radius of the lever post. The mirror is
therefore pulled away from the beam-splitter by the
amount, r 0.
In this way, a relatively large displacement of the lever
(d = R 0) results in a much smaller displacement of the
mirror (d
m
= r 0). By selecting appropriate values for
is controlled so that each
division on the micrometer dial corresponds to 1
micron of mirror movement.
Figure 3 INTERFEROMETER
Figure 4 MIRROR MOVEMENT MECHANISM
, the motion of M
and
r
R
2
MYLAR
STRIP
θ
r
θ
M2
r
θ
MICROMETER
KNOB
r
θ
R
d
The Interferometer
The Michelson Interferometer is shown in
Figure 3. The alignment of the beam-
splitter and the movable mirror, M
2
, is
easily adjusted by loosening the thumb-
screws that attach them to the interferom-
eter. The fixed mirror, M
1
, is mounted on
an alignment bracket. The bracket has two
alignment screws to adjust the angle of the
mirror.
The movement of M
2
toward and away
from the beam-splitter is controlled and
measured using the micrometer knob.
Each division of the knob corresponds to 1
micrometer
(10
-6
meter) of mirror movement.
The Movable Mirror
To measure the wavelength of light, the movement of
M
2
must be measurable for distances about 10
-6
meters. Also, as the mirror moves, its reflective
surface must remain perpendicular to the axis of the
incident light beam.
A taut-band carriage is used to maintain the alignment
of the reflective surface of M
2
as it moves. The mirror
is mounted in a cradle that is fixed to two semi-rigid
aluminum bands. With this set-up the mirror is free to
move, but its movement is constrained to a line
parallel with the beam axis.
The micrometer mechanism controls and measures the
movement of M
2
. The cradle of M
2
is attached to a
mylar strip that is attached to a lever arm. The dis-
placement of the lever is controlled with the microme-
ter knob.
Suppose the micrometer knob is turned so it pushes the
lever in by a distance d (see Figure 4). The angle of
the lever arm changes by an amount 0 such that d = R
tan 0, as shown. Since the angle change is always
small,
R tan 0 = R 0, to a close approximation. This change
in the lever arm angle causes the mylar strip to be
pulled further around the lever post by an amount r 0,
