PASCO OS-8501 Interferometer - Manual (Page 7)

scientific
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012-02675B Interferometer
In this way the original beam of light splits, and
portions of the resulting beams are brought back
together. The beams are from the same source and
their phases highly correlate. When a lens is placed
between the laser source and the beam-splitter, the
light ray spreads out. An interference pattern of dark
➦
NOTE: Do not be concerned if your pattern
shows irregularities or has fewer fringes. As
long as fringes are clearly visible, measurements
will be accurate.
Since the two interfering beams of light were split
from the same initial beam, they were initially in
phase. Their relative phase when they meet at any
point on the viewing screen, therefore, depends on the
difference in the length of their optical paths in reach-
ing that point.
By moving mirror M
2
, the path length of one of the
beams can be varied. Since the beam traverses the
path between M
2
and the beam-splitter twice, moving
M
2
1/4 wavelength nearer the beam-splitter will reduce
the optical path of that beam by 1/2 wavelength. The
interference pattern will change; the radii of the
maxima will be reduced so they now occupy the
position of the former minima. If M
2
is moved an
additional 1/4 wavelength closer to the beam-splitter,
the radii of the maxima will again be reduced so
maxima and minima trade positions. However, this
new arrangement will be indistinguishable from the
original pattern.
By slowly moving M
2
a measured distance d
m
, and
counting m, the number of times the fringe pattern is
restored to its original state, the wavelength of the light
(
λ
) can be calculated as:
λ
=
If the wavelength of the light is known, the same
procedure can be used to measure d
m
.
Figure 2 INTERFERENCE PATTERN
and bright rings, or fringes, is seen on the viewing
screen, as shown in Figure 2.
2d
m
m