3B SCIENTIFIC PHYSICS U17303 Instructions D'utilisation page 9

5.1.2 Michelson interferometer
•
Position the laser half way along the longer side
of the metal board (Fig. 2) and adjust the laser
beam parallel to the ground plate (see Notes).
•
Position mirror M2 on the opposite side of the
board, ensuring that the side without the screw
is facing the laser. Direct the beam back to the
laser by adjusting the screws on the laser holder
and the mirror holder.
•
Position the semi-transparent mirror between
the laser and mirror M2 as shown in Figure 2.
The angle between the semi-transparent mirror
plane and the axis of the laser beam should be
45°. Correct positioning is vital.
•
Position the ground screen as shown in Figure 2,
on the shorter side of the board. The beam spot
should appear in the centre of the screen.
•
Place mirror M1 opposite the ground screen on
the other side of the board.
•
Merge the spots on the screen by moving mirror
M1 slightly and adjusting the screw on the mir-
ror holder and place the spots on the same
height as the laser source (see Notes).
•
Position the lens between the semi-transparent
mirror and the laser. This causes a typical inter-
ference pattern to appear.
Fig. 2 Experimental set-up Michelson's interferometer. (M1,
M2 = mirrors, SM = semitransparent mirror, S = ground
screen)
Notes:
Before the beginning of the experiment clean the
lens, so that no parasite interference on dust parti-
cles attached to lens surface occur. You can easily
identify such parasite interference as a number of
concentric circles. An interference of beams coming
from only one of the mirrors M1 or M2 can occur.
This interference is easy to identify as occurring even
by covering of one the mirrors M1 or M2.
By adjusting the set up according to Figure 2 it is
very important that the interfering spherical waves
are containing only a small angle (see Figure 3a),
than we can observe the interference in the area I. If
the spherical waves contain too big an angle (Figure
3b) the interference can not be observed. Therefore
it is important to adjust the laser beam parallel to
the ground plate and try to keep it parallel after
reflection from mirrors M1 and M2.
Fig. 3 Interference of two spherical waves
After placing and adjusting the mirrors observe two
images of laser beams on the screen. By adjusting
the mirrors and the laser place these two images in
the same height as the laser source and on the same
place on the screen. This way it is secured that the
beam axis will be parallel to the ground plate and
will intersect the image screen. It is better to carry
out this adjustment without the lens placed.
It is very helpful to adjust the mirrors by positioning
the laser very close to the semitransparent mirror.
The mirror images should be nearly of the same
shape and position. After finding the interference
image we can move the laser source freely, without
affecting the success of the interference.
In accordance to the high sensibility of the Michel-
son interferometer is it very important to place the
base plate very stable and resistant to shake disrup-
tion. Touch the whole set up very carefully during
adjustments.
If there is no interference image on the screen, re-
move the lens and ensure that the interfering beams
are parallel to the ground plate and are hitting the
screen at the same place. If that's correct and there
is still no interference image, move one of the mir-
rors on the optical axis back or forward about 1 mm.
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