LEGO Interferometer User manual

Abbildung 1: Das Interferometer aus LEGO-Steinen.
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LEGO-Interferometer
INTRODUCTION
The interferometer built from LEGO-bricks was developed for usage in
schools. All parts can be build b students using LEGO-manuals in about
15 minutes.
This manual will describe how the additional optical components can
be added to the LEGO-construction. In addition to that, the calibration
process of the interferometer is explained.
PREPARATION
 Function of a Michelson interferometer.
 Mathematical relation between the wavelength of light and the
translation of the mirror (see the end of this manual).
COMPONENTS
 LEGO-bricks or alread built components
 1x Hama Laserpointer LP18
 2x Front surface mirror 30mm x 20mm,
 1x Semi-transparent front surface mirror
 Glass lens, diameter d=18mm, f=26,5mm
 Hexagon screw M5 x 40mm
 Rubber bands
IMPORTANT INFORMATION
Before adding the parts, please be aware that the optical components
are very prone to damage and therefore have tob e handled with care.
The quality of the interference patterns will be disrupted by
fingerprints on the lens or the mirrors. The experiments use front face
mirrors  why?). These are coated and the coating should not be
touched if possible. Cleaning of the optical components is not to be
done by using common handkerchiefs  why?)
0. ASSEMBLY
Build the components of the LEGO interferometer b using manuals 1-
4. Should the components already be assembled, please connected
them as shown in figure 1 and continue with the steps for adding the
optical components.

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1. ADDING THE LASER
Put the laserpointer into the laser mount as shown and fixate it at the
back end b using the movable parts A, B and C.
B using the gear, adjust the laserpointer into a horizontal
position.
2. ADDING THE SCREW
Remove the bricks as shown.
Guide the M5 screw through the holes in the blue bricks.
Put a rubber band several times around the two angles to give
additional support to the screw.
Put the bricks with the screw back at its former place. The head of the
screw will be put between the two angles as shown.

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Wind two rubber bands around the sled and the screw’s mount so
that the sled is pressed against the screw as shown.
3. ADDING THE MOVABLE MIRROR
Wind a rubber band around the LEGO bricks that the mirror will be
placed against. Pull down the rubber band to the base of the bricks.
Determine front and backside of the front surface mirror. Hold the
mirror as shown in the picture.
Slide the mirror between the rubber band and the bricks. The back
surface should point towards the bricks.
Wind a second rubber band around the top of the bricks and the mirror.
4. CALIBRATING THE LASER
Switch on the laser pointer. Use the clamp to permanentl have the
button pressed.

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Adjust the laser pointer b using the gear D. The laser must be reflected
back to the pointer at the same height as it is leaving the pointer.
Correct the laser’s horizontal angle b using parts A and B so that the
laser is reflected exactl back into the opening of the pointer. Switch off
the laser pointer.
5. ADDING CALIBRATABLE MIRROR
Secure the mirror mount with a rubber band against the pla during
sidewa s rotations.
Place a rubber band in front of the movable arm so that it is pressed
against the Lego screw.
Locate the front side of the front surface mirror (see above). Place the
mirror on the intended location and fixate it with the T-link (E). During
this step, it is important that the mirror is placed exactl as shown.
Adjust the mirror mount b using the two gears so that the mirror is
perpendicular to the ground plate and parallel to the bobble pattern.

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6. ADDING THE BEAMSPLITTER
Hold the beam splitter at the intended place.
Fixate the beam splitter b pressing the two rods tot he splitter b using
the T-link.
Adjust the beam splitter b light pusshing and pulling at the two rods. It
should be in a 45° angle tot he bubble pattern rows on the ground plate.
7. CALIBRATING CALIBRATABLE MIRROR
Place the screen at the position shown.
Switch on the laser pointer. There should be two points visible at the
screen.
Adjust the calibratable mirror so that the two points are on top of each
other.
Place the screen at a larger distance awa from the construction and
adjust the mirror, if two points are visible again.

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. ADDING THE LENS
Hold the lens at the intended place and fixate it on both sides. Adjust
the mount so that the beam hits the lens at its center. The height can
be adjusted at the places marked b the red arrows.
FINE TUNING
There should now be an interference pattern on the screen. The center
of the pattern can be shifted bei carefull adjusting the calibratable
mirror mount and the lens.
TIPS AND POSSIBLE PROBLEMS
1. Should the front surface mirrors be placed at equal distances, the
rings of the interference pattern might be too far apart and mostl
not recognizable. In this case, it might help to take out the Lego-
part with the screw and turn it for an additional one or two
windings.
2. The interferomter is susceptible for all kinds of vibrations.
Therefore, it is advised to place it on a stable surface. It might be
helpful to use a clamp for fixating the interferometer to the table.
3. The stabilit can often be increased b making sure to press the
bricks into each other whilst building the parts. This can be
especiall helpful at the links between the ground plates.
CALCULATING THE WAVELENGTH
To determine the wavelength of the laser, the gear is turned b the lever
at the end of the transmission and the change of rings in the
interference pattern is counted. The ration between rings and turns can
be calculated b using the transmission ratio of the screws.
The transmission is constructed so that one full turn of the lever means


of a turn of the screw.
The screw has a pitch of 0,8mm. This means a shift of the sled b



   per turn.
All in all, one can get the wavelength  b using
 








.
 ist he distance that the sled travels,  the number of ring shifts and
the number of turns of the lever.