Experiment Evaluation; Twyman-Green Interferometer; Experiment Setup; Fabry-Perot Interferometer - 3B SCIENTIFIC PHYSICS U10350 Manuel D'utilisation

5.3.3 Experiment evaluation

•
At pressure p = 0 the refractive index n(p = 0) = 1.
For increasing pressure, the refractive index in-
creases according to the following equation:
∆
n
( ) = +
n p 1 p
.
∆
p
•
Therefore, to determine the refractive index at
normal air pressure, the gradient ∆n / ∆p must
be derived first. To a first approximation:
∆ λ ∆
n m
=
∆ ∆
p 2 l p
z
•
In this case m is the number of rings appearing or
disappearing, λ is the wavelength of the light and
l is the internal length of the vacuum cell (here
Z
41 mm). If, for example, for a pressure ∆p = 90 kPa,
a corresponding value of ∆m = 25 is determined,
then ∆n / ∆p = 2.14×10
•
This means that the refractive index of air at at-
mospheric pressure (100 kPa) is n = 1.00021. This
compares to a value of n = 1.00029 as quoted in
2)
the literature .

6. Twyman-Green Interferometer

• This experiment determines the surface quality of
optical components. Normally a Twyman-Green
interferometer is considered to be one where one
(laser) beam is diverging and the other parallel. In
order to understand the principle, however, it is
possible to use a diverging but not a parallel beam
(as in the previous experiments.

6.1 Experiment setup

•
As an example of a poor optical surface, trans-
parent sticky tape is used. The tape is glued to
a glass plate and appears homogenously trans-
parent when peered through in the normal
way.
•
The experiment setup is initially the same as that
of the standard experiment (see Section 5.1.1).
Thereafter the glass plate with rotating holder
introduced into the beam at front as in Fig. 5.
The adjustable mirror is then minimally read-
justed so that the interference rings appear in the
middle of the screen.
6.2 Experiment procedure and evaluation of
observations
•
On the right-hand side of the observation screen,
regular interference rings can be seen as in Ex-
periment 5.1. On the left-hand side, however, the
rings are fuzzy and bright points appear in what
are really dark regions and vice versa. Since we
know from Experiment 5.2 that even tiny changes
in the thickness of coatings (accomplished in Ex-
periment 5.2 by rotating the glass plate) can cause
the interference rings to shift, the implication is
2) H. Stöcker, Taschenbuch der Physik, Deutsch, 1998
–9
1/Pa.
that the distortion in the rings can be explained
by the bumpy, uneven surface of the sticky tape.
Fig. 5: Experiment setup for measuring the surface quality of optical components

7. Fabry-Perot Interferometer

7.1 Experiment setup
• The setup of the components on the base plate is
shown in Fig. 6. It is assumed in the following de-
scription of the setup, that all the basic settings have
to be made in advance.
• Screw the He-Ne laser into its holder (see above
instruction for assembling laser) and position it so
that it is more or less straight in front of the diverg-
ing lens.
• Remove the adjustable mirror and the diverging
lens and place the laser in front of the base plate
so that it is slightly turned about a vertical axis.
Then adjust the inclination of the laser so that part
of the beam that is reflected back onto the laser is
at the same height as the emitted beam. If the la-
ser is now turned straight in front of the base plate
again, two points appear on the screen. Adjustments
should then be made to cause the two points to
coincide.
• Place the diverging lens in the beam and, if neces-
sary, adjust the height of the beam by turning all
three knurled screws by the same amount. Then
return to the second one as required. (It may be
that weak interference rings are apparent on the
screen since even the ostensibly transparent side
of the glass of the finely adjustable mirror also re-
flects the beam slightly.)
• Turn the diverging by 90° again so that it is out of
the beam.
• Mount the adjustable mirror with great care. The
distance between the two mirrors should be about
2 mm.
• Set up the adjustable mirror so that all the points
and any interference patterns coincide on the
screen.
• Turn the diverging lens back into the beam and
secure it in position so that the image is in the
middle of the screen (i.e. the bright part of the beam
should be centered not the interference rings). Tilt
the screen away from the vertical enough so that
the observer sees the brightest and clearest image.
11