PASCO OS-9257A PRECISION INTERFEROMETER - Manual (Page 26)

Precision Interferometer 012-07137A
22
Experiment 3: The Index of Refraction of Glass
Reference to – Procedure
1.
The glass plate must be absolutely perpendicular to the
laser for accurate measurement of the index of refrac-
tion. When the plate is perpendicular, there will be a
faint secondary fringe pattern (Fabry-Perot interference
between the front and back surfaces of the plate) vis-
ible in the center of the view screen.
2. It is important to measure as large an angle as possible,
and measure the angle as carefully as possible.
Reference to – Analysis
1.
The actual equation, which is derived in Optics of the
Electromagnetic Spectrum, by C.L. Andrews
(Prentice-Hall, 1960) is
The second term is negligible for visible wavelengths,
and may be ignored.
Notes – General
It is often difficult to count large numbers of fringes due to
eyestrain. If you find this to be the case, you may want to
make a circuit such as this:
+
-
7 4 1
+ 5 -1 5 v
5 6 k 2 0 k
1 0 k
1 k 1k
1 k
The phototransistor should be mounted in a plate of sheet
steel, which can then be held in the magnetic viewscreen
holder. Mask the transistor with a piece of electrical tape
with a pinhole at the center. Adjust the sensitivity of the
circuit with the 20k potentiometer so that the flashes of the
LED can be counted instead of the actual fringes.
This circuit may also be used in conjunction with the
PASCO Series 6500 computer interface so that the fringes
can be counted by computer, if desired.
n
g
=
(2t – N
λ
0 )(1 – cos
θ
) + (
N
2 λ
0
2
4t
)
2t(1 –cos
θ)
– N
λ
0