Pasco OS-9257A PRECISION INTERFEROMETER Manuel d'utilisateur télécharger pdf (Page 26)

Precision Interferometer 012-07137A

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:

741

+5-15v

56k 20k

10k

1k 1k

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.

(2t –Nλ

)(1 – cosθ)+(

)

2t(1 –cosθ) –Nλ

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Pasco OS-9257A PRECISION INTERFEROMETER Manuel d'utilisateur Page 26