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Thermodynamics

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Jamin Refractometer

It is used to determine the refractive index of a gas at different pressures. A and B are two glass plates silvered at their back surfaces. The two plates are sufficiently thick and two identical glass tubes T₁and T₂ are placed in the path of the beams 1 and 2 respectively. A source S is placed at the focal plane of the lens L and a parallel beam of light is incident on the front surface at the plate A. It is divided into two beams by the plate A. The beam 1 is reflected by the front surface and the beam 2 is reflected by the back surface. The two beams are incident on the plate B and the beam 2 is reflected by the front surface and the beam 1 is reflected by the back surface. The emergent beams interfere and they are viewed by a telescope T which is focused at infinity. Interference fringes are obtained. Here, the planes of A and B are inclined at a small angle.

The tubes T₁ and T₂ are evacuated and the fringes are observed in the field of view of the telescope. The gas is allowed to enter one of the tubes and the number of fringes that cross the centre of the field of view is counted. Suppose, n fringes have crossed the field of view. If the length of the tube is l, the path difference introduced = ( – 1)l

∴ ( – 1)l = nλ

Therefore, the refractive index of the gas at a desired pressure can be determined.

In order to avoid the counting of fringes every time, two compensating plates C₁ and C₂ of equal thickness cut from the same piece, are introduced in the beams 1 and 2. The plates C₁ and C₂ can be rotated about a common horizontal axis (at a fixed angle θ between them) with the help of a calibrated circular disc D. When the disc D is rotated, the interfering beams passing through C₁ and C₂ are affected such that in one case the path increases and in the other case it decreases. The circular disc is calibrated by counting the number of fringes directly and is marked in terms of the refractive index and the number of wavelengths. Here the tubes T₁ and T₂ are evacuated and using white light the telescope is focused such that the central white fringe is in the field of view. The gas is introduced at a desired pressure and temperature, into the tube T₁. The central fringe shifts. With the help of circular disc D, the plate C₂ is rotated to bring the central fringe back to its original position. The reading on the
calibrated circular disc directly gives the refractive index of the gas.