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Thermodynamics

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Pulfrich Refractometer

It is used to measure the refractive indices of solids and liquids. It consists of a right angled prism A having its two faces perfectly plane. One of the faces is horizontal and the other is vertical. The solid B whose refractive index is to be determined is taken having two faces cut perpendicular to one another. The faces are made perfectly plane. The solid B is placed on the prism A. To bring perfect contact between the horizontal faces of A and B a few drops of a liquid whose refractive index is greater than the solid is used. In the case of a liquid, whose refractive index is to be determined, a glass cell is used.

Light is incident in a direction parallel to the horizontal surface so that light entering the prism A is incident at the critical angle c with the normal. Finally it emerges from the prism at an angle i.

Let the refractive index of the solid be and that of the material of the prism A be

Here and is the critical angle.

Squaring and adding equations (i) and (ii)

Knowing and i, the value of can be calculated.

To measure i, the apparatus has a collimating telescope with a circular scale attached to it. The telescope is adjusted so that the cross-wires lie on the dark edge of the field of view. This gives the position of the minimum angle of emergence i. the material of the prism A has a refractive index of 1.74. In modern instruments, the circular scale is calibrated in terms of the refractive index and readings can be read directly from the scale. In some cases, a table is provided with the instruments which gives the value of corresponding to i.