pHYSICS

Experiment 7

Aim of the Experiment

1. To find refractive index of the given liquid samples.

2. To study the variation of refractive index with

3. (a) temperature of the liquid sample.
   (b) wavelength of the light source.

To determine the polarisability of the given liquid samples at a given temperature.

Principle

Abbe’s Refractometer

The Abbe instrument is the most convenient and widely used refractometer, Fig(1) shows a schematic diagram of its optical system. The sample is contained as a thin layer (~0.1mm) between two prisms. The upper prism is firmly mounted on a bearing that allows its rotation by means of the side arm shown in dotted lines. The lower prism is hinged to the upper to permit separation for cleaning and for introduction of the sample. The lower prism face is rough-ground: when light is reflected into the prism, this surface effectively becomes the source for an infinite number of rays that pass through the sample at all angels. The radiation is refracted at the interface of the sample and the smooth-ground face of the upper prism. After this it passes into the fixed telescope. Two Amici prisms that can be rotated with respect to another serve to collect the divergent critical angle rays of different colors into a single white beam, that corresponds in path to that of the sodium D ray. The eyepiece of the telescope is provided with crosshairs: in making a measurement, the prism angle is changed until the light-dark interface just coincides with the crosshairs. The position of the prism is then established from the fixed scale (which is normally graduates in units of n_D). Thermosetting is accomplished by circulation of water through the jackets surrounding the prism.

The Abbe refractometer is very popular and owes its popularity to its convenience, its wide range (n_D = 1.3 to 1.7), and to the minimal sample is needed. The accuracy of the instrument is about ±0.0002; its precision is half this figure. The most serious error in the Abbe instrument is caused by the fact that the nearly glazing rays are cut off by the arrangement of to prisms; the boundary is thus less sharp than is desirable.

A precision Abbe refractometer, that diminishes the uncertainties of the ordinary instrument by a factor of about three, is also available; the improvement in accuracy is obtained by replacing the compensator with a monochromatic source and by using larger and more precise prism mounts. The former provides a much sharper critical boundary, and the latter allows a more accurate determination of the prism position.

Procedure

To switch the views of the equipment, click the left and right arrows on the bottom of the simulator.

1. Turn on the light source.

2. Choose the liquid sample.

3. Water is used first to calibrate the instrument.

4. Turn the refractometer scale knob to get the clear interface between the illuminated and dark regions.

5. Read the index of refraction using the telescope scale.

6. Temperature of the sample can be varied by using the slider.

7. Repeat the experiment with different liquids and study the variation of refractive index with temperature and wavelength.

Observation Table

Polarisibilty ,

Where,

ε₀ - permitivity in free space = 1.

n - refractive index of the given liquid at a particular temperature.

N_A - Avogadro number = 6.023 x 10 ²³ mol^-1.

M and ρ are the molecular mass and density of the given liquid.

For water, M - 18.01g/mol and ρ - 1000Kg/m³.

Benzene, M - 78.11 g/mol and ρ - 0.8765g/cm³.

Nitrobenzene, M - 123.06g/mol and ρ - 1.199g/cm³.

Toluene, M - 92.14g/mol and ρ - 0.8869g/mL.

Aniline, M - 93.13g/mol and ρ - 1.0217g/mL.

Ethanol, M - 46.07g/mol and ρ - 0.789g/cm³.

Example: For sodium light as source, water is the liquid the refractive index at 20⁰C is 1.333 and polarisibility is 3.25*10^-23.

Calculation

Refractive index of the material ‘n‘ for given temperature = ……………………

Result

1. Refractive index of the given liquid sample at a particular temperature is……………………..

2. Polarisability of the given liquid sample at a particular temperature is………………………….

Reference Material

1. A Treatise on Heat, Meghnad Saha, and B. N. Srivastava, 1969, Indian Press

2. Virtual Lab (http://vlab.amrita.edu/?sub=3&brch=194&sim=802&cnt=1)

Questions

1. Why refractive index changes with temperature?

2. What is working principle of Abbe’s Refractometer?