REFRACTIVE INDEX

 "Refractive index in) of a transparent substance is defined as the ratio of the lity of light in warm to the elocity of light in the given medium" The refractive indes of any suletanee depends upon, the nature of substance, temperature and the wave length of light used. Whenever a beam of manschromatic light passes from one medium into the other, it suffers a change in the direction of propagation. This change of direction is called refraction. depends on the nature (refractive index) of the twe media, and the direction which the light travels When a ray of light passes from a rarer (air) to a denser medium liquid or glass), it bends towarde the normal at the point of incidence, as shown in Fig 3.12. According to Snells law, the ratio of the sine of angle of incidence and that of refraction in onstant and characteristic for that medium.

                          n=Sin i / Sin r         (Snell's law)

Arding to the way theory of light, the ratio of the sines of the angles of incidence and refracton is identical with the ratio of the velocities of light in the two media. Thus

          

                 n = (sin i)/(sin r) = velocity in air / velocity ia liquid

According to the law of refraction,

             

                     (sin i)/sin r. = n2/ n1

 

where the refractive index of the rarer and the refractive indes of the denser of refraction also increases. ronches maximum value and is known as the critical angle. Since sin 90 = 1 Eq (27) becomes

                    Sinr  =n1/n2

If the angle of incidener is grunter than 90, the ray is totally reflected. Most of the refractometers work on the principle of critical angle for measuring refractive index di mediam.

MEASUREMENT OF REFRACTIVE INDEX :

The refractive indices of liquids can be measured directly with calibrated mutrument called refractometer In practice, Abbe and Pulfrich refractometere are unly used

 Abbe Refratometer

A general sketch of the Ahe refractometer is shown in Fig. 3.13 the ptical system of the Abbe refractometer consists of three parts (1) a mirror M. (2) two prisms A and B had in a box hinged at H and (3) a fixed telescope Tand an eye-piece O The two prism faces can be held in contact with knob C. To the box carrying the prisme, is attached an arm R which moves along a graduated scale S, a direct reading on the scale given the refractive index.

Procedure

 The priam box is opened, a drop of the eliqund is placed between the two priams and then the box is closed. The cross-wires of the telescope are focused by rotating the eye-piece and the mirror is bus is slowly moved backwards and forwarda by means of the knoh C until the field of view becomes partly dark and partly bright. When the white light is used the moured fring alserved are removed by rotating the compensatori consisting of twe prime sctached st the use of the telescope and a sharp line will divide the bright and dark portions The priame bax is then rotated until the end of the bright portion coincides with pint ef intersection of the cross-wires of the telescope, and the refractive indes is directly anted on the scale through the eye piece 0.

Since refractive indes is affected by the temperature and wavelength of light. In order to maintain consistency of temperature and wavelength, prisms A and Bar mclused in a water jacket J and sodium or mercury light is preferably used.

 

 

PULFRICH REFRACTOMETERE:

 Pulfrich refractometer is very accurate and simple is principle for measuring the refractive andises of liquids The basic  design of the strument is shown in Fig (3:14) The essential part of the instrument is a right angled glass prism with a small glass cell cemented to its top.

The liquid to be examined is placed is the glass coll and beam of Лалабивних light is made to enter at grading taxideas along the surface beiteteny che big and the prim. It follows the path ABCD and is observed by a telescope at D If be the angle of refraction when the angle af incidence is 90 degree

                                sin r = n1/n2

 where n1 the refractive index of the liquid and n2 that of the glass prism. It is clear fem Fig. 14) that

        Sin I/ sin (90-r)  = n2

        But sin(90-r)=cos r , thus we have

        Sin i/ Sin r = n2.  Or. Cos r = sin I/n2

But. Sin r   = 1-Sin2i/n2 2 = n2 2 - sin2 I/n2 2

Or n2 sin r = n2 2-sin2i

          n1 = n2 2 - sin 2 i ( since n1=n2 sin r from (eq28)

 If the refractive index n2 of the prism is known and the angle is measured, the refractive index of the liquid, can be calculated. In practice, tables of sqrt n2 2-sin2i putting the refractive indices from the values of the measured angle i.