Dispersive power. of Prism

Transcription

1 Dispersive power of Pris 1

2 Dispersive Power of Pris Experient No. Object : To deterine the dispersive power of the aterial of a pris by spectroeter. Apparatus : Spectroeter, pris, spirit level, reading lens and ercury lap. Forula : Where The dispersive power of the aterial of a pris is given by v 1) efractive index of the aterial of a pris for extree violet colour V efractive index of the aterial of a pris for extree red colour efractive index of the aterial of a pris for yellow (ean) colour position. efractive index is given by sin( A ) / 2 sin( A/ 2) Where A Angle of pris Angle of iniu deviation Procedure : To obtain pure spectru by spectroeter the following adjustents ust be ade : The axis of the telescope and that of colliator sut intersect the central axis of rotation of telescope. 2

3 The pris table is leveled with the help of spirit level. The slit of the colliator is ade narrow, vertical and syetrical on both sides. Note the lest count of vernier scales. Adjustent of the telescope. i) Turn the telescope towards a white wall and the distance between its objective and the eyepiece is so adjusted that the field of view becoes copletely luinous. Now the eyepiece is displaced inside the tube till the cross-wire becoes distinctly visible. ii) Now the telescope with objective is directed towards a distant tree or pole and they are viewed through the telescope. The distance between the objective and the eye piece is adjusted with the help of rack and pinion arrangeent such that a distinct and clear iage of the object is seen. Thus the telescope is ready to focus all the parallel rays at the cross wire. Adjustent of Colliator : Place the ercury lap in front of the slit of colliator and align the telescope with the colliator such that the iage of the slit is seen through telescope. Thej distance between the slit and the lens of the colliator is adjusted with the help of its rack and pinion arrangeent until a distinct iage is seen through the telescope. In this position the light rays coing out of the colliator will be parallel to each other. Adjustent of Pris table : The height of the pris table is adjusted in such a way that the light rays coing out of the colliator fall axiu on the refracting surface of the pris when it is placed on the pris table. Schuster s ethod : There is another ethod of adjusting the telescope and the colliator for parallel rays. This ethod is called Schuster s ethod. The ethod is as follows : i) First of all pris table is adjusted at the sae height of the colliator and telescope. ii) iii) iv) Now the pris is placed on the pris table in such a position that its one of the refracting surface faces the colliator. The light eerging fro the other refracting surface of the pris is viewed through the telescope. The pris table and the telescope both are then rotated slowly and siultaneously so that the spectral lines always reain the cross-wire. A state is reached when on rotating the pris table further, the direction of rotation of the spectral lines reserved. This position is the position of iniu deviation. Now, keeping the telescope fixed, the pris table is rotated through a sall angle. The spectral lines are seen to be blurred. The telescope is adjusted with the help of rack and pinion arrangeent so that spectral lines becoe distinct and clear. v) table is rotated through a sall angle in the colliator is adjusted with the help of a rack and pinion arrangeent so that spectral lines becoe distinct and clear. 3

4 vi) The ethod described in steps (iv) and (v) are repeated attain and again till the spectral lines becoe distinct and clear through out the entire rotation of the telescope. In this position the spectroeter will be set for parallel rays coing out of the colliator, focusing parallel rays by telescope at its cross-wire and the pris in the position of angle of iniu deviation. Measureent of the angle of iniu deviation : i) Follow the procedure as given in steps (i), (ii), and (iii) of the Schuster s ethod. ii) iii) iv) Now fix the vertical cross-wire on one of the extree spectral line of the spectru with the help of tangent screw of the telescope. Note the reading of both the verniers on the ain scale. Siilarly fix the vertical cross wire on the ean line (yellow line) and extree spectral line on the other end of the spectru and in each position note the reading of both the verniers on the ain scale. The position of the pris table is kept fixed and the pris is reoved. The telescope is rotated till the direct iage of the slit is viewed at its vertical cross-wire. The position of the telescope is noted by the reading of both the verniers. v) The difference of readings in steps (ii), (iii) with (iv) is calculated for both the circular verniers. The ean of these two differences will be angle of iniu deviation. Measureent of the angle of Pris A : i) Place the pris on the pris table with its angle of pris A towards the colliator and with its refracting edge A at its centre. In this position soe of the light falling on each refracting surface AB and AC will be reflected and can be received by the telescope. ii) iii) iv) The telescope is oved to one side to receive the light reflected fro the refracting surface AB and the iage of the slit is focused at the vertical cross-wire. The reading of two verniers are noted on the ain scale. Now the telescope is oved to another side to receive the light reflected fro the refracting surface AD and the iage of slit is focused again at the vertical cross-wire. The reading of two verniers are noted again on the ain scale. The difference of readings in steps (ii) and (iii) is calculated for both the verniers. The ean of these two differences will be twice the angle of pris. Therefore, half of this angle gives the angle of pris. The refractive index and then dispersive power of the pris are calculated fro these observations. 4

5 Figure: Observations : One division of ain scale x.. Nuber of vernier divisions n.. Least count of the vernier n x.. For iniu deviation : eading in the S.No. Colour Circular Scale iniu deviation position 1. ed V 1 2. ellow V 1 3. Violet V 1 eading in the position of direct iage.s. c.s. Total a.s. c.s. Total b a - b Mean 5

6 S.No. For Angle of Pris A : eading for eading for reflection Vernier eflection fro face fro fact AC AB.s. c.s. Total a.s. c.s. Total a Difference a - b Mean Value 2A Mean Value A 1. V 1 2. V 1 Calculation : Angle of pris A Angle of iniu deviation for extree violet colour V sin( A ) / 2 for violet V sin A / 2 Angle of iniu deviation for extree red colour for ed sin( A ) / 2 sin A / 2 Angle of iniu deviation for extree red colour sin( A ) / 2 for ellow sin A / 2 Dispersive power of aterial of a pris V a esult : The dispersive power of aterial of a pris Standard value for the flint glass Crown Glass % error Sources of Error : The slit ay not be vertical and narrow. The edge of slit and pris ay not be perfectly vertical. The axis of rotation of the telescope ay not pass through the centre of circular scale. While taking observations, telescope and pris table are not fixed. 6

7 Pris is not clean. Precautions : The slit should be as narrow as possible but the knife edge s of the slit should not touch each other. The telescope and the colliator should be separately set for parallel rays. The height of the pris table should be so adjusted that the axiu light ust fall on the entire surface of the pris. While taking observations the telescope and the pris table ust be claped. The reading lens should be used for taking readings on both the verniers. Viva-Voce : What is the use of collilator in the spectroeter? The colliator akes the light rays coing fro the light source parallel to each other. Why are lines and circles drawn on the pris table? For placing the grating or pris properly on the pris table. Why do you take readings fro both the verniers? This is done in order to eliinate the error arising due to non coincidence of the axis of rotation of the pris table or the telescope with the centre of the circular ain scale. * * * 7

8 8