SPECTROMETER- PRISM.
Aim:
/
(o determine refractive Index of the material of solid, prism (glass) using
spectrometer)by measuring the (i) Angle of the prism () Angle of minimum
deviationy
/Apparatus
Spectrometer, glass prism, sodium vapour lamp, etc.
Description :
A spectrometer essentially consists of three components as shown in
nd
Fig. 41.1. (i). Collimator. (ii). Telescope and (iii). Prism Table.
CS
L
D base
Collimator
o Source
Fig. 41.1
Telscopê
Collimator is an arrangement of two coaxial tubes. There is a vertical
adjustable slit, S at one end and a convex lens L, at the other end. The narrow slit
of the collimator is illuminated by a source of light. Thedistance between the slit
and the lens is adjusted using a rack and pinion arrangement to focus the slit. The
rays of light through the slit, after refraction by the lens are rendered parallel.
This procedure is usually referred as adjusting the collimator to produce parallel
rays. The collimator is fixed to a rigid stand.
An astronomical telescope with its axis in same horizontal plane as that of
collimator, consists of on objective O and an eye piece E. The distance between the
objective and eye piece can be adjusted using a rack and pinion arrangement. In
front of the eye piece, a horizontal and vertical cross wires are fixed. The telescope
is attached to a circular scale CS and can be
rotated about a vèrtical axis, passing
through the center of circular scale. The telescope can be fixed at any
position by
means of radial screw and fine adiustmentsare done'usiig-a .
tangential screw.
The scale is graduated in degrees from 0° to 860. Each division of the scale
corresponds to half a degree.
A prism table P with adjustable height is provided with three léveling
screws.
It can be rotated about same vertical axis as that of telescope. The prism
table can
he raised or lowered and fixed at any position using a
long screw.
The position of prism table can be read w ith the help of tw o vemiers V, and
V.. Each vernier scale is divided into thirty divisions. 2.
Procedure :
(a). Least Count: of Verniers
The value of each division in circular scale is found. The number of divisions
in vernier scale V, or, V, is noted. The zero of vernier scale is made to coincide
with any one of main scale divisions and total number of main scale divisions
required to cover complete vernier scale is determined. The Least Count (L.C) of
the vernier can be calculated as follows:
The value of 1 MSD = l2o = 30'
Number of divisions iD vernier scale = 30 VSD
Number of MSD to cover V.S. = 29 MSD
30 VSD = 29 MSD
Hence, 1 VSD 29/30 MSD
Therefore, Least Count (L.C.) of the vernier
L.C. = 1,M.S.D-1 V,S.D
= 1/30 MSD = 1.
(b)., Preliminary Adjustments :
1). The eye piece,of the telescope is adju_ted to see the vertical and
horizontal cross wires clarly and distinctly,
(ii). The telescope is then turned towards a distant object. The distanee
between the eye piece and the objective is adjusted using the rack and pinion
arrangement to get aclear and well defined image of the distant object. The ima9e
w ill be a dim in ish ed and inverted im age. Once the telescope is adjugted
(focussed for the distant object), the rack and pinion adjustment Bcrew
should not be disturbed.
(iii). The spectrometer is placed in front of a source of monochromatic light
(Sodium vapour lamp). The slit of collimator is opened narrowly. Telescope is
brought on line with collimator. The lens of collimator is then adjusted so that the
clear, well defined image of narrow slit obtained and coincides with the vertical
cross wire without parallax. The collimnator als0 need not be disturbed hereafter.
(iv). Levelling of prism table -
Optical Method -. The initial
levelling of the table is done by CO,
using spirit level and three
levelling screws. Prism is placed
on the table with its edge nearer
to the collimator and base at right
BO
b bae
angles to it. One of the refracting Ao
faces of the prism, say, ac must
be perpendicular to the line
joining any two of the leveling Fig. 41.2
screws (A and B), as shown in
Fig.41.2.
The rays from collimator are get reflected from both the faces ab and ac.
image of the slit due to reflection is formed on each face. Now, after locating
the imagefrom face ac, the telescope is turned to view this image. By working on
the leveling screws A and B, the image is bought to the centre of the field of view
of eye piece. Next, the image from the face ab is centralized in the field of view by
adjusting the third screw C. By these adjustments, the prism table is leveled.
Measurement of the Angle of Prism A
/The prism is placed on the prism table and mounted vertically with a prism
holder. The edge of the prism is facing the collimator, so that parallel rays of light
fall almost cqually on two refracting faces ab and ac, as shown in Fig. 41.3.
The telescope is turned towards one of the faces (ab) and the reflected image
from the face is observed through the telescope. Using tangential screw, the vertical
cross wire is made tocoincide with the image. The positions of prism table, vernier
and telescope are fixed. The main scale (circular scale) readings and coinciding
vernier scale divisiöns are noted for both the verniers V, and V.2 Now, releasing
the telescope, it is tuned towards other face ac, experiment is performed and
corresponding readings are noted. Obsèrvations are tabulated as in Table 41.1.
Thedifference in readings between Iand I face gives twice the angle of prism A.
160
conics
(d). Angle of Minimum Deviation D:
The prism is placed on the table
so that its base (groundéd face) is almost !
in line with the collimator. The parallel
rays falling on one of the refracting face
ab are refracted through other face [Fig.
base
41.4). The telescope is adjusted to obtain
this refracted imageof the slit. The prism
table and hence the prism is slightly
D
rotated in either direction, so that the
image tends to move towards the direct
image side. The image should be followed
by the telescope. In a particular position
for further rotation in same direction, the T
image just turns back in opposite
direction. This corresponds to minimum Fig. 41.4
deviation of the prism.
The prismn is fixed in this position and with finer adjustment using tangential
SCrew, the vertical cross wire at the center of field of view is made to coincide with
the image. Telescope is fixed and readings of verniers, V, and V, are noted. The
prism then is removed, telescope is released and is brought in line with the
collimator, the direct readings of verniers are taken. The difference between these
two readings for each vernier gives the angle of minimum deviation D and hence
the mean D is calculated.
HOLLOW PRISM
Refractive index of liquid
The refractive index of a transparent liquid (water, glycerine) can be
determined with same experimental procedure as in the case of solid glass prism.
A hollow prism is filled with the given liquid, say,
water. Two faces (refracting
faces) of hollow prism are made up of optically plane, thin parallel sided glass
plates. The angle of prism A and the angle of minimum deviation D are
lsee Table 41.1 and Table 41.2]. The refractive index u of the determined
from the formula liquid is calculated
Sin A+D
2
=
Sin)
