To Learn the Parts of a Spectrometer and to Read a Reading.

Aim of the Experiment

To learn the parts of a Spectrometer and to read a reading.

Apparatus Required

Reading Lens
Spirit Level


Spectrometer has the following three main parts:
1. The Collimator
2. The Telescope
3. The Prism Table

Spectrometer Parts

1. The Collimator

Its purpose is to produce a parallel beam of light. It consists of a lens and slit. The slit faces the source of light. The distance between the slit and the lens can be adjusted by a screw fixed to the collimator tube C to obtain parallel beam of rays. The slit consists of two sharp edges. One of the edges is fixed while the other can be moved parallel to it by working the screw provided at its side. The collimator cannot be rotated.

2. The Telescope

The telescope T consists of an objective 0 at one and an eyepiece fixed with the cross wires on the other end. The telescope is fixed to the circular scale graduated in degrees. The circular scale is in between the collimator and the telescope. The telescope can be turned with the scale about a vertical axis passing through the center of the spectrometer. The eyepiece of the telescope is provided with cross — wires. The telescope can be fixed in any position by a main screw. Fine adjustment can be made by a fine adjustment screw which is tangential to the main screw. The focusing of the telescope is done by a screw to the side of the telescope.

3. The Prism Table

This consists of a upper plate and a lower plate separated by three springs. Three screws pass through these springs. There are lines engraved in the upper plate so as to mount the prism in proper position. The prism table P can be mounted in any position by means of a screw at its bottom. The prism table can be rotated about the same yertical axis as the telescope.

Initial Adjustments

Telescope Adjustment

The telescope is turned towards a distant object and its focusing screw is adjusted till the image of the object is clearly seen. In this position, the telescope is capable of receiving parallel rays.

Collimator Adjustment

The slit is illuminated with sodium vapour lamp or Hg vapour lamp. The telescope is turned so that the telescope and the collimator are in a line. In this position one can see the image of the slit through the telescope. The clear image of the slit is obtained by adjusting the collimator screw. The slit must be adjusted to be narrow and vertical.

Levelling of the Prism Table

This is done with a spirit level. The spirit level is kept on the prism table and the three leveling screws of the prism table are adjusted till the air bubble comes to the centre.


1. To find the least count of the spectrometer:

The main scale is a circular scale graduated in degrees. The value of one MSD is half degree. Each vernier scale consists of 30 divisions as shown in the figure below:

Circular Scale of Spectrometer

29 MSD are equal to 30 VSD.
LC = I MSD - 1 VSD
1 MSD = 0.5o = 30´ (10 = 60´ )
30 VSD = 29 MSD
1 VSD = 29/30 MSD
= (29/30) X 30´
= 29´
1 LC = 30´ - 29´
L.C. = 1´

2. To read a reading:

Main scale reading (MSR) and vernier scale reading (VSR) are noted as explained in the vernier calipers when the telescope and the disc (prism table) are fixed at the required position by the main or fine adjustment screw.

The two vernier scales VA and VB are fixed to the disc, which can be rotated about the same vertical axis. A main screw and a fine adjustment screw as in the telescope control the movement of the vernier scales.


Least Count L.C = 1´

S.No. M.S.R V.S.C (div) V.S.R = V.S.C x LC T.R = M.S.R + V.S.R
1 110.5o= 110o 30´ 9 110o 39´


The parts and functions of the Spectrometer are studied and a few readings are taken.

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