The Spectrometer


In order to measure , the angle between zero order image and observed image, an instrument called the spectrometer is commonly used. It consists of the following parts:


A spectrometer consists of the following parts:


• A collimator, whose function is to make the light that enters through the slit come out as a parallel beam. The word ‘collimate’ means ‘to make parallel’.


• A heavy base onto which the collimator is fixed • A turntable onto which the diffraction grating or prism is placed


• A telescope which is free to rotate. The image of the slit will be viewed through this.


• A Vernier scale which is attached to the telescope so that the angle through which it rotates relative to the fixed base can be measured accurately.


Adjustments to the spectrometer before use


• Look into the telescope; adjust the telescope to bring the cross threads into focus. • View a distant object with the telescope; adjust the telescope to bring that distant object into focus.


• Illuminate the slit with light and view the slit through the telescope. Adjust the colli- mator to bring the slit into focus.


• Adjust the width of the slit so that it is narrow but is clearly visible. • Level the turntable by means of the levelling screws.


Vernier Scale on a Spectrometer


The spectrometer uses a Vernier scale to increase the accuracy of the measurement of angle to the nearest minute or one or two decimal places of degree. The Vernier scale is a movable scale which can slide or rotate past a main scale. As mentioned previously, the Vernier callipers, the micrometer and the spectrometer all use this scale.


Measuring angle using the Vernier scale on a spectrometer


Step 1: Locate the image of the slit at the centre of the cross threads by rotating the telescope.


Step 2: Read the angle using the Vernier scale. Step 2A: Note the line on the main scale that is directly in front of the first line on the Vernier scale. This gives the angle in degrees.


Step 2B: Note the line on the Vernier scale that lines up precisely with any line on the main scale. This improves the accuracy to one decimal place or to the nearest minute.


Step 2C: Combine the two readings taken to give the angle in degrees, accurate to one decimal place or to the nearest minute.


SAMPLE PROBLEM 14A What is the angle on the spectrometer shown in Fig 14.5?


30


The Vernier scale on a spectrometer


Telescope


Adjustable slit Collimator Turntable


Levelling screws Diffraction grating


Vernier scale Heavy base A spectrometer


VER I 20 10 0


15 (on the vernier scale) 155° (on the main scale) 155° + 15 =155°15


160


Fig 14.5:Vernier scale on a spectrometer, Sample Problem 14A


WAVE NATURE OF LIGHT 239


170


15


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