# An experiment is performed to find the refractive index of glass using a travelling microscope. In this experiment distances are measured by Option 1) a screw gauge provided on the microscope Option 2) a vernier scale provided on the microscope Option 3) a standard laboratory scale Option 4) a meter scale provided on the microscope.

As we learnt in.

To determine refractive index of a glass slab using travelling microscope -

Calculation

$\mu=\frac{Real\ thickess\ of\ the\ slab}{Apparent\ thickness\ of\ the\ slab}$

$\mu=\frac{R_{3}-R_{1}}{R_{3}-R_{2}}$

$Mean=\mu=\frac{\mu_{1}+\mu_{2}+\mu_{3}}{3}$

Result: $\frac{R_{3}-R_{R_{1}}}{R_{1}-R_{2}}= Const.$

It gives R.I of material of the glass slab.

* Const. of proportionality in Snell's Law is called R.I.

$\mu_{g}^{a}=\frac{Real\ thickness}{Apparent\ thickness}$

- wherein

< SPQ = i

sini = $\frac{SQ}{PQ}$

<SPQ=r

sinr = $\frac{SQ}{P_{1}Q}$

$\mu_{g}^{a}=\frac{P_{1}Q}{PQ}$

$\mu_{a}^{g}=\frac{PQ}{P_{1}Q}$

Procedure

1.    The least of the vertical scale of the travelling microscope are determined.

2.    Make a black-ink cross-mark on the base of the microscope.

3.    The whole microscope tube is moved by loosening the vertical fixing screw till image of the cross is visible in the piece.

4.    The sharp image of the cross is focused using the rack and pinion screw provided with the microscope tube. (The position of racks and pinion screw once fixed is not to be disturbed throughout the experiment).

5.    The reading on the main and vernier scales of the icroscope are taken. This is the first observation.

6.    Two more sets are recorded by reseting the tube and refocusing the cross, without disturbing the rack and pinion screw. The average of these gives the first reading of the microscope.

7.    Place the glass slab on the cross mark. The microscope tube is adjusted by loosening the tightening screw to see the clear image of the cross and the tightening screw is tight.

8.    The image of the cross is brought into sharp focus using vertical fine motion screw and the reading on vertical (main and vernier) scale are noted.

9.    Two more readings are taken.

10.    Average of the above three observations give the second reading.

11.    Spread little lycopodium powder on the top surface of the glass slab and following these powder granules.

12.    Average of three observations taken in the same way as described in earlier step 8 and 11 gives the third reading.

13.    For better result, the experiment may be repeated with glass slab of different thickness but of the same material.

A vernier scale provided on the microscope

Option 1)

a screw gauge provided on the microscope

This option is incorrect

Option 2)

a vernier scale provided on the microscope

This option is correct

Option 3)

a standard laboratory scale

This option is incorrect

Option 4)

a meter scale provided on the microscope.

This option is incorrect

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