Reason (R) : Optical density of a medium is directly proportionate to its mass density which results in a proportionate refractive index.
In the light of the above statements, choose the most appropriate answer from the options given below :

(A) is not correct but ( R ) is correct

Both (A) and (R) are correct and (R) is the correct explanation of (A)

(A) is correct but (R) is not correct

Both (A) and (R) are correct but (R) is not the correct explanation of (A)

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Concepts Covered - 1

To determine refractive index of a glass slab using travelling microscope

Aim-
To determine the refractive index of a glass slab using a traveling microscope.

Apparatus-
Three "glass slabs of different thickness but same material, a traveling microscope, lycopodium powder.

A slab is a piece of transparent material with rectangular faces. All faces are transparent and opposite faces
are parallel. The dimension along with the light travels inside the slab is called its thickness.

A Short Description of a Travelling Microscope
It is a compound microscope fitted vertically on a vertical scale. It can be moved up and down, carrying a
vernier scale moving along the main scale.
In any position, the reading is taken by combining the main scale and vernier scale reading.

Theory-

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

Diagram-

Procedure-
Adjustment of the travelling microscope

1. Place the travelling microscope (M) on the table near a window so that sufficient light falls on it.
2. Adjust the levelling screws so that the base of the microscope becomes horizontal.

3. Make microscope horizontal. Adjust the position of the eyepiece so that the cross wires are clearly
visible.
4. Determine the vernier constant of the vertical scale of the microscope.

Other steps
5. Make a black-ink cross-mark on the base of the microscope. The mark will serve as point P.

6. Make the microscope vertical and focus it on the cross at P, so that there is no parallax between the
cross-wires and the image of the mark P.
7. Note the main scale and the vernier scale readings (R₁) on the vertical scale.
8. Place the glass slab of least thickness over the mark P.

9. Raise the microscope upwards and focus it on the image P₁ of the cross-mark
10. Note the reading P₂ on the vertical scale as before (Step 7 )
11. Sprinkle a few particles of lycopodium powder on the surface of the slab.

12. Raise the microscope further upward and focus it on the particle near S.
13. Note the reading R₃ on the vertical scale again (Step 7)
14. Repeat the above steps with another glass slab of more thicknesses.
15. Record your observations.

Calculation-

Vernier constant (least count) for the vertical scale of microscope = .....

$
\begin{aligned}
& \mu=\frac{\text { Real thickess of the slab }}{\text { Apparent thickness of the slab }} \\
& \mu=\frac{R_3-R_1}{R_3-R_2} \\
& \text { Mean }=\mu=\frac{\mu_1+\mu_2+\mu_3}{3}
\end{aligned}
$

Result-

$
\underline{R_3-R_1}
$

The ratio $\overline{R_3-R_2}$ is constant.
It gives the refractive index of the material of the glass slab.