# What is. the experiment about refractive indexâ€‹

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.

Theory-

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

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 = .....

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

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

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

Result-
The ratio $\frac{R_{3}-R_{1}}{R_{3}-R_{2}}$  is constant.
It gives the refractive index of the material of the glass slab.

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