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

 

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

Aim
To determine refractive index of a glass slab using a travelling microscope.

Apparatus
Three “glass slabs of different thickness but same material, a travelling 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 main scale and vernier scale reading.

Theory

Diagram

Procedure
Adjustment of 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 eye piece 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 (R2) 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 above steps with other glass slab of more thicknesses.
15. Record observations in tabular form as given below.

Observations and calculations
Vernier constant (least count) for vertical scale of microscope = 0.001cm.

S  No	Reading on vertical scale while Focused microscope in			Real thickness (  R3 – R1) cm	Apparent thickness (  R3 – R2) cm	Refractive index n = (  R3 – R1)/ (  R3 – R2)
	Cross mark without glass slab R1  cm	Cross mark with glass slab R2 cm	Lycopodium Powder R3  cm
1 2 3	3.4 3.425 3.35	3.845 4.052 3.95	4.6 5.226 5.1	1.2 1.8 1.75	0.775 1.173 1.15	1.54 1.53 1.52

 

Precautions

1. In microscope, the parallax should be properly removed.
2. The microscope should be moved in upper direction only to avoid back lash error.

Sources of error
The microscope scale may not be properly calibrated.

Result

The ratio (  R₃ – R₁)/(  R₃ – R₂) = constant

It gives refractive index n= (1.54+1.53+1.52)/3 = 1.53