To Determine Refractive Index of a Glass Slab Using a Travelling Microscope

Aim/Aim of Experiment

To determine Refractive Index of a glass slab using a Travelling Microscope.

Apparatus/Material Required

  • A Marker
  • A Paper
  • Three “Glass slabs” of different thickness but same material
  • A Travelling Microscope
  • Lycopodium powder.


Refraction – Refraction is the phenomenon of propagation of light from one transparent medium into other medium, such that the light deviated from its original path. (Usually the first medium always be air).

Glass Slab – A slab is a piece of transparent material with rectangular faces. All faces are transparent and opposite faces are parallel.

Thickness of Glass Slab – The dimension along with the light travels inside the slab is called its thickness.

Traveling 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.

Refractive Index – The ratio of velocity of light in first medium to its velocity in second medium, is called refractive index of second medium with respect to first medium. It is represented by the symbol p or n.

If a glass slab is placed in air on a horizontal surface and its bottom surface is viewed from top, it appears to be elevated due to the phenomenon of refraction. The distance between this apparent bottom and the top surface of the slab gives the apparent thickness of the slab. In case of normal observation, it can be shown that the refractive index of glass with respect to the medium, air is,

Refractive index, n = real thickness of slab/apparent thickness of slab.


to determine refractive index of a glass slab using a travelling microscope.
Refractive index of a glass slab using a travelling microscope


Adjustment of Travelling Microscope:

  1. To get sufficient light which falls on the travelling microscope (M), place the travelling microscope on the table near a window.
  2. To make the base of the travelling microscope horizontal, Adjust the levelling screws.
  3. Adjust the position of the eye piece so that the cross wires are clearly visible.
  4. Now determine the vernier constant of the vertical scale of the microscope.

Other Steps:

  1. On the base of the Microscope make a black-ink cross-mark. The mark will serve as point P.
  2. Ensure there is no parallax between the cross-wires and the image of the mark P. So that make the microscope vertical and focus it on the cross at P.
  3. Note the main scale and the vernier scale readings (R1) on the vertical scale.
  4. Place the glass slab of least thickness over the mark P.
  5. Raise the microscope upwards and focus it on the image P1 of the cross-mark.
  6. Note the reading (R2) on the vertical scale as before (Step 3).
  7. Sprinkle a few particles of lycopodium powder on the surface of the slab.
  8. Raise the microscope further upward and focus it on the particle near S.
  9. On the vertical scale again note the reading (R3), (Step 3).
  10. Repeat above steps with different glass slab of more thicknesses.
  11. Record your observations.


Table for Microscope Readings:

Sr. No.(RVS) Cross Mark without slab R1 (cm)(RVS) Cross Mark with slab R2 (cm)(RVS) Lycopodium powder R3 (cm)Real Thickness (R3-R2) (cm)Apparent Thickness (R3-R2) (cm)Refractive Index (n=R3-R1/R3-R2)

(RVS) – Reading on Vertical scale when Microscope is focused on.


  • Vernier constant (least count) for vertical scale of microscope= 0.001 cm.
  • Mean n = (n1+n2+n3/3) =(1.15+1.27+1.24/3) =(3.66/3), therefore n=1.22.


The ratio (n=R3-R1/R3-R2) is constant. It gives the refractive index of the glass slab.


  • Least count of the scale of travelling microscope should be calculated carefully.
  • In Travelling Microscope, the parallax should be properly removed.
  • To avoid back lash error. The microscope should be moved in only upper direction.
  • Only a thin layer of powder should be spread on the top of the slab.

Sources of Error

  • The parallax may not be fully remoued.
  • The microscope scale may not be properly calibrated.

Viva Voice Questions with Answers

1. Define a slab.

Answer: A slab is a piece of transparent material with rectangular faces. All faces are transparent and opposite faces are parallel.

2. Define thickness of a slab.

Answer: The dimension along with the light travels inside the slab is called its thickness.

3. Define lateral displacement.


4. Why a slab does not deviate and disperse light, where as a prism does?

Answer: In a slab, the refracting faces are parallel. The emergent ray is parallel to the incident ray. There is no deviation and dispersion. In a prism, the refracting faces are not parallel. The emergent ray is not parallel to incident ray. There is a deviation and hence dispersion.

5. Why lycopodium power is spread over the glass surface?

Answer: To focus the microscope accurately, otherwise the bottom surface will be focused because of transparency of glass slab.

6. What is normal shift?

Answer: It is the difference between actual depth and apparent depth. Its S.I. unit is metre.

7. What is cause of normal shift?

Answer: Due to refraction of light.

8. On what factors, apparent depth depends?

Answer: On following factors apparent depth depends:

  1. Nature of medium (R.I.),
  2. Thickness of medium (actual depth),
  3. Colour of light.

9. In general for which colour we take the refractive index of a material in lens and glass slabs?

Answer: Yellow colour. Since it is the mean colour of visible spectrum.

10. What may be refractive index for hollow glass slab?

Answer: n=1.

Class 12 Physics Practicals:

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