Determine Refractive Index of a Glass Slab using a Travelling Microscope

Students often face trouble while conducting a travelling microscope experiment. Here, we will discuss the correct procedure to conduct this experiment, ensuring the best possible outcome.

However, before proceeding with the travelling microscope experiment class 12, let us learn some of the important factors necessary for the same. 

Defining Refractive Index

Index of refraction, or refractive index, is defined as the measure of deviation of a light ray when it passes from one medium to another. In simpler terms, suppose you have a glass full of water. If you place it in sunlight, the light bends upon entering the water. If you measure the angle of such a bend, you will get its refractive index.

You can calculate a refractive index if the velocity of light c for a particular wavelength in empty space is known. Additionally, you must also know the value of ‘v’, which represents light’s velocity in a substance. In such a case, refractive index n = c/v

What is a Travelling Microscope?

Before you can use a travelling microscope experiment effectively, you must understand the functionality of such a device. Travelling microscopes act as simple microscopes, with one exception. 

Where a simple microscope remains fixed for the duration of a study or experiment, a travelling microscope’s head is fitted onto a slider. Therefore, it can move along a scale, studying an object from various distances. Readings are taken by combining the readings from the Vernier and main scale. 

Now, let us proceed to determine the refractive index of glass slab using travelling microscope.

Apparatus Necessary

  • Three glass slabs, each varying in thickness. Material for each slab must be identical.

  • Travelling microscope, and

  • Lycopodium powder

Theory for Refractive Index Experiment Report

Refractive Index (n) = Slab’s real thickness/slab’s apparent thickness

Procedure to Follow

To ensure accuracy in this refractive index of glass slab using travelling microscope readings, follow the process mentioned below.

  • Step 1: Place a travelling microscope near a light source.

  • Step 2: Adjust screws to ensure that the base of this microscope is horizontal.

  • Step 3: Position the microscope horizontally, check the eyepiece to see whether the cross wires are visible clearly.

  • Step 4: Check the Vernier Constant of this scale when it is kept vertically.

  • Step 5: Use a marker to draw a mark at the microscope’s base. Consider this point as P.

  • Step 6: Now, focus the vertical microscope on point P in such a way that there is no chance of a parallax between this image of P and the cross wires.

  • Step 7: Now, note the vernier scale, as well as the main scale reading. Consider this as R1.

  • Step 8: Place the thinnest glass slab on point P.

  • Step 9: Lift the microscope and focus the image of P1 of the cross-mark.

  • Step 10: Make a note of the reading on the vertical scale (R2).

  • Step 11: Sprinkle lycopodium powder on the slab.

  • Step 12: Lift the microscope further, focusing it on this particle near S. 

  • Step 13: Make a note of R3 on this vertical scale.

  • Step 14: Follow the same procedure to take readings of the other glass slabs.

Note down the results in a tabular format for increased ease of calculations.

Table for Readings

Serial Number

Vertical scale reading  when it is focused on

Real Thickness

Apparent Thickness

Refractive Index








Refractive index calculation = R3 – R1/R3 – R2

Mean refractive index = n1 + n2 + n3/3

Precaution – Ensure that you remove the parallax properly in step 6, failing which results of this travelling microscope experiment can be erroneous.

To know more about refractive index and experiments in general, consult our live online classes. Our experienced teachers guide you toward proper understanding with an expertly devised curriculum. Furthermore, now you can also download our Vedantu app for added convenience.

FAQ (Frequently Asked Questions)

1. How can I define a normal shift?

Normal shift is nothing but the difference between actual depth and apparent depth of an image.

2. What is the refractive index of a glass slab, which is hollow?

For such a glass, the refractive index or n is always 1.

3. What is the significance of lycopodium powder in this experiment?

Lycopodium powder ensures that the microscope’s focus is correct, and not at this bottom surface of this transparent slab.