Determine Refractive Index of a Glass Slab using a Travelling Microscope

Objective: We will design a system to measure the refractive index of a glass slab using a moving microscope.

Introduction: A glass slab is a piece of glass with thickness ranging from a few millimeters to several meters. The refractive index of a glass slab is the ratio of the index of refraction to the thickness of the glass. Measurement of the refractive index of glass is important for optical applications like optical fiber and photonic crystal.

Structure: There are various methods for measuring the refractive index. We will measure the refractive index using a moving microscope.

Step 1: What will we need?

(Note: the same items are also required to create a light microscope)

We will start the design by looking for an item that can be used as both a lens and a holder. The most ideal item would be a microscope lens since we can use it to focus light on the glass slab and to measure the distance. We can also use it as a holder if we have to insert another lens to take a refractive index measurement.

We can use a microscope lens holder to attach the microscope lens in place. The microscope lens we will use is called an objective lens. The objective lens has a central convex lens that magnifies an object to be magnified.

Since the lens holder is circular, we can make the convex lens circular. We can use a microscope objective lens as a holder because the thickness of the lens holder is quite small.

We can buy the lens holder at a local store. However, we may have to make it from scratch. We will need a microscope, a microscope objective lens, a stepper motor, a potentiometer, and an Arduino board. We will describe the building process in the next section.

Step 2: Building process

Step 2.1: Choosing a Microscope 

We should buy a microscope to work with. First, we should consider the following when we are buying a microscope.

Whether it is a microscope made in a lab or one in a store. Amount of microscope that we want to buy. What brand name is most popular in stores, but can be bought for a cheaper price Next, we should decide where to mount the microscope. We should decide the size of the slab.

Also, if we use the same microscope for different experiments, we should decide whether we should buy a holder that can hold different lenses. If so, which size is most convenient?

Also, a microscope has to be able to magnify the object we are measuring the refractive index. We can decide whether the microscope has a magnification of 1x, 4x, 10x, 20x, and 40x.

2.1.1 Choosing a microscope

You can buy a microscope in the market.

Or, if you don't want to buy one, then you can ask your teacher for help. In any case, we can choose a microscope of the following three types.

• Microscope made in a laboratory

• Microscope made in a store

• Electron microscope

Now we will start the buying process.

1. Microscope Made in a Lab

The easiest way to start the process of buying a microscope is to ask your teacher to help. In this case, your teacher is going to give you instructions on buying a microscope for your experiments. There are many types of microscopes. Your teacher will give you some instructions.

2. Microscopes Made in a Store

If you don't have enough money to buy the microscope made in a laboratory, then you can buy one in a store. First, you have to buy the microscope itself. To do this, you have to visit a store of the type you want. Once you have seen the types of microscopes, the price of each one, and its functions, you will be able to choose the microscope that you need.

3. Electron Microscope

An electron microscope is an extremely high-tech microscope. We can't get an electron microscope at home. We have to find the store of the type we want to buy.

The advantage of an electron microscope over a general microscope is the resolution. We can obtain the magnification of the electron microscope of a hundred thousand times the magnification of the microscope we normally use. There are many types of electron microscopes. You have to find the store of the type you want to buy.

How to Do a Test With a Microscope?

There are two methods to test how the microscope works:

1. Direct Observation

When you test a microscope, you have to place an object in the objective lens, which is the lens where we'll see the object. The objective lens is very big, so it is not possible to directly observe its function from the camera. You have to buy a microscope with a camera. If you have bought a microscope, you can test its performance by looking at an object through the objective lens.

In this way, you can test if you can correctly see the object you are going to use. It is easier to take pictures with a small lens. For example, it is easier to take a test with the 30X objective, and it is difficult to do a test with the 400X objective. It is better to test an objective lens that has a strong lens.

2. Test With an Object that You Already Have

If you already have an object, it is easy to test the working principle of a microscope. However, it is not easy to test the microscope's performance in this way. In this case, you have to find an object that has sharpness and contrast.

What is Sharpness?

If the object that you want to observe has sharpness, you can better observe its details. If there is no sharpness, you cannot observe the fine details of the object. There are many kinds of sharpness in the object, such as

• Line (edge sharpness)

• Curve (curve sharpness)

• Point (dots sharpness)

There are also different types of sharpness in the observer's eyes, but this is not very important.

What is Contrast?

If the object that you want to observe has contrast, the edges of the object will be very clear and the objects can be distinguished. When you observe an object, it will be bright and dark, and we can judge whether the object is soft or hard. However, sometimes the object is not bright, and then it will be difficult to see the contrast.

How to test the Microscope?

You must try to find an object that has sharpness and contrast. If you do not find such an object, then you can buy one from a supermarket.

You can also refer to the specification of the microscope. First, make sure that you are familiar with how the microscope works. Second, make sure that you understand what a working principle means. If you are not familiar with a working principle, you must refer to the documentation. If you think it is appropriate, you can buy a kit or assemble the kit yourself. Third, we will learn how to use the microscope.

You must put a microscope slide (a white plate), a cover glass, a mirror, a camera lens, a light source, a tripod, a flashlight, a microscope, a micro-adjustment tool, a slide pen, and an adhesive tape in front of you.

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 the 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 the glass slab using a 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 a 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 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

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.