Detailed Procedure and Key Concepts for Accurate Measurement
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.
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.
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.
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:
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.
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.
FAQs on How to Determine the Refractive Index of a Glass Slab Using a Travelling Microscope
1. What is the fundamental principle behind using a travelling microscope to find the refractive index of a glass slab?
The core principle is the phenomenon of refraction, which causes a shift in the apparent position of an object when viewed through a denser medium. A travelling microscope accurately measures the real depth (actual thickness of the slab) and the apparent depth (the depth at which an object below the slab appears to be). The refractive index (μ) is then calculated as the ratio of these two values, based on the bending of light.
2. What is the specific formula used to calculate the refractive index from the readings taken in this experiment?
The refractive index (μ) of the glass slab is calculated using the formula derived from the real and apparent depths:
μ = Real Depth / Apparent Depth
Using the standard notations for the microscope readings, the formula becomes:
μ = (R₃ - R₁) / (R₃ - R₂)
Where:
- R₁ is the microscope reading of the mark without the slab.
- R₂ is the reading of the mark's image seen through the slab.
- R₃ is the reading of the slab's top surface.
3. What are the three essential readings that must be recorded with the travelling microscope?
To calculate the refractive index, three precise readings are required, each corresponding to a different focal plane:
- Reading 1 (R₁): The microscope is focused on a reference mark (e.g., an ink cross) on paper without the slab.
- Reading 2 (R₂): The glass slab is placed over the mark, and the microscope is refocused on the image of the mark as seen through the slab. This determines the apparent position.
- Reading 3 (R₃): The microscope is focused on fine particles, like lycopodium powder, sprinkled on the top surface of the slab. This determines the actual top position of the slab.
4. What is the importance of using lycopodium powder in this experiment?
Lycopodium powder is sprinkled on the top surface of the glass slab to provide a clear, opaque reference point. Since the glass surface itself is transparent, it is difficult to focus the microscope on it accurately. The fine powder creates a distinct plane, allowing for a precise measurement of the reading (R₃), which is essential for calculating the real depth of the slab.
5. What are the most important precautions to take to ensure an accurate result in this experiment?
To minimise errors and obtain an accurate value for the refractive index, a student must:
- Eliminate Parallax Error: Ensure there is no relative shift between the microscope's cross-wire and the focused image when the eye is moved sideways. This is the most critical step for correct readings.
- Use a Sharp Mark: The reference mark should be very fine and clear to allow for precise focusing.
- Clean Apparatus: The glass slab must be clean and free from scratches to avoid distorted images.
- Vertical Movement Only: Adjust the microscope using only the vertical screw to avoid lateral shifts while focusing.
6. Why is a travelling microscope preferred over a simple scale or ruler for measuring the slab's thickness?
A travelling microscope is preferred due to its significantly higher precision. Its least count (typically 0.001 cm) is much smaller than that of a ruler (0.1 cm). This allows it to accurately measure the very small difference between the real and apparent depths, a measurement that would be highly inaccurate or impossible with a standard scale. It also helps in systematically removing parallax error, which is not possible with a ruler.
7. How does the concept of 'normal shift' relate to the real and apparent depths measured in this experiment?
The 'normal shift' is the vertical distance by which the reference mark appears to be lifted when viewed through the glass slab. It is the direct result of refraction. Mathematically, it is the difference between the real depth and the apparent depth. This shift occurs because light rays travelling from the mark (denser medium) to the air (rarer medium) bend away from the normal, making the object appear closer to the surface than it actually is.
8. What would be the effect on the final result if a student forgets to remove parallax error before taking readings?
Failing to remove parallax error leads to incorrect readings for R₁, R₂, or R₃ because the focus is not exact. An inaccurate reading for the apparent position (R₂) or real position (R₃) will directly lead to an incorrect calculation of the apparent depth or real depth. This systematic error will propagate through the formula, resulting in a calculated refractive index that is either significantly higher or lower than the true value, making the experiment's result unreliable.
9. Can this experiment be performed with a transparent liquid like water instead of a glass slab? If so, what changes are needed?
Yes, the refractive index of a liquid can be determined using a similar procedure with a few key changes. A beaker is used to hold the liquid. The readings required are:
- R₁: Focus on a mark at the bottom of the empty beaker.
- R₂: Focus on the mark after filling the beaker with the liquid.
- R₃: Focus on the top surface of the liquid.
10. If you perform this experiment with a hollow glass slab, why is the calculated refractive index equal to 1?
A hollow glass slab contains air. When it is placed on a surface, the medium inside the slab (air) is the same as the medium outside (air). Since light rays do not change their medium while passing from the reference mark to the microscope, no refraction or bending occurs. Consequently, the apparent depth is exactly equal to the real depth. When these equal values are used in the formula μ = Real Depth / Apparent Depth, the ratio is 1, correctly indicating the refractive index of air relative to air.
