Refractive index of a glass slab using a travelling microscope

Summary

This video script outlines an experiment to determine the refractive index of a glass slab using a traveling microscope. It explains the concept of real and apparent depth and how they relate to the refractive index. The process involves setting up the microscope, finding the least count, and measuring the real and apparent thickness of the glass slab. The refractive index is then calculated as the ratio of these two measurements. The experiment is repeated for accuracy, and the mean refractive index is determined.

Takeaways

• 🔍 The refractive index is a dimensionless number that indicates how light bends when it passes from one medium into another.
• 📏 The refractive index is calculated as the ratio of the sine of the angle of incidence to the sine of the angle of refraction.
• 🌟 The real and apparent depth concept is used to determine the refractive index of a glass slab.
• 🔬 A traveling microscope is used to measure the real and apparent thickness of a glass slab to calculate its refractive index.
• 📏 The least count of the microscope is determined by the ratio of the length of one main scale division to the number of divisions on the Vernier scale.
• 🛠️ The microscope must be leveled using a spirit level to ensure accurate measurements.
• 📋 A mark is made on a paper to serve as a reference point for measuring the thickness of the glass slab.
• 🔭 The microscope is adjusted to focus on the mark, and readings are taken before and after placing the glass slab.
• 🌌 Lycopodium powder is used to observe the top surface of the glass slab and to take additional readings.
• 📊 The real thickness of the glass slab is calculated by subtracting the initial reading from the reading after placing the slab.
• 🔄 The process is repeated multiple times to find the mean refractive index of the glass slab.

Q & A

What is the refractive index and how does it relate to the propagation of light?

The refractive index is a dimensionless number that describes how light propagates through a medium. It is the ratio of the speed of light in a vacuum to its speed in the medium. When light travels obliquely from one transparent medium into another, it changes direction due to the difference in refractive indices of the two media.

What is the relationship between the angle of incidence and the angle of refraction in terms of refractive index?

The relationship is given by the equation where the ratio of the sine of the angle of incidence (I) to the sine of the angle of refraction (R) is a constant, which is the refractive index of the second medium with respect to the first medium.

How can the real and apparent depth be used to determine the refractive index?

The refractive index of glass with respect to air can be determined by the ratio of the real thickness of the glass slab to its apparent thickness when viewed from above. The apparent depth appears elevated due to the refraction of light through the glass.

What is the aim of the experiment described in the script?

The aim of the experiment is to determine the refractive index of a glass slab using a traveling microscope by measuring the real and apparent thicknesses of the slab.

What materials are required to perform the experiment?

The materials required for the experiment include a traveling microscope, a glass slab, a spirit level, lycopodium powder, paper, a marker, and adhesive tape.

What are the main components of a traveling microscope mentioned in the script?

The main components of a traveling microscope include the eyepiece, objective, vertical main scale, vertical Vernier scale, horizontal Vernier scale, and horizontal main scale.

How is the least count of the microscope determined?

The least count of the microscope is determined by the ratio of the length of one main scale division to the number of divisions on the Vernier scale.

Why is a spirit level used in the experiment and how is it used?

A spirit level is used to ensure that the base of the microscope is horizontal, which is crucial for accurate measurements. It is used by adjusting the leveling screws until the bubble in the spirit level is centered.

How is the real thickness of the glass slab calculated in the experiment?

The real thickness of the glass slab is calculated by taking the difference between the readings R3 (when the microscope is focused on the particles on the top surface of the glass slab) and R1 (the initial reading before placing the glass slab).

How is the apparent thickness of the glass slab determined?

The apparent thickness of the glass slab is determined by the difference between the readings R3 and R2, where R2 is the reading when the microscope is focused through the glass slab on the mark on the paper.

What is the final step to calculate the refractive index of the glass slab?

The refractive index of the glass slab is calculated as the ratio of the real thickness to the apparent thickness of the glass slab. The experiment is repeated multiple times to find the mean refractive index for increased accuracy.

Outlines

00:00

🔬 Introduction to Refractive Index and Experimental Setup

This paragraph introduces the concept of the refractive index, which is a dimensionless number that indicates how light propagates through a medium. It explains that when a light ray travels obliquely from one transparent medium to another, its direction changes due to refraction. The refractive index is the ratio of the sine of the angle of incidence to the sine of the angle of refraction. The paragraph then describes an experiment to determine the refractive index of a glass slab using a traveling microscope. The experiment involves understanding real and apparent depth and requires materials such as a traveling microscope, a glass slab, a spirit level, lycopodium powder, and other tools. The setup includes adjusting the microscope to be level and ensuring the lens system is correctly positioned over a marked point on a sheet of paper. The procedure also involves finding the least count of the microscope by comparing the main scale division to the Vernier scale.

05:00

🔍 Measuring Refractive Index Using a Traveling Microscope

This paragraph details the steps to measure the refractive index of a glass slab using a traveling microscope. It begins with taking the initial reading (R1) with the microscope focused on a mark on a paper. Then, the glass slab is placed over the mark, and the microscope is adjusted to refocus on the mark through the glass, taking a new reading (R2). Lycopodium powder is sprinkled on the glass slab to observe the particles clearly when the microscope is raised and focused again, recording another reading (R3). The real thickness of the glass slab is calculated as the difference between R3 and R1, while the apparent thickness is the difference between R3 and R2. The refractive index is then determined as the ratio of the real thickness to the apparent thickness. The experiment is repeated multiple times to ensure accuracy, and the mean refractive index is calculated from the observations.

Keywords

💡Refractive Index

The refractive index is a dimensionless number that characterizes the speed of light in a medium relative to its speed in a vacuum. It is crucial in understanding how light bends when it passes from one medium to another, such as from air into a glass slab. In the video, the refractive index of glass is determined by comparing the real and apparent thickness of a glass slab, which is a primary focus of the experiment.

💡Angle of Incidence

The angle of incidence is the angle at which a ray of light strikes the surface of a medium. It is a fundamental concept in the study of refraction, as it directly influences the angle at which light bends. The video mentions this concept when discussing how the direction of light changes when it enters a different medium.

💡Angle of Refraction

The angle of refraction is the angle at which light bends after entering a new medium, relative to the normal (a line perpendicular to the surface). It is calculated using Snell's Law, which relates the angles of incidence and refraction to the refractive indices of the two media. The video script uses this concept to explain the change in direction of light as it enters the glass slab.

💡Real Depth

Real depth refers to the actual physical thickness of an object, such as a glass slab. In the context of the video, the real depth is measured using a traveling microscope to determine the thickness of the glass slab when it is not viewed through another medium, which is essential for calculating the refractive index.

💡Apparent Depth

Apparent depth is the perceived depth of an object when viewed through a medium that bends light, such as a glass slab. Due to refraction, the depth appears different than the actual depth. The video script describes how the apparent depth is measured and used to calculate the refractive index of the glass.

💡Traveling Microscope

A traveling microscope is an instrument used for measuring the thickness of transparent objects with high precision. In the video, it is used to measure both the real and apparent thickness of the glass slab, which is key to determining the refractive index.

💡Least Count

The least count of a microscope is the smallest division that can be measured by the instrument, indicating its precision. The video script describes how to find the least count of the traveling microscope, which is necessary for accurate measurements of the glass slab's thickness.

💡Spirit Level

A spirit level is a tool used to check if a surface is level or horizontal. In the video, it is used to ensure that the traveling microscope is set up correctly before taking measurements, which is important for obtaining accurate results.

💡Lycopodium Powder

Lycopodium powder is a fine, lightweight powder used in the video to create a thin layer on the glass slab. This helps in focusing the microscope on the top surface of the slab, which is necessary for measuring the apparent thickness.

💡Vernier Scale

A Vernier scale is a device used for precise measurement by aligning a secondary scale with a primary scale. In the video, the Vernier scale of the microscope is used in conjunction with the main scale to determine the exact position of the crosshair, which is crucial for accurate thickness measurements.

💡Refraction

Refraction is the bending of light as it passes from one medium to another with a different refractive index. The video script describes how refraction causes the apparent depth of the glass slab to appear elevated when viewed from above, which is the basis for the experiment to determine the refractive index.

Highlights

The refractive index is a dimensionless number that affects light propagation through a medium.

Light direction changes when it obliquely travels from one transparent medium to another.

The sine ratio of the angle of incidence to the angle of refraction equals the refractive index.

Real and apparent depth concepts are used to determine the refractive index.

The refractive index of glass to air is the ratio of real to apparent thickness of the glass slab.

A traveling microscope is used to measure the real and apparent thickness of a glass slab.

The experiment's aim is to determine the refractive index of a glass slab.

Materials required include a traveling microscope, glass slab, spirit level, lycopodium powder, and adhesive tape.

The microscope's least count is calculated by the ratio of main scale division to Vernier scale division.

The microscope must be leveled using a spirit level for accurate measurements.

Marking a point on paper serves as a reference for measuring the glass slab's thickness.

The microscope's fine adjustment is used to focus on the mark for initial readings.

The refractive index is calculated by comparing readings with and without the glass slab.

Lycopodium powder is used to observe the glass slab's top surface for accurate measurements.

Three readings (R1, R2, R3) are taken to calculate the real and apparent thickness of the glass slab.

The mean refractive index is determined by repeating the experiment and averaging the results.

The experiment concludes with the calculation of the glass slab's refractive index.

Transcripts

[Music]

refractive index of a glass slab

the refractive index of a material is a

dimensionless number that describes how

a light propagates through the medium

a ray of light that travels obliquely

from one transparent medium into another

will change its direction in the second

medium

if I is the angle of incidence R is the

angle of refraction then the ratio of

the sine of the angle of incidence to

the sine of angle of refraction is a

constant called the refractive index of

the second medium with respect to the

first medium

here we use the concept of real and

apparent depth to determine the

refractive index Theory

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

in case of normal observation it can be

shown that the refractive index of glass

with respect to air is the ratio of real

thickness of glass slab to the apparent

thickness of the glass slab

a traveling microscope can be used to

find the real and apparent thickness of

the glass slab and hence calculate its

refractive index

aim to determine refractive index of a

glass slab using a traveling microscope

materials required

a traveling microscope

a glass slab

spirit level

lycopodium powder

paper

a marker

adhesive tape

the traveling microscope has the

following parts

eyepiece

objective

vertical main scale

vertical Vernier scale

horizontal Vernier scale

and horizontal main scale

first find the least count of the

traveling microscope the least count of

microscope is the ratio of length of one

main scale division to the number of

division on the Vernier scale

place the traveling microscope on the

table

now place a spirit level on the

horizontal surface of the traveling

microscope

adjust the leveling screws if required

to make base of the microscope

horizontal so that the bubble in spirit

level comes exactly in the center

put a mark on a sheet of paper and place

it on the horizontal platform of the

microscope

use an adhesive tape to stick it in

place the Mark will serve as our Point p

make sure that the lens system of the

microscope is vertically above the mark

rotate the screw in front of the

traveling microscope anti-clockwise to

loosen it

adjust the vertical position of the

microscope and tighten it

for Clear visibility of the crossfire

adjust the position of the eyepiece

using the rack and pinion Arrangement do

not change the screw for the rest of the

experiment

use the fine adjustment screw on top of

the microscope to focus on the mark such

that the cross of cross wires coincides

with the mark on the paper note the main

scale reading just before the zero Mark

of Vernier scale

use the magnifying lens attached to

microscope and note the Vernier scale

division that coincides with the

division of the main scale as the

Vernier scale reading VSR

now calculate the total reading of

microscope R1

next place the glass slab over the mark

on the sheet of paper

the microscope will now be out of focus

without disturbing the microscope setup

loosen the screw for vertical adjustment

and gently raise the entire lens system

upward until mark on the paper seen

through the slab is sharp and clear

tighten the screw

make the fine adjustment

take the reading R2 with the main scale

and coinciding division of the Vernier

scale

spread a little lycopodium powder over

the top surface of the glass slab

make sure that the layer of powder is

very thin

now gently raise the microscope and

focus it to see some of the particles

clearly

foreign adjustment

record reading R3

now we have got three readings R1 R2 and

R3

calculate the real thickness of glass

slab as R3 minus R1

the apparent thickness of glass slab is

R3 minus R2

find the refractive index of the glass

slab as the ratio of real thickness of

the glass slab to apparent thickness of

the glass slab

repeat the above steps two more times

and record your observation

find the mean refractive index of the

glass slab

[Music]